Amino Acid Residues 489–503 of Dihydropyridine Receptor (DHPR) β1a Subunit Are Critical for Structural Communication between the Skeletal Muscle DHPR Complex and Type 1 Ryanodine Receptor*

Background: Dihydropyridine receptor (DHPR) β1a subunit is essential for muscle contraction. Results: Deletion of residues 489–503 in the β1a C terminus prevents calcium signaling and DHPR tetrad formation. Conclusion: β1a C terminus is critical for structural communication with the Ca2+ release channel. Significance: The β1a C-terminal tail is as important as the DHPR α1S II–III loop for skeletal EC coupling. The β1a subunit is a cytoplasmic component of the dihydropyridine receptor (DHPR) complex that plays an essential role in skeletal muscle excitation-contraction (EC) coupling. Here we investigate the role of the C-terminal end of this auxiliary subunit in the functional and structural communication between the DHPR and the Ca2+ release channel (RyR1). Progressive truncation of the β1a C terminus showed that deletion of amino acid residues Gln489 to Trp503 resulted in a loss of depolarization-induced Ca2+ release, a severe reduction of L-type Ca2+ currents, and a lack of tetrad formation as evaluated by freeze-fracture analysis. However, deletion of this domain did not affect expression/targeting or density (Qmax) of the DHPR-α1S subunit to the plasma membrane. Within this motif, triple alanine substitution of residues Leu496, Leu500, and Trp503, which are thought to mediate direct β1a-RyR1 interactions, weakened EC coupling but did not replicate the truncated phenotype. Therefore, these data demonstrate that an amino acid segment encompassing sequence 489QVQVLTSLRRNLSFW503 of β1a contains critical determinant(s) for the physical link of DHPR and RyR1, further confirming a direct correspondence between DHPR positioning and DHPR/RyR functional interactions. In addition, our data strongly suggest that the motif Leu496-Leu500-Trp503 within the β1a C-terminal tail plays a nonessential role in the bidirectional DHPR/RyR1 signaling that supports skeletal-type EC coupling.

The ␤ 1a subunit is a cytoplasmic component of the dihydropyridine receptor (DHPR) complex that plays an essential role in skeletal muscle excitation-contraction (EC) coupling. Here we investigate the role of the C-terminal end of this auxiliary subunit in the functional and structural communication between the DHPR and the Ca 2؉ release channel (RyR1). Progressive truncation of the ␤ 1a C terminus showed that deletion of amino acid residues Gln 489 to Trp 503 resulted in a loss of depolarization-induced Ca 2؉ release, a severe reduction of L-type Ca 2؉ currents, and a lack of tetrad formation as evaluated by freeze-fracture analysis. However, deletion of this domain did not affect expression/targeting or density (Q max ) of the DHPR-␣ 1S subunit to the plasma membrane. Within this motif, triple alanine substitution of residues Leu 496 , Leu 500 , and Trp 503 , which are thought to mediate direct ␤ 1a -RyR1 interactions, weakened EC coupling but did not replicate the truncated phenotype. Therefore, these data demonstrate that an amino acid segment encompassing sequence 489 QVQVLTSLRRNLSFW 503 of ␤ 1a contains critical determinant(s) for the physical link of DHPR and RyR1, further confirming a direct correspondence between DHPR positioning and DHPR/RyR functional interactions. In addition, our data strongly suggest that the motif Leu 496 -Leu 500 -Trp 503 within the ␤ 1a C-terminal tail plays a nonessential role in the bidirectional DHPR/RyR1 signaling that supports skeletal-type EC coupling.
Excitation-contraction (EC) 2 coupling in skeletal muscle involves a direct interaction between the L-type Ca 2ϩ channel, 1-4-dihydropyridine receptor (DHPR) complex, located in the surface membrane and the type 1 ryanodine receptor (RyR1), Ca 2ϩ release channel in the sarcoplasmic reticulum (SR) membrane (1)(2)(3). This interaction does not require Ca 2ϩ permeation through the DHPR but instead relies on direct reciprocal communication between the two channels. Although there is compelling supporting evidence that both the DHPR-␣ 1S (Cav1.1) and DHPR-␤ 1a (Cav␤1) subunits are essential components for the DHPR/RyR1 communication (4 -11), the mechanisms by which conformational changes of the DHPR complex translate into activation of RyR1 during skeletal EC coupling remain unclear.
Structure/function studies in cultured myotubes have revealed that the cytosolic loop linking repeats II and III (the II-III loop) of the DHPR-␣ 1S subunit plays an essential role in the bidirectional signaling with RyR1. Within this intracellular loop, residues 720 -765 are critical to support both depolarization-induced Ca 2ϩ release, also known as the orthograde signal (1,12,13), and the ability of the DHPR complex to receive a retrograde activation signal from RyR1 (1,14). Importantly, the same region is also required to support the stereospecific positioning of DHPR complexes in relation to RyR1 that result in the assembly of ordered DHPR tetrad arrays characteristic of skeletal muscle (15).
The role of the DHPR-␤ 1a subunit in skeletal EC coupling stems from studies in ␤ 1 -null mice showing that a lack of ␤ 1a expression eliminates electrically evoked Ca 2ϩ release from the SR (9). Most revealing are recent observations on the paralyzed zebrafish mutant relaxed that lacks expression of the DHPR-␤ 1a subunit. In muscle fibers from this fish model, the DHPR complex, although reduced in number, is appropriately targeted to the junctional SR but is not arranged in tetrads (16,17). These studies confirm that functional skeletal-type EC coupling requires the appropriate alignment of DHPRs with RyR1 and strongly suggest that the ␤ 1a subunit is essential in this positioning. It is currently unclear whether the ␤ 1a subunit is additionally involved in transmitting the activation signal from the DHPR to RyR1 and/or directly modulating RyR1 function. Studies on chimeric ␤ 2a /␤ 1a and truncated ␤ 1a subunits have shown that deletion, or substitution, of 35 residues within the ␤ 1a C-terminal end produces a severe reduction in voltageevoked Ca 2ϩ transient amplitude and DHPR Ca 2ϩ current density (18,19), suggesting a critical contribution of the ␤ 1a C-terminal tail to skeletal-type EC coupling signaling. However, because no structural information was obtained in these studies, it is unclear whether the effects of ␤ 1a in EC coupling are simply due to an effect on DHPR positioning. Recent in vitro studies have shown that peptides fragments from the C-terminal domain of ␤ 1a modulate RyR1 channel function, giving support to the idea of a direct functional interaction between ␤ 1a and RyR1 (20). Mutational analysis of these peptides identified the critical motif responsible for RyR1 activation in a hydrophobic pocket formed by amino acid residues Leu 496 -Leu 500 -Trp 503 (21). However, it is currently unknown whether the alleged ␤ 1a -RyR1 interaction mediated by this motif plays any role either in the bidirectional signaling between RyR1 and DHPR or in the DHPR/RyR1 physical linkage that supports DHPR tetrad arrays.
In this study, we examine these questions by assessing the effect on EC coupling signaling of deletions and mutations of the amino acid sequence within the Leu 496 -Leu 500 -Trp 503 hydrophobic pocket of mouse ␤ 1a subunit. We find that progressive truncations of ␤ 1a C-terminal tail significantly affected depolarization-induced Ca 2ϩ release, retrograde signaling, and the arrangement of DHPR into tetrads. Moreover, although the disruption of motif Leu 496 -Leu 500 -Trp 503 weakened EC coupling, it did not prevent bidirectional signaling or DHPR tetrad formation. In summary, our data again establish a direct correspondence between DHPR positioning and DHPR/RyR functional interactions, revealing a key role for amino acid sequence Gln 489 -Trp 503 in this process. In addition, our data indicate that although the Leu 496 -Leu 500 -Trp 503 motif contributes to normal bidirectional communication between the DHPR and RyR1, it does not appear to constitute a critical determinant for skeletal EC coupling.

EXPERIMENTAL PROCEDURES
cDNA Constructs and Virus Packaging-Full-length cDNA of mouse ␤ 1a subunit (GenBank TM , NM_031173), as well as the ␤ 1a truncated constructs, were cloned into a retroviral vector by inserting the AgeI-NotI cloning cassette from vector pSG5T7-AgeNot␤ 1a (gift from Dr. R. Coronado) into the corresponding restriction sites of the bicistronic retroviral vector pCMMP-MCS-IRES-Puro carrying a Puromycin resistance gene (Addgene 36952 (22)). Truncation of the C-terminal tail of ␤ 1a subunit was performed by inserting a set of two complementary oligonucleotide primers containing the desired truncated sequence in frame into pSG5T7-AgeNot␤ 1a within restriction sites BsmBI and NotI (for ␤ Ϫ21 , ␤ Ϫ36 , and ␤ LLW mutant clones) or SacII and NotI (for ␤ Ϫ14 ). A stop codon was engineered at the 3Ј end of each primer upstream of the NotI site. All clones were confirmed by sequencing prior to use. Details of the deleted sequence and mutated residues of the clones analyzed in this study are summarized in Fig. 1. Virus production was per-formed with a set of three packaging vectors as described elsewhere (23).
Cell Culture and Calcium Imaging-Primary myoblasts from ␤ 1 -null muscles (5,9) infected with ␤ 1a cDNA-containing virions at a multiplicity of infection of 0.5 were selected with 1.5 g/ml Puromycin for 2 weeks to obtain stably transduced myoblasts. ␤ 1 -Expressing myoblasts were then grown and differentiated in 96-well plates as described previously (24). Calcium imaging was performed 4 -5 days after differentiation in myotubes loaded with 5 M Fura2-AM (Molecular Probes, OR) in imaging buffer (125 mM NaCl, 5 mM KCl, 2 mM CaCl 2 , 1.2 mM MgSO 4 , 6 mM glucose, and 25 mM Hepes/Tris, pH 7.4). Membrane depolarization was performed by a 5-s perfusion with 5-7 volumes of imaging buffer containing increased concentrations of KCl supplemented with or without 0.5 mM CdCl 2 and 0.1 mM LaCl 3 . To preserve osmolarity of the depolarization buffer, the increased K ϩ concentration was compensated with an equivalent reduction in total NaCl concentration. Cells were imaged with an intensified 10-bit digital CCD camera (XR-Mega-10; Stanford Photonics, Stanford, CA) using a DG4 multiwavelength light source. Fluorescent emission at 510 nm was captured from regions of interest within each myotube at 33 frames per second using Piper-controlled acquisition software (Stanford Photonics) and expressed as ratio of signal collected at alternating 340/380-nm excitation wavelength.
Freeze Fracture Replicas-The cells were grown and differentiated on Thermanox TM coverslips (Nunc Inc., Naperville, IL) coated with extracellular matrix gel (25). Differentiated myotubes were washed twice in PBS at 37°C, fixed in 3.5% glutaraldehyde in 100 mM sodium cacodylate buffer (pH 7.2), and then infiltrated with 30% glycerol, frozen, and fractured as described previously (15,26). Association of particles with RyR1 orthogonal arrays and frequency of tetrad formation were assessed as described (15,27). For each truncated construct, we used digitized images from micrographs taken at a magnification of 33,900ϫ and selected clusters that were most highly populated with particles and showed most evidence for order. We further limited the measurements within each cluster to areas that had either coherent arrays of tetrads with the same orientation or an evenly distributed set of particles. The density of DHPR particles at the peripheral coupling of each tested construct was estimated by counting all large particles that are clearly clustered and located over slightly raised mounds, which represent the peripheral coupling site.
Measurement of Ionic Currents-Macroscopic Ca 2ϩ currents were measured using the whole cell patch clamp technique according to previously described protocols (28). Myotubes were voltage-clamped with the use of an Axopatch 200B amplifier (Axon Instruments, Foster City, CA). Effective series resistance was compensated up to the point of amplifier oscillation with the Axopatch circuit, and the leak current was subtracted by using a -P/6 protocol before each sweep. The patch pipettes were coated using Sylgard and had a resistance of ϳ2 M⍀ when filled with the pipette solution. The patch pipette internal solution consisted of 140 mM cesium aspartate, 5 mM MgCl 2 , 10 mM Cs-EGTA, and 10 mM Hepes titrated with CsOH to pH 7.2. The composition of the external bath solution was 145 mM TEA-Cl, 10 mM CaCl 2 , 10 mM Hepes titrated with TEA(OH) to pH 7.4.
Intramembrane charge movement was determined following the protocol described previously (29). The internal and external pipette solutions had the same ionic composition described above. However, the external solution was supplemented with 0.5 mM CdCl 2 and 0.1 mM LaCl 3 to block L-type Ca 2ϩ currents. A 1-s prepulse to Ϫ30 mV to inactivate both sodium and transient calcium channels preceded the test pulses. The leak current and other passive components of the total current was subtracted by using a -P/6 protocol before each sweep. The patch pipettes were Sylgard-coated and had a tip resistance of 1.5-2.0 M⍀ when filled with the internal solution. The serial resistance was corrected to 70% using the Axopath circuit with cell capacitance Ͻ100 pF in most recordings. The recorded signal was filtered to 5 kHz and was digitalized at 20 kHz. All experiments were performed at room temperature.
The voltage dependence of the Ca 2ϩ currents was fitted to the following equation, where G max is the maximal conductance, V corresponds to the test potentials, V1 ⁄ 2 is the potential at which G ϭ 1/2 G max . k represents a slope parameter, and V r is the reversion potential.
The voltage dependence of the charge movement was fitted to the following equation, where Q max is the maximal charge, V corresponds to the test potentials, V1 ⁄ 2 is the potential at which Q ϭ 1/2 Q max , and k represents a slope parameter.
Data Analysis-Statistical significant differences among data sets were calculated using one-way analysis of variance (GraphPad Software, San Diego, CA). The data were expressed as means Ϯ S.D. or means Ϯ S.E.

C-terminal Truncation of ␤ 1a Does Not Affect Targeting of ␣ 1S
Subunit to the Plasma Membrane-To assess the role of the C-terminal domain of the ␤ 1a subunit in skeletal-type EC coupling, we constructed a series of truncated DHPR-␤ 1a subunits bearing progressive deletions of its C-terminal tail (Fig. 1) and then confirmed their expression and ability to target the DHPR complex to the surface membrane using immunocytochemical analysis (Fig. 2). No ␤ 1a or ␣ 1S expression was detected in ␤ 1 -null myotubes (Fig. 2, A and B). In contrast, expression of wild type ␤ 1a in these myotubes restored proper targeting of the ␣ 1S subunit to the surface membrane in discrete foci sometimes organized in a semilinear longitudinal orientation (Fig. 2, C and  D). Because myotubes at this stage of differentiation have few internal Ca 2ϩ release units, most of the foci represent peripheral couplings where the SR cisternae bind the plasmmalema. Overall, immunocytochemistry revealed no obvious differences in expression and distribution of the ␤ 1a constructs used in this study (Fig. 2, E-L). Noticeably, the ␤ 1a truncations that support skeletal EC coupling signaling (see below) displayed the same clustered distribution pattern of ␣ 1S and ␤ 1a expression as those that diminish or prevent EC coupling.
Progressive Truncation of the ␤ 1a C-terminal Tail Impairs Depolarization-induced Ca 2ϩ Release-The role of the ␤ 1a C-terminal tail in skeletal EC coupling was evaluated by comparing the effect of each truncation on depolarization-induced Ca 2ϩ release signals (Fig. 3). Calcium release was estimated from the average peak of the Ca 2ϩ transient of Fura2-loaded myotubes depolarized with increased K ϩ concentrations. Calcium signals were measured either in the presence of Cd 2ϩ and La 3ϩ in nominal free Ca 2ϩ to measure skeletal-type EC coupling, which depends entirely on the direct DHPR-RyR interaction (Fig. 3A), or in the presence of 2 mM CaCl 2 (Fig. 3B). In the presence of Cd 2ϩ and La 3ϩ , Ca 2ϩ transients measured from ␤ 1a -expressing myotubes displayed a classic sigmoidal K ϩ dose-response curves. (Fig. 3, black circles). The average peak amplitude of the Ca 2ϩ transients for clone ␤ Ϫ14 (lacking 14 amino acids of the C-terminal tail) were quite similar to those restored by ␤ 1a (Fig. 3A). Construct ␤ Ϫ21 (lacking 21 amino acids of the C-terminal tail) induced a small but statistically  Table 1) without a change in sensitivity to depolarization. The construct ␤ Ϫ36 (lacking 36 amino acids of the C-terminal tail) elicited a severe disruption of the Ca 2ϩ signal rendering myotubes unresponsive to depolarization (Fig. 3, gray asterisks). On the other hand, in the presence of 2 mM extracellular Ca 2ϩ , both ␤ Ϫ14 and ␤ Ϫ21 constructs restored the Ca 2ϩ transient peak to wild type levels ( Fig. 3B and Table 1). However, myotubes expressing ␤ Ϫ36 did not respond to K ϩ treatment even in the presence of extracellular Ca 2ϩ . This is not due to a lack of expression or mistargeting of either ␣ 1S or ␤ Ϫ36 subunits because immunolabeling demonstrates a clear presence of both subunits at sites of peripheral couplings (Fig. 2). These findings confirm the critical role of the C-terminal tail of ␤ 1a subunit in supporting DHPR/   Table 1.  DECEMBER 26, 2014 • VOLUME 289 • NUMBER 52

JOURNAL OF BIOLOGICAL CHEMISTRY 36119
RyR1 communication and indicate that amino acid region encompassing sequence 489 QVQVLTSLRRNLSFW 503 , missing in clone ␤ Ϫ36 , contains a determinant of ␤ 1a that is essential to support skeletal-type EC coupling. Disruption of the Hydrophobic Motif Leu 496 -Leu 500 -Trp 503 Affects EC Coupling Signaling-The motif conformed by amino acids Leu 496 , Leu 500 , and Trp 503 of the ␤ 1a C terminus region has been recently identified in in vitro studies as a putative region of interaction with the RyR1 (21). To test whether the absence of residues Leu 496 -Leu 500 -Trp 503 in our construct ␤ Ϫ36 is directly responsible for the failure of this truncated subunit to restore EC coupling, we designed the construct ␤ LLW (Fig. 1). In this clone, residues Leu 496 , Leu 500 , and Trp 503 were substituted to alanine, a triple mutation that prevented RyR1 activation by peptides derived from the ␤ 1a C-terminal tail (21). Here we expressed this mutation within the context of construct ␤ Ϫ14 that contains only the critical sequence 489 QVQVLTSLRRNLS-FWGGLEASPR 511 of the C-terminal domain but that still displays normal EC coupling. The expression of clone ␤ LLW in ␤ 1 -null mouse myotubes restored strong depolarization-induced Ca 2ϩ release both in the presence and in the absence of extracellular Ca 2ϩ . However, in comparison with wild type ␤ 1a and the control clone ␤ Ϫ14 , ␤ LLW -expressing myotubes showed significant reduction in peak Ca 2ϩ transient amplitude in response to depolarization (25-30%; Table 1) and an evident rightward shift in Ca 2ϩ release sensitivity to depolarization (Fig. 3).

Truncation of ␤ 1a C-terminal Tail Amino Acids Gln 489 -Trp 503 Prevents Organization of the DHPR Complex into
Tetrads-To correlate the effect of ␤ 1a C-terminal truncations on EC coupling with the structural link between DHPR and RyR1, we also analyzed the ultrastructural organization of the DHPR complex using freeze-fracture electron microscopy. Replicas from myotubes expressing each of the ␤ 1a constructs displayed numerous small and slightly domed patches of membrane containing clusters of large particles characteristic of DHPR complex (3,15,26,27,30,31) (Fig. 4). Myotubes expressing constructs ␤ 1a , ␤ Ϫ14 , ␤ Ϫ21 , or ␤ LLW displayed DHPR particles arranged in arrays of complete and incomplete tetrads that followed an overall orthogonal alignment. In contrast, DHPR particles in ␤ Ϫ36 -expressing myotubes were loosely arranged, and no tetrads were present, an arrangement similar to that seen either in dyspedic myotubes that lack RyRs (27) or in ␤ 1anull zebrafish (17). Table 2 shows counts of all large particles, presumably representing DHPR complexes, within identified membrane patches. Although DHPR particles in myotubes expressing ␤ 1a , ␤ Ϫ14 , ␤ Ϫ21 , and ␤ LLW are clustered at comparable densities, the density is ϳ50% lower in ␤ Ϫ36 myotubes with no tetrads evident. The combined immunolabeling and freeze-fracture analyzes demonstrate the presence of ␣ 1S /␤ Ϫ36 complexes at peripheral couplings, confirming that the negative results with ␤ Ϫ36 construct are not due to lack of expression or targeting of the DHPR complex to the surface membrane. In addition, the data also reveal that disruption of the Leu 496 -Leu 500 -Trp 503 motif of ␤ 1a does not significantly affect the DHPR/RyR1 structural link responsible for the arrangement of DHPR into tetrads.

␤ 1a C-terminal Truncation of Amino Acids Gln 489 -Trp 503
Reduces L-type Ca 2ϩ Currents-The effects of ␤ 1a truncations on Ca 2ϩ channel function of the DHPR complex were assessed using whole cell patch clamp analysis. Fig. 5 shows representative traces of Ca 2ϩ currents and voltage dependence of the peak Freeze-fracture replica images showing examples of particle arrangements within the plasmalemma at sites of peripheral couplings in ␤ 1 -null cells expressing various ␤ subunits. Intramembranous particle distribution in the ␤ 1 -null cells is completely random. In all cells expressing ␤ subunits, a small raised platform caused by the apposition of a junctional SR element to the plasmalemma identifies the position of peripheral couplings. ␤ 1a , ␤ Ϫ14 , ␤ Ϫ21 , and ␤ LLW rescue the formation of DHPR tetrads, whose center is indicated by a yellow dot in a duplicate of each image. Tetrad formation, even if somewhat incomplete because of a limited level of expression, indicates that the normal stereospecific association of DHPR with RyR1 is re-established. On the contrary, the few large particles representing the location of DHPRs at a peripheral coupling (indicated by arrows) in a ␤ Ϫ36 cell are randomly disposed, indicating a lack of specific DHPR to RyR association. Ca 2ϩ current measured in ␤ 1 -null myotubes expressing each of the ␤-truncated constructs. ␤ 1 -null myotubes had marginal to nondetectable whole cell Ca 2ϩ currents in response to depolarization (Fig. 5, left panel). In contrast, ␤ 1a -expressing myotubes showed high density Ca 2ϩ currents with slow activation kinetics and fast deactivation, characteristic of L-type Ca 2ϩ channels (1,28,32). Overall Ca 2ϩ current density, maximal conductance (G max ), and other Boltzmann parameters recorded in myotubes expressing wild type ␤ 1a subunit (Table 3) were similar to those reported previously by other groups (2,(33)(34)(35)(36). Expression of constructs ␤ Ϫ14 , ␤ Ϫ21 , and ␤ LLW in ␤ 1 -null myotubes rescued high density Ca 2ϩ currents similar to ␤ 1a (Fig. 5). By contrast, myotubes expressing construct ␤ Ϫ36 displayed significantly reduced Ca 2ϩ current density (Fig. 5, middle panels). Under our experimental conditions, the average peak Ca 2ϩ current density displayed by construct ␤ Ϫ14 appeared slightly higher than the one in ␤ 1a -expressing myotubes. However, further analysis revealed no significant difference in Ca 2ϩ conductance (G max ) values between these two clones (Table 3). Likewise, longer deletion ␤ Ϫ21 or mutation of the Leu 496 -Leu 500 -Trp 503 motif (construct ␤ LLW ) did not result in significant differences in maximal Ca 2ϩ current (Fig. 5, middle and right panels) or G max (Table 3) when compared with ␤ 1a -expressing myotubes. However, Ca 2ϩ current density in ␤ LLW -expressing cells was ϳ25% smaller than the Ca 2ϩ current recovered by cells expressing the control construct ␤ Ϫ14 (Fig. 5,  right panel). Consistently, an equivalent difference in average G max was observed between these two clones (p Ͻ 0.01; Table  3). Deletion of the last 36 amino acids of ␤ 1a C-terminal tail (␤ Ϫ36 ), on the other hand, resulted in a severe reduction in peak Ca 2ϩ current (Fig. 5, left and middle panels). Comparison of Ca 2ϩ conductance revealed a reduction in average G max of ϳ65% from 301 Ϯ 14 pS/pF in ␤ 1a -expressing myotubes to 105 Ϯ 13 pS/pF in myotubes expressing ␤ Ϫ36 (Table 3). This G max value is fully consistent with the Ca 2ϩ conductance previously reported for a ␤ 1a construct lacking 35 amino acids of the C-terminal region (88 pS/pF) (18). Further analysis of the recovered Ca 2ϩ current indicates that truncation of the ␤ 1a C-terminal tail or mutational substitution of the Leu 496 -Leu 500 -Trp 503 motif resulted in no detectable alterations of the Ca 2ϩ current kinetics (data not shown). These results suggest that a loss of depolarization-induced Ca 2ϩ release signal in myotubes expressing truncation ␤ Ϫ36 is associated with a severe reduction of retrograde signaling from RyR1, as expected for a mutant that do not support tetrad formation ( Table 2).
Truncation of ␤ 1a C-terminal Tail Does Not Affect Intramembrane Charge Movement-Immobilization-resistant intramembrane charge movement is a measure of DHPR voltage sensor density. Fig. 6 shows charge versus voltage relationships for the five tested constructs obtained by integration of the ON component. Wild type and mutant ␤ 1a -expressing myotubes presented maximal charge movement (Q max ) and other Boltzmann parameters (Table 3) similar to those reported by others groups (5, 18, 37, 38). The average Q max values measured in myotubes FIGURE 5. ␤ 1a C-terminal truncation reduces L-type Ca 2؉ current density. Left panel, representative traces of whole cell Ca 2ϩ currents of ␤ 1 -null myotubes expressing the indicated ␤ 1a constructs. Myotubes were depolarized for 200 ms from a holding potential of Ϫ50 mV to test potentials between Ϫ40 and ϩ90 mV applied in 10-mV increments. Test sweeps from Ϫ20 to ϩ30 mV are shown. Middle panel, comparison of voltage dependence of peak Ca 2ϩ current densities recorded from ␤ 1 -null myotubes expressing either wild type ␤ 1a (black circles), ␤ Ϫ14 (blue squares), ␤ Ϫ21 (green triangles), and ␤ Ϫ36 (gray asterisks). Right panel, the same data from ␤ 1 -null myotubes expressing ␤ Ϫ14 (blue squares) is compared with ␤ 1 -null myotubes expressing ␤ LLW subunit (red diamonds). The I/V relationships were fitted using Equation 1 (see "Experimental Procedures"). Fitting parameters for each plot between Ϫ40 and ϩ90 mV are presented in Table  3. The data are expressed as means Ϯ S.E., n ϭ 3 independent experiments.

TABLE 3 Ca 2؉ conductance and charge movement parameters of myotubes expressing ␤ 1a C-terminal truncations
The values were derived by one-way analysis of variance (Tukey's test). The data are presented as means Ϯ S.E. of Boltzman parameters fitted between Ϫ30 and ϩ90 mV. expressing the different truncated ␤ 1a or mutated ␤ LLW subunits were not significantly different from the Q max values observed in wild type ␤ 1a -expressing cells ( Fig. 6 and Table 3). Therefore, truncations or mutation of the Leu 496 -Leu 500 -Trp 503 motif of ␤ 1a did not affect either the voltage-sensing properties of the DHPR or the trafficking and density of the DHPR-␣ 1S subunit to the cell surface.

DISCUSSION
The essential role of DHPR-␤ 1a subunit in EC coupling signaling of skeletal muscle cells is supported by compelling evidence (18,19,37,39,40), yet the molecular determinant(s) involved in this process are still undefined. In this study, using progressive truncations of the C-terminal region of mouse ␤ 1a subunit, we found that the domain encompassing amino acid residues Gln 489 -Trp 503 plays an essential role in supporting bidirectional signaling between the DHPR complex and RyR1 in cultured myotubes. Deletion of this region resulted in a loss of depolarization-induced Ca 2ϩ release signal, a severe reduction of L-type Ca 2ϩ currents (retrograde signal), and the loss of DHPR organization into arrays of tetrads despite seemingly normal expression and targeting of the DHPR complex to the junctional couplings. These results are fully consistent with work from zebrafish ␤ 1 -null mutant model showing that the lack of expression of ␤ 1a subunit precludes the skeletal musclespecific arrangement of DHPR particles into arrays of tetrads, resulting in a loss of EC coupling (16,17). Thus, our data demonstrate that, like the II-III loop of ␣ 1S , the ␤ 1a C terminus constitutes an essential physical and functional link between the DHPR and RyR1.
Whether the loss of a structural link between DHPR and RyR1 by deletion of the Gln 489 -Trp 503 sequence of ␤ 1a is the direct result of disruption of a yet unidentified ␤ 1a -RyR1 interaction is unknown. Currently, the strongest evidence supporting a direct link between ␤ 1a and RyR1 comes primarily from reports of in vitro interactions. Previous co-immunoprecipitation studies defined a cluster of positively charged amino acids within domain 3490 -3523 of RyR1 as a ␤ 1a -binding domain (41). The disruption of this region not only prevents ␤ 1a -RyR1 interactions in vitro but also weakens electrically evoked Ca 2ϩ release signals in cultured myotubes (41). Interestingly, ␤ 1a peptides from the same C-terminal region identified in our study were found to activate native and purified RyR1 channels fused into lipid bilayers (20,21). Moreover, microinjection of these peptides into flexor digitorum brevis fibers was reported to specifically enhance depolarization-induced Ca 2ϩ release (42), further supporting the idea that the C-terminal domain of ␤ 1a subunit directly interacts with RyR1 and that this interaction is involved in EC coupling signaling. More recently, extensive mutagenesis analysis has found that the active motif in these peptides are residues Leu 496 -Leu 500 -Trp 503 because alanine substitutions at these positions completely eliminate the activating properties of the peptides (21). In the present study, we show that similar disruption of the Leu 496 -Leu 500 -Trp 503 motif by site-directed mutagenesis did not abolish depolarization-induced Ca 2ϩ release, providing strong indication that the amino acid residues Leu 496 , Leu 500 , and Trp 503 are not essential for EC coupling. Nonetheless, our data indicate that alanine substitution of these residues significantly affects the efficiency and voltage sensitivity of the EC coupling signal. These changes were linked to a small but statistically significant reduction in G max of the DHPR complex, suggesting an influence of the Leu 496 -Leu 500 -Trp 503 motif on channel conductance. Thus, although not essential, the Leu 496 -Leu 500 -Trp 503 motif appears to contribute, to some extent, to the structural determinant of ␤ 1a subunit that modulates bidirectional signaling in muscle cells.
Previous studies have demonstrated a close correlation between DHPR positioning into tetrads and EC coupling (15,26,27), and the data from this study further confirm these findings. Nonetheless, because of the effects of C-terminal truncations of ␤ 1a (␤ Ϫ36 ) on both DHPR current density and SR Ca 2ϩ release, we cannot completely rule out a direct involvement of ␤ 1a on signal transduction between the DHPR and RyR1. However, the severe reduction in G max of the DHPR complex associated with ␤ 1a truncation is suggestive of an important conformational alteration of the ion-conducting channel, the ␣ 1S subunit. Thus, our data seem to be consistent with the hypothesis that ␤ 1a functions as an allosteric modulator of the ␣ 1S subunit, restoring the functional conformation that enables ␣ 1S both proper anchorage to RyR1 enabling organization into tetrads and normal channel conductive properties (16,39). FIGURE 6. Truncation of the C-terminal tail of ␤ 1a does not affect DHPR charge movement. A, representative charge movement (Q) recording in response to 20-ms depolarizing test pulses between Ϫ40 and ϩ50 mV from a holding potential of Ϫ50 mV. Test sweeps from Ϫ30 to ϩ40 mV are shown. B, comparison of voltage dependence of the integrated outward gating current (Q on ) recorded from ␤ 1 -null myotubes expressing either wild type ␤ 1a (circles), ␤ Ϫ14 (squares), ␤ Ϫ21 (triangles), ␤ Ϫ36 (hexagons), or ␤ LLW subunit (diamonds). The Q/V relationships were fitted to Equation 2, and the parameters are summarized in Table 3. The data are expressed as the means Ϯ S.E., n ϭ 3 independent experiments.
Recent studies in muscle cells from zebrafish ␤ 1a -null mutant have reported that domain cooperativity between a conserve proline-rich motif at the C-terminal tail ( 464 PXXP 467 ) and the Src homology 3 (SH3) domain of ␤ 1a are essential to restore the voltage sensing properties of the DHPR complex and, thus, EC coupling signaling (39). Whether similar domain cooperativity is required in mouse skeletal EC coupling is unknown. Crystal structure analysis of mammalian DHPR ␤ 2a and ␤ 3 subunits indicates that the SH3 domain would not be compatible with canonical modes of proline-rich ligand binding (43,44), suggesting that direct interaction between the SH3 domain and 464 PXXP 467 motif of ␤ 1a would be unlikely. Interestingly, all truncated ␤ 1a subunits tested in our study, including those that do not support skeletal EC coupling, had an intact 464 PXXP 467 motif. This finding suggests that either domain cooperativity mediated by the PXXP motif is not essential for mammalian EC coupling or that in addition to the 464 PXXP 467 motif, other determinant(s) of the C-terminal region, like the one identified in this study, would also be required for functional domain cooperativity.
Overall, our work defines the region of the ␤ 1a subunit encompassing amino acids 489 QVQVLTSLRRNLSFW 503 as an essential structural determinant for the physical and functional communication between DHPR complex and the RyR1. The putative ␤ 1a /RyR1-interacting motif Leu 496 -Leu 500 -Trp 503 enclosed within this segment influences the efficiency of EC coupling signal, but it appears to be not essential for either DHPR-RyR1 bidirectional signaling or DHPR tetrad organization.