SpoIIQ Anchors Membrane Proteins on Both Sides of the Sporulation Septum in Bacillus subtilis*

During the process of spore formation in Bacillus subtilis, many membrane proteins localize to the polar septum where they participate in morphogenesis and signal transduction. The forespore membrane protein SpoIIQ plays a central role in anchoring several mother-cell membrane proteins in the septal membrane. Here, we report that SpoIIQ is also responsible for anchoring a membrane protein on the forespore side of the sporulation septum. Co-immunoprecipitation experiments reveal that SpoIIQ resides in a complex with the polytopic membrane protein SpoIIE. During the early stages of sporulation, SpoIIE participates in the switch from medial to polar division and co-localizes with FtsZ at the polar septum. We show that after cytokinesis, SpoIIE is released from the septum and transiently localizes to all membranes in the forespore compartment. Upon the initiation of engulfment, it specifically re-localizes to the septal membrane on the forespore side. Importantly, the re-localization of SpoIIE to the engulfing septum requires SpoIIQ. These results indicate that SpoIIQ is required to anchor membrane proteins on both sides of the division septum. Moreover, our data suggest that forespore membrane proteins can localize to the septal membrane by diffusion-and-capture as has been described for membrane proteins in the mother cell. Finally, our results raise the intriguing possibility that SpoIIE has an uncharacterized function at a late stage of sporulation.

Despite their small size and apparent simplicity, bacteria exhibit remarkably complex spatial organization, in which proteins localize to particular sites within the cell. Well-characterized examples include the cell division proteins, which assemble into a cytokinetic ring at mid-cell (1)(2)(3); the chemotaxis receptors, which localize to the flagellated cell pole (4); the actin-like cytoskeletal proteins that form spiral-like structures along the long-axis of the cell (5); and the secretion apparatus, which localizes at discrete sites in the cell membranes (6 -8). Although the number of proteins that display specific patterns of localization continues to grow, our knowledge of the cellular landmarks that anchor them at particular sites remains poorly understood.
A powerful system in which to identify the anchors responsible for localizing proteins at particular sites is the process of sporulation in Bacillus subtilis. Many proteins involved in spore formation localize to defined subcellular positions and this localization is critical for their activity. Upon the initiation of sporulation, the developing cell (called the sporangium) divides into two compartments of unequal size and dissimilar fates. The smaller compartment (the forespore) matures into a dormant spore, while the larger cell (the mother cell) nurtures the spore, packaging it in a protective coat. Through out the course of this developmental process, the two cells follow completely different programs of gene expression that are set in motion by the compartment-specific transcription factors F in the forespore and E in the mother cell.
The first landmark event in the process of sporulation is the formation of an asymmetric septum. The switch from medial to polar division is brought about by a change in the subcellular position of the cytokinetic FtsZ-ring from mid-cell to the cell poles (9). The membrane phosphatase SpoIIE, which is synthesized at the onset of sporulation, co-localize with FtsZ and plays an important role in re-positioning the FtsZ ring (10 -13). After the septum is complete, SpoIIE has a second function: the activation of the forespore-specific transcription factor, F (14 -16). The localization of SpoIIE at the polar septum plays a critical role in coupling developmental gene expression to the completion of this morphological event (17).
Shortly after polar division, the mother cell engulfs the forespore in a phagocytic-like process generating a cell-within-acell. As a result of engulfment, the forespore is surrounded by two membranes: its own referred to as the inner forespore membrane and one derived from the mother cell called the outer forespore membrane. Many proteins involved in spore morphogenesis and signal transduction specifically localize to the engulfing septal membranes (18 -23). Mother cell membrane proteins achieve this localization by insertion into the cytoplasmic membranes followed by diffusion to and capture in the septum (24). However, in most cases it is not clear how these proteins are captured at this particular site. One example in which the anchoring mechanism is well understood is the mother-cell membrane protein SpoIIIAH. SpoIIIAH localizes to the engulfing septal membranes and this localization depends on a forespore membrane protein SpoIIQ (19,20). Importantly, the extracellular domains of SpoIIIAH and SpoIIQ interact in the space between the mother cell and forespore membranes (19,20). Thus, the zipper-like interaction between SpoIIQ and SpoIIIAH anchors SpoIIIAH in the engulfing septum. SpoIIQ itself localizes to the septal membrane on the forespore side but the mechanism by which it is anchored at this site is unknown (21). Interestingly, SpoIIIAH and SpoIIQ are also required for the proper localization of several other mother-cell membrane proteins (20,25). However, a direct link between these proteins and SpoIIIAH or SpoIIQ has not been established.
In this report, we show that SpoIIQ is responsible for anchoring membrane proteins on the forespore side of the septal membrane. We demonstrate that SpoIIQ resides in a membrane complex with SpoIIE. Re-examination of the subcellular localization of SpoIIE revealed that after asymmetric division, SpoIIE is released from the polar septum and transiently localizes to all membranes of the forespore compartment. Upon the initiation of engulfment, it specifically re-localizes to the septal membrane. We show that SpoIIQ is required to anchor SpoIIE in the engulfing septal membrane. These results indicate that SpoIIQ plays a major role in organizing membrane proteins on both sides of the division septum. Moreover, they suggest that forespore membrane proteins can achieve proper localization by diffusion-and-capture. Finally, these data raise the possibility that SpoIIE has a third function in sporulation during or after the morphological process of engulfment.

EXPERIMENTAL PROCEDURES
General Methods-All B. subtilis strains were derived from the prototrophic strain PY79 (26) and are listed in supplemental Table S1. All strains used for the immunoprecipitation experiments contain a spoIVB mutation to prevent degradation of proteins that have domains in the intermembrane space. 2 Sporulation was induced by resuspension at 37°C according to the method of Sterlini-Mandelstam (27).
Anti-GFP Antibody Resin-Affinity-purified anti-GFP 3 antibodies (4 mg) (22) were batched absorbed to 1 ml of protein A-Sepharose (Amersham Biosciences) for 1 h at 4°C. The antibody resin was washed four times with phosphate-buffered saline (PBS) and the antibody was covalently cross-linked to the protein A-Sepharose by the addition of disuccinimidyl suberate (Pierce) to a final concentration of 5 mM. After 30 min, the reaction was quenched by the addition of Tris, pH 7.5 to a final concentration of 100 mM. The antibody resin was washed with 100 mM glycine pH 2.5 to remove uncross-linked antibody and then neutralized with 1ϫ PBS.
Preparation of Crude Membranes and Detergent Solubilization of Membrane Proteins-50-ml cultures were harvested at hour 2.5 after the initiation of sporulation and washed two times with 1ϫ SMM (0.5 M sucrose, 20 mM MgCl 2 , 20 mM maleic acid pH 6.5) at room temperature. Cells were resuspended in 1:10 volume 1ϫ SMM and protoplasted with lysozyme (0.5 mg/ml). Protoplasts were collected by centrifugation and flash-frozen in N 2 (l). Thawed protoplasts were disrupted by osmotic lysis with 3 ml hypotonic buffer (Buffer H) (20 mM Hepes pH 8, 200 mM NaCl, 1 mM dithiothreitol, with protease inhibitors: 1 mM phenylmethylsulfonyl fluoride, 0.5 g/ml leupeptin, 0.7 g/ml pepstatin). MgCl 2 and CaCl 2 were added to 1 mM and lysates were treated with DNAse I (10 g/ml) (Worthington) and RNAseA (20 g/ml) (USB) for 1 h on ice. The membrane fraction was separated by centrifugation at 100,000 ϫ g for 1 h at 4°C. The supernatant was carefully removed, and the membrane pellet was dispersed in 200 l of Buffer G (Buffer H with 10% glycerol). Crude membranes were aliquoted and flash-frozen in N 2 (l). 50 -100 l crude membranes were diluted 5-fold with Buffer S (Buffer H with 20% glycerol and 100 g/ml lysozyme), and membrane proteins were solubilized by the addition of the nonionic detergent digitonin (Sigma) to a final concentration of 0.5%. The mixture was rotated at 4°C for 1 h. Soluble and insoluble fractions were separated by centrifugation at 100,000 ϫ g for 1 h at 4°C.
Co-immunoprecipitation from Detergent-solubilized Membrane Fractions-The soluble fraction from the digitonintreated membrane preparation (the load) was mixed with 20 l of affinity-purified anti-GFP antibody resin and rotated for 4 h at 4°C. The resin was pelleted at 5 krpm, and the supernatant (the flow-through) was removed. The resin was washed four times with 1 ml of Buffer S ϩ 0.5% digitonin. Immunoprecipitated proteins were eluted by the addition of 50 l of sodium dodecyl sulfate (SDS) sample buffer (0.25 M Tris, pH 6.8, 6% SDS, 10 mM EDTA, 20% glycerol) and heated for 15 min at 50°C. The resin was pelleted, and the supernatant (the IP) was transferred to a fresh tube and 2-mercaptoethanol was added to a final concentration of 10%. The load, flow-through, and immunoprecipitate were analyzed by immunoblot and SDS-PAGE followed by silver staining (28).
Mass Spectrometry Analysis-Individual bands were excised from silver-stained gel and trypsinized. Extracted peptides were then separated on a nanoscale C18 reverse-phase HPLC capillary column, and were subjected to electrospray ionization followed by MS using an LCQ DECA ion-trap mass spectrometer.
Fluorescence Microscopy-Fluorescence microscopy was performed with an Olympus BX61 microscope as previously described (31). SpoIIE-GFP (and SpoIIE-YFP) was visualized on agarose pads as described previously (12). All other fluorescent proteins were visualized on glass slides with poly-L-lysinetreated coverslips. Fluorescent signals were visualized with a phase contrast objective UplanF1 100ϫ and captured with a monochrome CoolSnapHQ digital camera (Photometrics) using Metamorph software (Universal Imaging). The lipophylic membrane dye TMA-DPH (Molecular Probes) was used at a final concentration of 0.05 mM and exposure times were typically 200 ms. Images were analyzed, adjusted, and cropped using Metamorph software.

SpoIIE Reside in a Membrane
Complex with SpoIIQ-The forespore membrane protein SpoIIQ interacts across the sporulation septum with the mother cell membrane protein SpoIIIAH. We and others (19,20) have shown that these two proteins are required for the efficient localization of proteins involved in morphogenesis and cell-cell signaling. To identify other proteins that reside in a complex with SpoIIQ and SpoIIIAH, we immunopurified SpoIIQ from detergent-solubilized mem-brane fractions. For these experiments, we took advantage of a high affinity anti-GFP antibody resin (32) and a strain (BTD665) harboring a functional CFP-SpoIIQ fusion (20). At hour 2.5 of sporulation, cells were harvested and disrupted by osmotic lysis. A crude membrane fraction was isolated and solubilized by the addition of the nonionic detergent digitonin. Immunoprecipitations were performed on the clarified detergent-solubilized fraction, and the immunoprecipitates were analyzed by SDS-PAGE followed by silver staining (Fig. 1A and see "Experimental Procedures"). As controls, immunoprecipitations were performed on detergent-solubilized lysates derived from a strain (BNC689) lacking a GFP fusion and a strain (BNC1384) harboring a GFP fusion to an unrelated E. coli membrane protein (MalF) (Fig.  1A) (21). The malF-gfp fusion was expressed from the spoIIQ promoter (P spoIIQ ). Accordingly, this heterologous membrane protein was synthesized at the same time and in the same cellular compartment as SpoIIQ.
Silver staining revealed that several other proteins co-purify with CFP-SpoIIQ (Fig. 1A). Importantly, no more than a few faint bands were detected in the immunoprecipitates from the lysate lacking a GFP fusion (Fig. 1A). Similarly, only a small number of proteins co-immunoprecipitated with the Mal-GFP fusion. We note that the heavy and light chains from the anti-GFP antibodies were barely detectable in these experiments due to the efficient cross-linking procedure we used to generate the antibody resin (see "Experimental Procedures"). To identify the proteins that were present in complexes with SpoIIQ, we cut out individual bands from the silver-stained gel, digested them with trypsin and analyzed the tryptic fragments by mass spectrometry (MS). By this method, we identified several mother-cell-specific proteins and several proteins that are made during vegetative growth. 4 In addition, we identified the sporulation membrane protein SpoIIE. SpoIIE is synthesized at the onset of sporulation prior to asymmetric division and is present in both the mother cell and forespore compartments (33)(34)(35). Most of the proteins that co- immunoprecipitated with SpoIIQ also co-purified with CFP-SpoIIIAH in support of the idea that many proteins reside in the zipper complex with SpoIIQ and SpoIIIAH. 4 Interestingly, SpoIIE did not co-immunoprecipitate with CFP-SpoIIIAH (supplemental Fig. S1) suggesting that the complex that contains SpoIIQ and SpoIIE is different from the zipper complex that contains SpoIIQ and SpoIIIAH.
To validate the MS results, we analyzed the immuno-affinity purifications by immunoblot (Fig. 1B). Anti-GFP antibodies revealed that the purifications were very efficient with greater than 80% of the GFP-fusion proteins immunodepleted from the detergent-solubilized lysates (Fig. 1B, compare load (L) and flow-through (FT)). Immunoblotting with anti-SpoIIE antibodies confirmed that SpoIIE efficiently co-immunoprecipitated with CFP-SpoIIQ but not with MalF-GFP or the untagged control (Fig. 1B). Moreover, an unrelated B. subtilis membrane protein (EzrA) was not detected in any of the immunoprecipitates (Fig. 1B). Finally, to determine whether the conditions used for immunoprecipitation maintained the reported interaction between SpoIIQ and SpoIIIAH (19,20), we probed the immunopurifications with anti-SpoIIIAH antibodies. Under our conditions, SpoIIIAH remained complexed with CFP-SpoIIQ and was efficiently immunodepleted (supplemental Fig. S2).
To more rigorously assess whether SpoIIQ and SpoIIE reside in a complex, we performed a reciprocal immunoprecipitation using a strain (BNC1203) harboring a functional SpoIIE-GFP fusion (33). Analysis of individual bands in the silver-stained gel by MS confirmed the presence of SpoIIQ among the proteins co-purifying with SpoIIE-GFP (Fig. 1A). Immunoblot analysis with anti-SpoIIQ antibodies further supported this conclusion (Fig. 1C). We note that a relatively small amount of SpoIIQ co-immunoprecipitated with SpoIIE-GFP since very little SpoIIQ was immunodepleted from the lysate (Fig. 1C, compare load (L) and flow-through (FT)). Nonetheless, the amount of SpoIIQ present in this immunoprecipitate was significantly higher than in control immunoprecipitations (Fig. 1C). Finally, SpoIIIAH could not be detected by immunoblot in the SpoIIE-GFP immunoprecipitate (data not shown). Altogether, these results indicate that SpoIIE resides in a complex with the forespore membrane protein SpoIIQ. Moreover, our data suggest that the SpoIIQ-SpoIIE complex is different from the zipper complex composed of SpoIIQ and SpoIIIAH.
SpoIIE Has a Dynamic Localization Pattern-SpoIIE is a polytopic membrane protein with ten transmembrane segments in the N-terminal portion of the protein and a PP2C-like phosphatase domain in its C terminus (36,37). SpoIIE plays two distinct roles during the early stages of sporulation. First, at the onset of sporulation, SpoIIE participates in the switch from medial to polar septation. Consistent with this activity, SpoIIE-GFP has been shown to co-localize with the cell division protein FtsZ forming polar "E-rings" (10,12,33,38). Second, after asymmetric division, SpoIIE remains associated with the polar septum (33,34) and plays a critical role in the activation of the forespore-specific transcription factor F (14 -16).
Because SpoIIQ is synthesized in the forespore under the control of F and the lysates for our immunoprecipitations were derived from cells at hour 2.5 of sporulation (well after the completion of both events that SpoIIE participates in), it was unexpected to find SpoIIE in a complex with SpoIIQ. With our interaction data in mind, we re-examined the localization pattern of SpoIIE-GFP. As described previously (10,33,34), prior to polar septation (hour 1.25), the fusion protein localized in bipolar "E-rings" at both potential division sites (Fig. 2). Surprisingly, immediately after asymmetric division but prior to the initiation of engulfment (flat polar septa), SpoIIE-GFP was not retained at the septum and instead was present throughout the forespore membranes (Fig. 2, hour 1.5). Importantly, this localization pattern was transient and as soon as engulfment initiated (slightly curved polar septa), SpoIIE-GFP re-localized to the septal membranes (Fig. 2, hour 1.75). SpoIIE-GFP remained present in the septal membranes throughout the FIGURE 2. SpoIIE has a dynamic localization pattern. Subcellular localization of SpoIIE-GFP in strain BDR1845 was analyzed by fluorescence microscopy at the indicated times (in hours) after the initiation of sporulation. The membranes from the same field were visualized using the fluorescent dye TMA-DPH. As indicated, the top row of the images shows SpoIIE-GFP fluorescence. The 2nd row of images shows fluorescence from the membrane dye TMA-DPH. The 3rd row shows the overlay of the two. The yellow carets at hour 1.25 indicate the localization of SpoIIE-GFP in "E-rings" at potential division sites prior to septation. The yellow carets at hour 1.75 and hour 2 highlight the SpoIIE-GFP concentrated at the engulfing septal membranes. The white carets in all four images show the transient localization SpoIIE-GFP throughout the forespore membranes. The membrane staining indicates that these cells have flat septa and have therefore just completed polar division. Schematic diagrams depicting SpoIIE-GFP (green) localization are shown below the images. Fluorescent images of individual cells from the same fields are shown below the schematic diagrams. Scale bar 1 m. engulfment process (Fig. 2, hour 2). Finally, when engulfment was nearly complete, weak SpoIIE-GFP foci could be observed in the forespore membranes (Fig. 2, hour 2). The presence of the SpoIIE-GFP fusion so late in sporulation was not due to stabilization of SpoIIE-GFP as has been observed for other GFP fusions (39). Native SpoIIE and SpoIIE-GFP were present at similar levels throughout the process of sporulation as assessed by immunoblot (supplemental Fig. S3). Altogether, these results show that the localization of SpoIIE is more dynamic than previously appreciated. We suspect that the transient nature of the intermediate SpoIIE localization pattern likely explains why this dynamic behavior had not been observed previously.
Rubio and Pogliano (21) have reported previously that all membrane proteins synthesized in the forespore (including heterologous ones like E. coli MalF) localize to the engulfing septal membrane. We therefore wondered whether the septal localization of SpoIIE-GFP during engulfment was specific or part of this default pathway for forespore membrane proteins. To investigate this, we compared the localization of SpoIIE-GFP with one of the membrane proteins (MalF-GFP) used in the original study by Rubio and Pogliano. The localization patterns of these two membrane proteins during sporulation were qualitatively different (supplemental Fig. S4). In the case of SpoIIE-GFP, once engulfment had initiated, virtually all the protein was present in the septal membrane with almost no fluorescence detectable in the cytoplasmic membranes of the forespore ( Fig. 2 and supplemental Fig. S4). By contrast, although there appeared to be some enrichment of MalF-GFP in the engulfing septal membrane in some of the cells, there was always fluorescent signal in the cytoplasmic membranes of the forespore (supplemental Fig. S4). These results suggest that the localization of SpoIIE to the engulfing septal membrane is specific and likely involves a protein or set of proteins that anchors it at this site.
The Localization of SpoIIE to the Engulfing Septum Requires Mother Cell Gene Expression-To determine whether the localization of SpoIIE to the engulfing septal membrane was dependent on forespore proteins under the control of F and/or mother cell proteins under the control of E , we visualized SpoIIE-GFP in strains lacking these transcription factors. In the absence of either sigma factor, sporulating cells fail to initiate engulfment and a second asymmetric division occurs at the distal pole, creating a sporangium with two forespores separated from the mother cell compartment by flat septa (40). Consistent with the idea that mother cell gene expression is required for the septal localization of SpoIIE, SpoIIE-GFP remained present in all forespore membranes in the E mutant (Fig. 3, middle column). Importantly, at this same time point (hour 2), SpoIIE-GFP specifically localized to the engulfing septum in wild-type cells (Fig. 3, left column). Similar results were obtained in the F mutant (data not shown). However, because F is required for E activity (40), this experiment cannot distinguish between a role for forespore proteins in anchoring SpoIIE or in activating mother-cell gene expression. From these experiments, we can conclude that the re-localization of SpoIIE to the septal membranes requires mother cell gene expression under the control of E .

SpoIIQ Is Required for the Septal Localization of SpoIIE during Engulfment-Our biochemical data indicate that SpoIIQ
and SpoIIE reside in a membrane complex. Moreover, the localization pattern of SpoIIE during engulfment closely resembles the one previously described for SpoIIQ (21). Finally, both proteins require E for septal localization (21). These data raised the possibility that one of these proteins might anchor the other in the engulfing septal membranes. Because SpoIIE is essential for the activation of F , and this transcription factor directs the synthesis of SpoIIQ, we were unable to test whether the localization of SpoIIQ requires SpoIIE. However, we were able to examine the localization of the SpoIIE-GFP fusion in a SpoIIQ mutant. In the absence of SpoIIQ, SpoIIE-GFP failed to re-localize to the septal membrane upon the initiation of engulfment (Fig. 3, right column). At this same time point (hour 2), SpoIIE-FIGURE 3. SpoIIQ is required to anchor SpoIIE in the engulfing septal membranes. SpoIIE-GFP localization was analyzed in a wild-type strain (wt, BDR1845), a E mutant (sigE⌬, BNC1264) and a spoIIQ mutant (IIQ⌬, BNC1266) at hour 2 of sporulation. The membranes from the same fields were visualized using the fluorescent dye TMA-DPH. As indicated, the top row of images shows SpoIIE-GFP fluorescence. The 2nd row of images shows fluorescence from the membrane dye TMA-DPH. The 3rd row shows the overlay of the two. The localization of SpoIIE-GFP at the engulfing forespore membranes in wildtype cells is indicated (yellow carets). In the absence of E , the sporulating cells contain two flat septa, and SpoIIE-GFP is present throughout the forespore membranes (white carets). Similarly, in the absence of SpoIIQ, SpoIIE-GFP fails to re-localize to the engulfing septal membranes (white carets). Schematic diagrams depicting SpoIIE-GFP (green) localization in the different strains are shown below the images. Scale bar 1 m.
GFP specifically localized to the engulfing septum in wild-type cells (Fig. 3, left column). These results indicate that SpoIIQ anchors SpoIIE in the engulfing septal membranes. Moreover, they nicely explain why SpoIIE requires E for proper localization (Fig. 3, middle column). It has been shown previously that the septal localization of SpoIIQ requires mother cell gene expression under the control of E (21). Thus, in the absence of E , SpoIIQ is mislocalized, resulting in the mislocalization of SpoIIE.
SpoIIE and SpoIIQ Co-localize in the Engulfing Septal Membranes-To compare the dynamic localization of SpoIIE and SpoIIQ in the same cells, we analyzed a strain (BKM1553) that contained a YFP-fusion to SpoIIE (SpoIIE-YFP) and a CFPfusion to SpoIIQ (CFP-SpoIIQ). Early during sporulation prior to asymmetric division, SpoIIE-YFP localized in polar E-rings as observed previously and CFP-SpoIIQ was undetectable (data not shown). Immediately after polar division, SpoIIE-YFP was visible throughout the forespore membranes (Fig. 4, white carets). At this time, F becomes active and the synthesis of SpoIIQ commences. Accordingly, CFP-SpoIIQ was undetectable at this stage (Fig. 4, white carets). Upon the initiation of engulfment, SpoIIE-YFP re-localized to the engulfing septal membrane. Consistent with the idea that SpoIIQ anchors SpoIIE at this site, CFP-SpoIIQ co-localized with SpoIIE-YFP (Fig. 4, yellow carets). Importantly, in all cells in which SpoIIE-YFP had re-localized to the engulfing septal membranes, CFP-SpoIIQ could be detected at this site. Finally, at late stages of engulfment when CFP-SpoIIQ forms discrete foci in the forespore membrane, SpoIIE-YFP was also present in these foci (supplemental Fig. S5).

DISCUSSION
We have shown that after asymmetric division, the SpoIIE protein present at the polar septum is transiently released into the membranes of the forespore. Upon the activation of F in the forespore and the subsequent synthesis of SpoIIQ, SpoIIE re-localizes to the engulfing septal membranes where it is held in place by SpoIIQ. These results indicate that SpoIIQ is not only involved in anchoring proteins on the mother cell side of the membrane (supplemental Fig. S2 and Refs. 20, 25) but also in anchoring proteins on the forespore side. Our data suggest that forespore membrane proteins can achieve proper localization by diffusion-and-capture as was reported for mother-cell membrane proteins (24). Furthermore, our observation that SpoIIE is released into the forespore membranes upon completion of polar division, coincident with F activation, raises the intriguing possibility that the release of SpoIIE from the polar septum is the trigger for sigma factor activation. Finally, the specific interaction between SpoIIE and a protein synthesized after SpoIIE has completed its two known functions leads us to hypothesize that this membrane phosphatase has a third function late in sporulation.
SpoIIE Localizes to the Engulfing Septal Membrane by Diffusion-and-Capture-We have shown that immediately after the completion of polar division, SpoIIE-GFP becomes uniformly distributed in the forespore membranes. Moreover, moments later, upon the activation of F , it specifically localizes to the septal membrane in a manner that depends on SpoIIQ. We have previously shown that membrane proteins that reside in the engulfing septal membrane on the mother-cell side achieve this subcellular localization by insertion into the cytoplasmic membrane followed by diffusion to and capture in the septum (24). We hypothesize that SpoIIE becomes localized on the forespore side of the engulfing septum by a similar mechanism (Fig. 5). In this scenario, once SpoIIQ is synthesized in the forespore, the SpoIIE protein that was released into the forespore membranes diffuses into the septal membranes where it is captured by SpoIIQ.
It is formally possible that the SpoIIE-GFP present in the engulfing septal membrane was derived from the mother cell and the SpoIIE-GFP present in the forespore was degraded upon its release from the septum. However, protoplast treatment to separate the mother cell and forespore compartments revealed that at least half of the SpoIIE-GFP in the sporangium was in the forespore membranes (supplemental Fig. S6 and Refs. 33,34). This result strongly suggests that the SpoIIE-GFP released into the forespore was not degraded and therefore re-localized to the engulfing septal membrane by diffusion-and-capture.
The Role of SpoIIE in Cell Type-specific Activation of F -The forespore transcription factor F is made early during sporulation prior to asymmetric division, yet, its activation is restricted to the forespore compartment. The SpoIIE phosphatase (and its phosphatase activity) is directly involved in the establishment of this cell type specificity but the mechanism by which it selectively and robustly triggers F activation in the forespore is still unresolved. One of the models to explain forespore-specific activation of F is that SpoIIE is equally distributed on both sides of the polar septum, which would result in a higher concentration of the membrane phosphatase in the forespore because of the smaller volume of this compartment (15,33). In this model, the increased concentration of SpoIIE is sufficient to tip the balance in favor of F activation in the forespore. Alternatively, it has been proposed that SpoIIE is sequestered to the forespore face of the septum (16,34,41). Our observations that SpoIIE is released into the forespore membranes upon completion of asymmetric division appear to support this second model. We note that we cannot rule out the possibility that half of the septum-associated SpoIIE was released into the mother cell membranes and that we were unable to detect this protein because the signal was too diffuse or masked by the SpoIIE-GFP present at the distal "E-ring." However, the SpoIIE-GFP signal at the division site was similar in total intensity to the signal in the forespore membranes after cytokinesis, suggesting that all (or most) of the SpoIIE at the septum ends up in the forespore compartment.
The observation that SpoIIE is released into the forespore membranes moments before F becomes active raises the intriguing possibility that release of SpoIIE from the division machinery serves as an activating step. One possibility that we favor is that release of SpoIIE into the forespore membranes triggers high phosphatase activity. Because SpoIIE does not activate F in strains depleted of the cell division protein FtsZ, this model would require that SpoIIE achieves a new (and more active) conformation through its association with the division machinery but that this high phosphatase activity would only be unleashed after release from the septum. In this scenario, the division machinery would serve as a chaperone for SpoIIE. In support of the idea that release from division machinery alters the conformation of SpoIIE, the ability to visualize SpoIIE-GFP in cytoplasmic membranes is only observed after release from the septum. For example, SpoIIE-GFP appears as a cytoplasmic haze in cells in which FtsZ is depleted (38). Moreover, in protoplast experiments, the SpoIIE-GFP in the mother cell, which is derived from the unused E-ring, appears cytoplasmic while SpoIIE-GFP in the forespore is membrane-associated (supplemental Fig. S6 and Refs. 33,34). In addition, when SpoIIE-GFP is artificially synthesized during vegetative growth under conditions that promote division at the cell poles and mini-cell formation, SpoIIE-GFP only localizes to the cytoplasmic membranes of the minicells. 5 Finally, we have found that the localization of SpoIIE-GFP to the cytoplasmic membranes of the forespore is only observed after the protein is released from the septum (Fig. 2). SpoIIE has ten transmembrane segments and it is therefore unlikely that it resides in the cytoplasm prior to division. Nonetheless, these cytological data are consistent with the idea that a new conformation of the protein is achieved 5 S. Ben-Yehuda and D. Z. Rudner, unpublished manuscript. FIGURE 5. SpoIIE localizes to the engulfing septal membrane by diffusion-and-capture. Schematic diagram of SpoIIE. Prior to polar division, SpoIIE (green balls) co-localizes with FtsZ (not shown) in "E-rings." Upon completion of cytokinesis, SpoIIE is released into the membranes of the forespore compartment. Moments later, F is activated in the forespore and directs the synthesis of SpoIIQ. SpoIIQ (Q) specifically localizes to the septal membranes by an unknown mechanism that depends on mother cell gene expression. SpoIIE present in the forespore diffuses into the septum where it is captured by SpoIIQ. The interaction between SpoIIE and SpoIIQ is depicted as direct but this need not be the case.
through association with and release from the division septum. Although very speculative, we hypothesize that the release of SpoIIE into the forespore membranes upon completion of cytokinesis triggers high phosphatase activity resulting in compartment-specific activation of F . In this model, the SpoIIE present in the predivisional cell and in the mother cell at the distal "E-ring" would have low phosphatase activity and thus F would remain inactive.
A Third Function for SpoIIE-Our co-immunoprecipitation and microscopy experiments indicate that SpoIIE is retained in the engulfing septal membranes long after it has fulfilled its two known functions in sporulation (participation in the switch from medial to polar Z-ring formation and activation of F ). Moreover, the re-localization of SpoIIE to the engulfing septal membrane depends on SpoIIQ, a forespore protein made under the control of F . The fact that SpoIIE is specifically recruited to the septum by a protein that is synthesized under the control of the transcription factor that SpoIIE itself activates, strongly suggests to us that this membrane phosphatase has another function at this late stage.
We hypothesize that SpoIIE is involved in a late step in the process of sporulation in the engulfing septal membranes. For example, SpoIIE might be necessary to anchor forespore and/or mother cell proteins in the engulfing septum. Alternatively, SpoIIE could play a direct role in the process of engulfment. It is also possible that SpoIIE participates in receiving a signal from the mother cell (or sensing the completion of engulfment) to activate G in the forespore. Finally, the interaction between SpoIIE and SpoIIQ could serve to modulate the phosphatase activity of SpoIIE. In this last model, interaction between SpoIIE and SpoIIQ might increase phosphatase activity to maintain F activity in the forespore. Conversely, once engulfment is complete or near complete, SpoIIQ could shut down phosphatase activity and turn off F to transition to late forespore gene expression under G control. Because SpoIIE is essential for the early stages of sporulation, any of these hypothetical late functions for SpoIIE would have been missed. The development of clever strategies for temporally controlled inactivation of proteins after their early roles are complete will be needed to investigate additional functions for this and other sporulation proteins.