INTRODUCTION

Csk is a 50-kilodalton (kDa)-cytosolic tyrosine protein kinase expressed in all cell types (1; reviewed in Refs. 2 and 3). As is the case for members of the Src family, Csk contains amino-terminal Src homology (SH)¹ 3 and SH2 domains, as well as a carboxyl-terminal catalytic domain. In contrast to Src-related enzymes, however, p50^csk does not possess an amino-terminal site of myristoylation, a site of autophosphorylation, or a carboxyl-terminal site of tyrosine phosphorylation (Fig. 1). Significant interest in p50^csk stems from its ability to phosphorylate the carboxyl-terminal tyrosine of Src family kinases in vitro, thereby repressing their enzymatic activity.

The importance of Csk in normal cellular physiology was first highlighted by the observation that Csk-deficient mice, generated by homologous recombination in embryonic stem cells, exhibited severe abnormalities in the central nervous system and early embryonic lethality (4, 5). Furthermore, Csk-deficient embryonic stem cells were unable to differentiate into T- or B-cells, when injected into blastocysts (6). Support for the notion that Csk functions in vivo by repressing Src family kinases was lent by the observation that fibroblasts derived from Csk-deficient embryos possessed hyperactive Src family kinases (p60^c-src, p59^fyn, and p56^lyn) and elevated levels of phosphotyrosine (4, 5). p50^csk also plays several critical roles in mature cellular physiology (7, 8, 9, 10). Notably, we reported that overexpression of Csk in an antigen-specific T-cell line (BI-141) caused a pronounced inhibition of T-cell receptor (TCR)-induced tyrosine protein phosphorylation and lymphokine secretion (7, 8).

Recently, several groups identified a second member of the Csk family (11, 12, 13, 14, 15, 16, 17). This enzyme, variably named Ntk, Matk, Ctk, Hyl, Lsk, and Batk, is now termed Chk (for sk omologous inase). We and others (11, 12, 18) have shown that Chk can phosphorylate the inhibitory carboxyl-terminal tyrosine of several Src-related enzymes in vitro, including Lck, Fyn, and c-Src. Unlike Csk, Chk only accumulates in brain and hemopoietic cells. Moreover, as a consequence of alternative splicing, the chk gene codes for two distinct proteins, p52^chk and p56^chk, which differ by the absence or presence of a 40-amino acid extension at their amino terminus (19; Fig. 1). Although the purpose of these two different Chk polypeptides is not determined, p52^chk primarily abounds in brain, whereas p56^chk is predominantly contained in hemopoietic cells. However, it should be pointed out that, while p56^chk is prominently expressed in human hemopoietic cells (15), it is generally a minor component of Chk proteins in mouse hemopoietic cells (19). The significance of this species difference is not understood.

Little is known of the role(s) of Chk in normal cellular physiology. Because it is seemingly always expressed with Csk, it is reasonable to speculate that the two enzymes may not serve fully identical functions. To begin dissecting these roles, we have tested the capacity of Chk to execute p50^csk-type functions in vivo. Our studies showed that, like Csk, Chk was apt at reducing the abundance of phosphotyrosine-containing proteins and the increased activity of p60^c-src and p59^fyn in Csk-deficient mouse embryo fibroblasts (MEFs). Unlike Csk, however, Chk was inefficient at repressing antigen receptor-induced signal transduction in T-cells. While these results supported the notion that Chk is also a negative regulator of Src family kinases in vivo, they implied that Chk may have a restricted Csk-like biological activity in mammalian cells.

MATERIALS AND METHODS

Csk-deficient and wild-type MEFs immortalized with simian virus (SV) 40 large T antigen were established by Imamoto and Soriano (5) and provided by Drs. Brian Howell and Jon Cooper (Fred Hutchinson Cancer Center, Seattle, WA). NIH 3T3 fibroblasts expressing a chimera bearing the SH3 and SH2 regions of Csk and the kinase domain of Chk (Csk-Chk chimera) will be described elsewhere.² BI-141 is an antigen-specific mouse T-cell hybridoma specific for the antigen beef insulin presented in the context of A^bA^k class II major histocompatibility complex (MHC) molecules (20). BI-141 derivatives expressing the neomycin phosphotransferase alone (Neo) or in combination with wild-type Csk or membrane-targeted Src-Csk were reported elsewhere (7). A BI-141 cell line expressing a myristoylation-defective version of Src-Csk, in which glycine 2 was mutated to alanine (A2Src-Csk), was generated by retrovirus-mediated gene transfer.² MEFs and NIH 3T3 cells were propagated in  minimal essential medium supplemented with 10% fetal calf serum (FCS), whereas BI-141 cells were grown in RPMI 1640 medium containing 10% FCS. Whenever indicated, puromycin or G418 was added to the growth medium.

cDNAs encoding mouse p52^chk or p56^chk or rat p50^csk were described elsewhere (1, 11, 19). A src-chk cDNA was engineered by fusing in-frame the sequences coding for the 15 amino-terminal residues of Src to the full coding sequence of p52^chk, through polymerase chain reaction. The first 15 residues of Src contain a myristoylation signal as well as polybasic sequences required for stable membrane association (21). The resulting chimeric cDNA was resequenced to ensure that no unwanted mutation had been introduced in the process (data not shown). For expression in MEFs from Csk-deficient mice (which contain an endogenous neomycin resistance gene (neo)), cDNAs were cloned in the multiple cloning site of the retroviral vector pBabePuro (22), which confers resistance to puromycin. In the case of BI-141 T-cells, cDNAs were inserted in the retroviral vector pLXSN (23), which bears the neo gene.

Retroviral constructs were transfected in -2 packaging cells by calcium phosphate precipitation (24). Transfected cells were selected by growth in medium containing either puromycin (1.0 µg/ml) or G418 (0.4 mg/ml). Retroviral supernatants were obtained from these cells and used to infect either MEFs or BI-141 T-cells, as described elsewhere (25). In both cases, monoclonal antibiotic-resistant derivatives were established by limiting dilution and screened by immunoblotting of total cell lysates with the appropriate antibodies.

Polyclonal rabbit antisera directed against the carboxyl-terminal tail or SH3 domain of p50^csk (7, 8, 26), the kinase domain of Chk (19), or the unique domain (27) or amino terminus (28) of p59^fyn were previously described. Affinity-purified rabbit anti-phosphotyrosine antibodies were reported elsewhere (28), whereas anti-phosphotyrosine monoclonal antibody (MAb) 4G10 was purchased from Upstate Biotechnology Inc., Lake Placid, NY. Polyclonal rabbit antibodies reacting against GTPase-activating protein (GAP) of p21^ras and Shc were obtained from Drs. Louise Larose and John Bergeron (Montréal, Québec, Canada), respectively. MAbs against the SH3 region of p60^src (MAb 327) were reported elsewhere (29), whereas MAbs against cortactin (MAb 4F11), Fak (MAb 2A7), and AFAP-110 (MAb F1) were generous gifts from Dr. Tom Parsons (Charlottesville, VA) (30). MAb LA-074 was generated against a synthetic peptide corresponding to amino acids 2-17 of the Src sequence and acquired from Quality Biotech, Camden, NJ. Anti-paxillin MAbs were purchased from Signal Transduction Laboratories, Lexington, KY. Anti-TCR MAb F23.1 was described elsewhere (31).

After washing in phosphate-buffered saline, cells were lysed in radio-immunoprecipitation assay (RIPA) buffer (20 mM MOPS, pH 7.0, 150 mM NaCl, 1% Nonidet P-40, 1% sodium deoxycholate, 0.1% SDS, 2 mM EDTA, pH 8.0) supplemented with 10 µg/ml each of the protease inhibitors leupeptin, aprotinin, N-tosyl-L-phenylalanine chloromethyl ketone, N-p-tosyl-L-lysine chloromethyl ketone and phenylmethylsulfonyl fluoride, as well as the phosphatase inhibitors sodium fluoride (50 mM) and sodium orthovanadate (1 mM). Polypeptides were recovered by immunoprecipitation using the indicated antibodies. Immune complexes were collected with Staphylococcus aureus protein A (Pansorbin; Calbiochem) coupled, if indicated, to rabbit anti-mouse immunoglobulin (Ig) G. Immunoprecipitates were washed three times with RIPA buffer containing 1 mM sodium orthovanadate. Proteins were then eluted in sample buffer, boiled, electrophoresed in 8% SDS-polyacrylamide gel electrophoresis gels, and transferred onto Immobilon membranes (Millipore, Mississauga, Ontario, Canada) for immunoblotting. Immunoblots were performed according to a previously described protocol (32). After incubation with ¹²⁵I-protein A (Amersham Canada, Oakville, Ontario, Canada) or ¹²⁵I-goat anti-mouse IgG (ICN Pharmaceuticals Canada Ltd., Montréal, Québec, Canada), immunoreactive products were detected by autoradiography and quantitated with a PhosphorImager.

These assays were done under linear conditions according to a previously described protocol (33). They were conducted in the presence of acid-denatured rabbit muscle enolase as a model substrate. Data were quantitated with a PhosphorImager.

For cell fractionation, cells were incubated for 15 min in hypotonic buffer (10 mM Tris, pH 7.4, 2 mM EDTA pH 8.0) supplemented with the protease and phosphatase inhibitors outlined above. Then membranes were mechanically broken using a Dounce homogenizer. In all cases, staining with trypan blue confirmed that over 95% of cells had been lysed (data not shown). After adjusting the homogenates to 0.15 M NaCl, intact cells, nuclei, and large membrane sheets were removed by two successive centrifugations at 480 × g for 5 min. Supernatants were then separated into soluble (S100) and particulate (P100) fractions by ultracentrifugation at 100,000 × g for 30 min. The various fractions were extracted in boiling sample buffer. Particulate fractions were washed once with hypotonic buffer prior to detergent extraction. Lysates corresponding to defined cell numbers were subjected to immunoblotting with anti-Src antibodies (see above). To avoid overloading the protein gels, lysates from 4 × lower cell numbers were used for the S100 fraction. This factor was taken into consideration in the subsequent calculation of the relative distribution of the chimeric proteins. The validity of the cell fractionation procedure was confirmed as reported previously (8, 19).

For myristic acid labeling, cells were incubated for 4 h at 37 °C with 1.0 mCi/ml [³H]myristic acid (DuPont NEN), in RPMI 1640 medium supplemented with 2% dialyzed FCS. For labeling with [³⁵S]methionine, cells were incubated for the same period of time with 1.0 mCi/ml Translabel (ICN Pharmaceuticals Canada Ltd., Montréal, Québec, Canada) in methionine-free RPMI 1640 medium containing 2% dialyzed FCS. After this incubation, cells were processed for immunoprecipitation as described above.

T-cells were activated by stimulation for 2 min at 37 °C with anti-TCR V8 mouse MAb F23.1 and sheep anti-mouse IgG, as outlined elsewhere (7, 28). Following activation, cells were lysed in boiling sample buffer, and lysates were processed for anti-phosphotyrosine immunoblotting. To measure lymphokine secretion, cells were incubated for 24 h with various concentrations of MAb F23.1 coated on plastic, according to a protocol detailed elsewhere (34). Release of interleukin-2 (IL-2) was determined by testing the ability of serial dilutions of the supernatants to support growth of the IL-2-dependent T-cell line HT-2. Units of IL-2 were calculated using a titration curve generated with recombinant IL-2 as reference.

Antigen stimulation assays were conducted by incubating BI-141 derivatives with irradiated spleen cells expressing A^bA^k class II MHC molecules (obtained from a cross between B10 and B10.BR mice; The Jackson Laboratories, Bar Harbor, ME) and pulsed with various concentrations of beef insulin. After 24 h of stimulation, supernatants were collected and assayed for lymphokine release by measuring their ability to support [³H]thymidine incorporation into HT-2 cells. Controls were without addition.

RESULTS

To test whether Chk can mediate Csk-type functions in mammalian cells, we examined whether it could restore the regulation of tyrosine protein phosphorylation in Csk-deficient MEFs (4, 5, 35). Cells were infected with retroviruses encoding the puromycin resistance gene (puro) alone or in combination with p52^chk, p56^chk, or p50^csk (Fig. 1). Monoclonal puromycin-resistant cell lines were screened by immunoblotting with anti-Csk or anti-Chk antibodies (data not shown). For cells expressing the puro gene alone, pools of antibiotic-resistant clones were used in our analyses.

Immunoblotting of total cell lysates with an anti-Csk serum (Fig. 2A, top panel) showed that csk^/ MEFs containing the puro gene alone (lane 2) lacked p50^csk. However, easily detectable amounts of p50^csk were present in derivatives in which a csk cDNA was re-introduced (lanes 3 and 4), as well as in MEFs derived from wild-type (csk^+/+) mice (lane 1). Parallel lysates were also probed by immunoblotting with anti-Chk antibodies (bottom panel). This experiment demonstrated that, contrary to control MEFs (lanes 1 and 2) which are devoid of endogenous Chk, cells infected with retroviruses encoding p52^chk (lanes 5 and 6) or p56^chk (lanes 7 and 8) contained appreciable quantities of the relevant Chk protein. In agreement with our earlier report (19), the expression levels of p52^chk were consistently greater than those of p56^chk in these cells. The basis for this difference is not known.

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To ensure that equivalent amounts of Chk and Csk were expressed in these cell lines, we took advantage of NIH 3T3 cells expressing a chimeric protein bearing the SH3 and SH2 domains of Csk and the catalytic region of Chk (Csk-Chk chimera). This polypeptide reacted both with antibodies directed against the SH3 domain of p50^csk (Fig. 2B, top panel, lanes 6-8) and with antibodies against the kinase domain of Chk (bottom panel, lanes 7-9). Hence, using Csk-Chk as an internal standard, we could show that the range of expression of either p52^chk or p56^chk was roughly equivalent to that of p50^csk (compare top panel, lanes 3 and 4 with bottom panel, lanes 2-5).

The abundance of phosphotyrosine-containing proteins in these cells was examined by anti-phosphotyrosine immunoblotting of total cell lysates (Fig. 3). Consistent with earlier reports (4, 5, 35, 36), MEFs lacking p50^csk (lane 2) contained markedly elevated levels of phosphotyrosine, in comparison with either MEFs derived from wild-type mice (lane 1) or csk^/ MEFs expressing p50^csk (lanes 3 and 4). Interestingly, Csk-deficient MEFs containing either p52^chk (lanes 5 and 6) or p56^chk (lanes 7 and 8) exhibited a striking reduction in tyrosine-phosphorylated proteins, in comparison with MEFs devoid of p50^csk (lane 2). Such a diminution affected all appreciable substrates. However, it is noteworthy that this decrease was slightly less than that observed in cells expressing p50^csk (compare lanes 5-8 with lanes 1, 3, and 4).

It has been demonstrated that components linked to the cytoskeleton, such as cortactin and paxillin, contain elevated amounts of phosphotyrosine in Csk-deficient MEFs (35, 36). To directly ascertain whether expression of Chk regulated these products, they were immunoprecipitated with specific antibodies and immunoblotted with anti-phosphotyrosine antibodies (Fig. 4, A and B, top panel). The abundance of these proteins was also monitored by immunoblotting of parallel immunoprecipitates with anti-cortactin or anti-paxillin antibodies, respectively (bottom panel). These analyses revealed that p52^chk and p56^chk markedly reduced the phosphotyrosine content of the 80-kDa cortactin (Fig. 4A) and 68-74-kDa paxillin (Fig. 4B). Similar observations were made for p125^fak and AFAP-110, two other components of the cytoskeleton (data not shown).

The differential ability of Csk and Chk to regulate tyrosine phosphorylation of predominantly cytosolic substrates was also assessed. GAP and its associated proteins (p62 and p190) as well as the adaptor molecule Shc were immunoprecipitated with the appropriate antibodies and probed by anti-phosphotyrosine immunoblotting (Fig. 4, C and D, top panel). Contrary to MEFs expressing p50^csk (lanes 1, 3, and 4), cells lacking Csk (lane 2) exhibited pronounced tyrosine phosphorylation of GAP-associated p62 and p190 (Fig. 4C), as well as of the 52- and 46-kDa isoforms of Shc (Fig. 4D). Little tyrosine phosphorylation of the 120-kDa GAP polypeptide was observed in these cells. Introduction of either Chk isoform in Csk-deficient MEFs dramatically reduced tyrosine phosphorylation of GAP-associated p62 and p190 (Fig. 4C, lanes 5-8). Similarly, the phosphotyrosine content of Shc (Fig. 4D, lanes 5-8) was diminished. Although the tyrosine phosphorylation of these substrates was clearly regulated by Chk, several polypeptides remained slightly hyperphosphorylated, especially in cells expressing p56^chk. This may relate to the lower levels of p56^chk expression generally achieved in these cells (Fig. 2).

To further prove that Chk has Csk-like activity in fibroblasts, its influence on the enzymatic activity of Src and Fyn was determined. The cell lines described above were lysed in RIPA buffer, and the enzymatic activity of Src or Fyn was determined in immune complex kinase reactions, using acid-denatured rabbit muscle enolase as a model substrate (Fig. 5, A and C). The relative specific activity of these enzymes was then calculated as described in the legend of Fig. 5. All experiments were conducted under linear assay conditions (data not shown). In agreement with other reports (4, 5, 35), the ability of p60^c-src (Fig. 5, A and B) and p59^fyn (Fig. 5, C and D) to phosphorylate enolase was increased ~5-fold in MEFs lacking Csk (lane 2), in contrast to MEFs containing p50^csk (lanes 1, 3, and 4). Expression of either p52^chk (lanes 5 and 6) or p56^chk (lanes 7 and 8) in Csk-deficient MEFs reduced the specific activity of p60^c-src by 2.5-3-fold, whereas that of p59^fyn was diminished by 3-5-fold (Fig. 5, B and D). Once again, this diminution was slightly less than that effected by Csk expression (lanes 3 and 4). We also noted that the abundance of Fyn was increased in cells expressing Csk or Chk (Fig. 5C, lanes 3-8), in comparison with csk^/ MEFs (lane 2). Perhaps, the abundance of p59^fyn was lowered in cells lacking Csk, in order to compensate for the augmented specific activity of the enzyme.

We also tested the impact of Chk on the physiology of BI-141 T-cells. Cells were infected with retroviruses encoding either p52^chk or p56^chk, in combination with the neomycin resistance gene (neo). Furthermore, a chimera encompassing the full sequence of p52^chk and the 15 amino-terminal residues of Src (Src-Chk chimera; Fig. 1) was engineered and also introduced in BI-141 cells. After selection in G418-containing medium, Chk-expressing monoclonal cell lines were detected by immunoblotting with an antiserum against Chk (data not shown).

Levels of Chk expression in representative cell lines are depicted in Fig. 6A. Like other transformed T-cell lines (11), control neomycin-resistant BI-141 cells (Neo) did not contain appreciable amounts of Chk (lanes 1 and 2). Nevertheless, all other cell lines expressed the appropriate Chk polypeptide (lanes 3-14). Furthermore, quantitative analyses similar to those described above (Fig. 2B) confirmed that p52^chk and p56^chk accumulated to levels similar to those achieved by overexpressed p50^csk in earlier studies (7, 8; data not shown). Finally, the expression levels of Src-Chk were compared with those of a Src-Csk chimera, expressed in the same manner in BI-141 cells (7). Immunoblotting of total cell lysates with an antibody against the Src-derived membrane targeting signal (MAb LA-074) revealed that Src-Chk (Fig. 6B, lanes 3-6) and Src-Csk (lanes 7 and 8) were expressed in comparable quantities in BI-141 cells.

To ascertain the effect of Chk on antigen receptor-mediated signals, cells were stimulated with anti-TCR MAb F23.1 and sheep anti-mouse IgG for 2 min, and changes in tyrosine protein phosphorylation were monitored by anti-phosphotyrosine immunoblotting of total cell lysates (Fig. 6, C and D). This experiment demonstrated that p52^chk (Fig. 6C, lanes 5-10), p56^chk (Fig. 6C, lanes 11-16), and Src-Chk (Fig. 6D, lanes 7 and 8) did not significantly repress TCR-induced tyrosine protein phosphorylation in BI-141 cells. While there was perhaps a tendency toward a lowering of TCR-induced phosphorylation in cells containing p56^chk (Fig. 6C, lanes 11-16), this difference was not maintained in more detailed time course analyses (data not shown). The result of Src-Chk expression was distinct from that of Src-Csk, which completely blocked TCR-induced tyrosine protein phosphorylation (Fig. 6D, lanes 5 and 6; Ref. 7).

The influence of Chk on TCR-induced lymphokine secretion was also determined (Fig. 7). Cells were first incubated with various concentrations of anti-TCR MAb F23.1 immobilized on plastic, and the release of lymphokines was monitored in an IL-2 bioassay (Fig. 7A), as outlined under "Materials and Methods." In these assays, BI-141 derivatives containing the neomycin phosphotransferase alone (Neo) exhibited a prominent anti-TCR-triggered release of lymphokines. This production was not measurably reduced in cells expressing either p52^chk or Src-Chk. p56^chk was also unable to repress IL-2 secretion, except at lower concentrations of anti-TCR antibodies (37 ng/ml). The lack of significant functional impact of the Chk proteins in BI-141 cells was contrary to the effect of Csk and Src-Csk, which dramatically inhibited IL-2 production at all antibody concentrations used (Fig. 7A; Refs. 7 and 8). We also evaluated the effects of Chk on the ability of BI-141 cells to produce IL-2 in response to antigen (beef insulin) and class II MHC-bearing spleen cells (Fig. 7B). This stimulus is a more physiological means of activating BI-141 cells. While Src-Csk completely blocked antigen/MHC-triggered lymphokine production, none of the Chk proteins, including p56^chk, reduced this response.

In order to understand the basis for the inefficiency of Src-Chk at repressing antigen receptor signaling, we examined whether addition of the Src myristoylation signal properly recruited Chk to cellular membranes. BI-141 derivatives expressing Src-Chk or Src-Csk were homogenized in hypotonic buffer, and particulate (P100) and cytosolic (S100) fractions were separated by differential centrifugation. While P100 contained cellular membranes, S100 possessed the cytosolic content. Cell lysates were probed by immunoblotting with anti-Src MAb LA-074 (Fig. 8A). In this assay, lysates corresponding to 4 × lower cell numbers were used for the S100 fraction, to avoid overloading of the protein gel. This factor was taken into consideration in calculating the proportion of proteins present in P100 and S100 (Fig. 8A, right-hand panel). As reported elsewhere (8), Src-Csk was mostly (approximately 90%) positioned in the particulate fraction of BI-141 cells (lane 3). In contrast, however, Src-Chk was primarily (~80%) located in the cytosolic fraction (lane 6). Similar results were obtained with other Src-Chk-expressing BI-141 clones (data not shown). It should be pointed out that under these conditions, both wild-type Csk and Chk primarily localized to the S100 fraction (data not shown; 8, 19).

Finally, we examined whether the lack of membrane targeting of Src-Chk was related to a defect in myristoylation. Cells were metabolically labeled with either [³H]myristic acid or [³⁵S]methionine, and the Src-tagged polypeptides were immunoprecipitated using MAb LA-074. Incorporation of radioactivity was monitored by fluorography (Fig. 8B). Similar to Src-Csk (lanes 2 and 3), Src-Chk (lanes 4 and 5) was efficiently labeled with both [³H]myristic acid (top panel) and [³⁵S]methionine (bottom panel). In contrast, a variant of Src-Csk in which the site of Src myristoylation (glycine 2) was mutated to alanine (A2Src-Csk; lane 6) did not incorporate the radioactive lipid (top panel), although it was labeled with [³⁵S]methionine (bottom panel). Together, these results demonstrated that Src-Chk failed to stably associate with cellular membranes even though it was myristoylated. This defect in membrane binding was not caused by mutations in the Src amino terminus, as revealed by careful re-sequencing of the src-chk cDNA (data not shown).

DISCUSSION

We and others (11, 12, 18) have shown that the two isoforms of the Chk tyrosine protein kinase (p52^chk and p56^chk) can phosphorylate the inhibitory carboxyl-terminal tyrosine of Src family kinases (Lck, Src, and Fyn) in immune complex kinase reactions and in yeast co-expression systems. Even though these phosphorylation assays are notoriously crude, the efficiency of Chk at regulating Src family kinases in this context was roughly equivalent to that of p50^csk. This finding raised the possibility that, like p50^csk, the Chk proteins may function in vivo by inhibiting Src family kinases. To begin understanding the function(s) of Chk in mammalian cells, we have evaluated its ability to carry out Csk-like functions in vivo, using two cellular systems in which Csk expression is known to have profound biochemical and/or biological consequences.

Our results demonstrated that p52^chk and p56^chk were efficient at repressing phosphotyrosine levels in Csk-deficient MEFs. Seemingly, the Chk kinases could down-regulate all the tyrosine phosphorylation events provoked by the absence of Csk. These involved components of the cytoskeleton such as cortactin, paxillin, Fak, and AFAP-110, as well as molecules implicated in the Ras pathway like Shc and the GAP-associated proteins. The effect of Chk was most likely due to repression of Src family kinases, as the activity of p60^c-src and p59^fyn in Csk-deficient MEFs was also diminished by Chk expression. As reported by others (5), however, we were not able to directly study the phosphorylation sites of Src family kinases in these cells, as they contain very low quantities of these enzymes. Nonetheless, on the basis of the documented activity of Chk against Src family kinases in vitro (11, 12, 18), it is fair to assume that Chk mediated these effects in csk^/ MEFs by inactivating Src family kinases. Hence, in combination, these data provided firm evidence that Chk exhibits Csk-like activity not only in vitro but also in vivo.

The impact of Chk in MEFs was consistently less marked than that of p50^csk. Since the abundance of Chk and Csk in these cells was roughly comparable, this raised the possibility that the intrinsic enzymatic activity of Chk may be lower than that of Csk, even though this notion may not be supported by earlier studies of their catalytic activity in vitro. Alternatively, it is plausible that Chk is recruited less efficiently in the vicinity of activated Src family kinases, as a consequence of differences in the affinity and/or specificity of its SH3 or SH2 domains. Or, Chk may not be able to physically interact with other proteins that normally participate in the function of Csk in cells. In keeping with this idea, we recently demonstrated that the SH3 regions of Csk and Chk have dramatically distinct binding specificities in vitro and in vivo (26). Finally, Chk may be physically restricted to a specific cellular compartment, thereby being unable to regulate all pools of Src family kinases in Csk-deficient MEFs. Further support for this concept will be presented below.

The capacity of Chk to behave as a negative regulator of antigen receptor signaling in T-cells was also evaluated. Even though the Chk proteins could be expressed in amounts comparable with those of p50^csk in Csk-overexpressing cells (this report; data not shown), they were inefficient at repressing TCR-triggered tyrosine protein phosphorylation and lymphokine secretion in the antigen-specific T-cell line BI-141. This is in contrast to p50^csk, which strongly repressed antigen receptor-induced signals in these cells (this report; 7, 8). In an attempt to enhance the activity of p52^chk, we also created a chimeric protein in which the sequence of p52^chk was fused to the membrane targeting signal of c-Src. While this chimera was myristoylated, it did not localize to cellular membranes in BI-141 T-cells. Moreover, it was unable to repress antigen receptor-mediated signal transduction. This was in contrast to Src-Csk, which efficiently associated with cellular membranes and completely blocked TCR-mediated signals (7, 8; this report).

While the inability of Src-Chk to bind cellular membranes prevented us from understanding the potential impact of membrane recruitment of this kinase, we feel that our findings are nonetheless informative. Possibly, they indicate that Chk contains dominant sequences that target the enzyme to a specific cellular locale. Consistent with this idea, we previously demonstrated that a significant proportion of p52^chk and p56^chk, but not p50^csk, was detergent-insoluble in various cell types, including hemopoietic cells (19). Since membrane recruitment may be essential for the action of Csk in T-cells (7, 8), constraints in the cellular distribution of Chk may interfere with its ability to negatively regulate TCR signaling. In contrast, they may not hamper the control of tyrosine protein phosphorylation in cells of fibroblastic lineage. Future studies will be necessary to clarify these issues.

Based on the data reported herein, it would appear that Chk can mediate some, but not all, of Csk-related functions in mammalian cells. While this may be due to differences in the kinase activity or regulation of the two Csk family members, our results also suggest that Chk may be a "spatially restricted" version of Csk, having Csk-like capabilities only in certain cellular locales. It should be pointed out though that our experiments did not address the possibility that Chk may also carry unique functions in neurons and hemopoietic cells, which are not provided by p50^csk. These may be consequent to a singular ability of Chk to accumulate in certain cellular compartments or to a yet unappreciated capacity to phosphorylate cellular targets other than Src family kinases.