Signalling through SH2 and SH3 domains

Functional dissection of p56lck, a protein tyrosine kinase which mediates interleukin-2-induced activation of the c-fos gene

Members of the newly identified receptor family for cytokines characteristically lack the intrinsic protein tyrosine kinase domain that is a hallmark of other growth factor receptors. Instead, accumulating evidence suggests that these receptors utilize nonreceptor-type protein tyrosine kinases for downstream signal transduction by cytokines. We have shown previously that the interleukin-2 receptor beta-chain interacts both physically and functionally with a Src family member, p56lck, and that p56lck activation leads to induction of the c-fos gene. However, the mechanism linking p56lck activation with c-fos induction remains unelucidated. In the present study, we systematically examined the extent of c-fos promoter activation by expression of a series of p56lck mutants, using a transient cotransfection assay. The results define a set of the essential amino acid residues that regulate p56lck induction of the c-fos promoter. We also provide evidence that the serum-responsive element and sis-inducible element are both targets through which p56lck controls c-fos gene activation.

Genetic analysis of a phosphatidylinositol 3-kinase SH2 domain reveals determinants of specificity

Phosphatidylinositol 3-kinase is an important element in both normal and oncogenic signal transduction. Polyomavirus middle T antigen transforms cells in a manner depending on association of its tyrosine 315 phosphorylation site with Src homology 2 (SH2) domains on the p85 subunit of the phosphatidylinositol 3-kinase. Both nonselective and site-directed mutagenesis have been used to probe the interaction of middle T with the N-terminal SH2 domain of p85. Most of the 24 mutants obtained showed reduced middle T binding. However, mutations that showed increased binding were also found. Comparison of middle T binding to that of the platelet-derived growth factor receptor showed that some mutations altered the specificity of recognition by the SH2 domain. Mutations altering S-393, D-394, and P-395 were shown to affect the ability of the SH2 domain to select peptides from a degenerate phosphopeptide library. These results focus attention on the role of the EF loop in the SH2 domain in determining binding selectivity at the third position after the phosphotyrosine.

Characterization of the interaction of natural proline-rich peptides with five different SH3 domains

The interaction of six different proline-rich peptides with five SH3 domains has been investigated by using spectroscopic techniques. These peptides correspond to natural sequences and have been implicated in the interaction of some SH3 domains with other proteins. We have determined the Kd values for all of the possible combinations between the peptides and the SH3 domains. Low specificity and low affinity (> 5 microM) are the most remarkable conclusions from these studies. None of the peptides tested here were found to bind with significant affinity to spectrin-SH3 or n-src-SH3. Abl-SH3 seems to be the most selective of the domains analyzed here, while Fyn-SH3 is the most promiscuous. CD and FTIR studies indicate that these peptides adopt to different extents a PPII-like structure in aqueous solution. However, analysis of the SH3 domain complexes with these peptides suggests that proline-rich peptides do not necessarily adopt an overall PPII structure over their entire length upon binding to the different SH3 domains.

Genetic analysis of a phosphatidylinositol 3-kinase SH2 domain reveals determinants of specificity

Phosphatidylinositol 3-kinase is an important element in both normal and oncogenic signal transduction. Polyomavirus middle T antigen transforms cells in a manner depending on association of its tyrosine 315 phosphorylation site with Src homology 2 (SH2) domains on the p85 subunit of the phosphatidylinositol 3-kinase. Both nonselective and site-directed mutagenesis have been used to probe the interaction of middle T with the N-terminal SH2 domain of p85. Most of the 24 mutants obtained showed reduced middle T binding. However, mutations that showed increased binding were also found. Comparison of middle T binding to that of the platelet-derived growth factor receptor showed that some mutations altered the specificity of recognition by the SH2 domain. Mutations altering S-393, D-394, and P-395 were shown to affect the ability of the SH2 domain to select peptides from a degenerate phosphopeptide library. These results focus attention on the role of the EF loop in the SH2 domain in determining binding selectivity at the third position after the phosphotyrosine.

Functional dissection of p56lck, a protein tyrosine kinase which mediates interleukin-2-induced activation of the c-fos gene

Members of the newly identified receptor family for cytokines characteristically lack the intrinsic protein tyrosine kinase domain that is a hallmark of other growth factor receptors. Instead, accumulating evidence suggests that these receptors utilize nonreceptor-type protein tyrosine kinases for downstream signal transduction by cytokines. We have shown previously that the interleukin-2 receptor beta-chain interacts both physically and functionally with a Src family member, p56lck, and that p56lck activation leads to induction of the c-fos gene. However, the mechanism linking p56lck activation with c-fos induction remains unelucidated. In the present study, we systematically examined the extent of c-fos promoter activation by expression of a series of p56lck mutants, using a transient cotransfection assay. The results define a set of the essential amino acid residues that regulate p56lck induction of the c-fos promoter. We also provide evidence that the serum-responsive element and sis-inducible element are both targets through which p56lck controls c-fos gene activation.

Binding of Bruton's tyrosine kinase to Fyn, Lyn, or Hck through a Src homology 3 domain-mediated interaction

Bruton's tyrosine kinase (Btk) is a recently described B-cell-specific tyrosine kinase. Mutations in this gene lead to human X chromosome-linked agammaglobulinemia and murine X-linked immunodeficiency. Although genetic evidence strongly suggests that Btk plays a crucial role in B-lymphocyte differentiation and activation, its precise mechanism of action remains unknown, primarily because the proteins that it interacts with have not yet been identified. Here, we show that Btk interacts with Src homology 3 domains of Fyn, Lyn, and Hck, protein-tyrosine kinases that get activated upon stimulation of B- and T-cell receptors. These interactions are mediated by two 10-aa motifs in Btk. An analogous site with the same specificity is also present in Itk, the T-cell-specific homologue of Btk. Our data extend the range of interactions mediated by Src homology 3 domains and provide an indication of a link between Btk and established signaling pathways in B lymphocytes.

CRK protein binds to two guanine nucleotide-releasing proteins for the Ras family and modulates nerve growth factor-induced activation of Ras in PC12 cells

It has been reported that growth factors activate Ras through a complex of an adaptor type SH2-containing molecule, Grb2, and a Ras guanine nucleotide-releasing protein (GNRP), mSos. We report on the involvement of another adaptor molecule, CRK, in the activation of Ras. Overexpression of wild-type CRK proteins CRK-I and CRK-II enhanced the nerve growth factor (NGF)-induced activation of Ras in PC12 cells, although the basal level of GTP-bound active Ras was not altered. In contrast, mutants with a single amino acid substitution in either the SH2 or SH3 domain of the CRK-I protein inhibited the NGF-induced activation of Ras. Two GNRPs for the Ras family, mSos and C3G, were coimmunoprecipitated with the endogenous Crk proteins in PC12 cells. The association between C3G and the CRK mutants was dependent upon the presence of intact SH3. The SH2 domain of CRK bound to the SHC protein phosphorylated on tyrosine residues by NGF stimulation. The results demonstrate that, in addition to Grb2, CRK participates in signaling from the NGF receptor and that two GNRPs appear to transmit signals from these adaptor molecules to Ras.

CRK protein binds to two guanine nucleotide-releasing proteins for the Ras family and modulates nerve growth factor-induced activation of Ras in PC12 cells

It has been reported that growth factors activate Ras through a complex of an adaptor type SH2-containing molecule, Grb2, and a Ras guanine nucleotide-releasing protein (GNRP), mSos. We report on the involvement of another adaptor molecule, CRK, in the activation of Ras. Overexpression of wild-type CRK proteins CRK-I and CRK-II enhanced the nerve growth factor (NGF)-induced activation of Ras in PC12 cells, although the basal level of GTP-bound active Ras was not altered. In contrast, mutants with a single amino acid substitution in either the SH2 or SH3 domain of the CRK-I protein inhibited the NGF-induced activation of Ras. Two GNRPs for the Ras family, mSos and C3G, were coimmunoprecipitated with the endogenous Crk proteins in PC12 cells. The association between C3G and the CRK mutants was dependent upon the presence of intact SH3. The SH2 domain of CRK bound to the SHC protein phosphorylated on tyrosine residues by NGF stimulation. The results demonstrate that, in addition to Grb2, CRK participates in signaling from the NGF receptor and that two GNRPs appear to transmit signals from these adaptor molecules to Ras.

High-resolution crystal structures of tyrosine kinase SH3 domains complexed with proline-rich peptides

Src-homology 3 (SH3) domains bind to proline-rich motifs in target proteins. We have determined high-resolution crystal structures of the complexes between the SH3 domains of Abl and Fyn tyrosine kinases, and two ten-residue proline-rich peptides derived from the SH3-binding proteins 3BP-1 and 3BP-2. The X-ray data show that the basic mode of binding of both proline-rich peptides is the same. Peptides are bound over their entire length and interact with three major sites on the SH3 molecules by both hydrogen-bonding and van der Waals contacts. Residues 4-10 of the peptide adopt the conformation of a left-handed polyproline helix type II. Binding of the proline at position 2 requires a kink at the non-proline position 3.

Identification of Src, Fyn, and Lyn SH3-binding proteins: implications for a function of SH3 domains

Src homology 3 (SH3) domains mediate protein-protein interactions necessary for the coupling of cellular proteins involved in intracellular signal transduction. We previously established solution-binding conditions that allow affinity isolation of Src SH3-binding proteins from cellular extracts (Z. Weng, J. A. Taylor, C. E. Turner, J. S. Brugge, and C. Seidel-Dugan, J. Biol. Chem. 268:14956-14963, 1993). In this report, we identified three of these proteins: Shc, a signaling protein that couples membrane tyrosine kinases with Ras; p62, a protein which can bind to p21rasGAP; and heterogeneous nuclear ribonucleoprotein K, a pre-mRNA-binding protein. All of these proteins contain proline-rich peptide motifs that could serve as SH3 domain ligands, and the binding of these proteins to the Src SH3 domain was inhibited with a proline-rich Src SH3 peptide ligand. These three proteins, as well as most of the other Src SH3 ligands, also bound to the SH3 domains of the closely related protein tyrosine kinases Fyn and Lyn. However, Src- and Lyn-specific SH3-binding proteins were also detected, suggesting subtle differences in the binding specificity of the SH3 domains from these related proteins. Several Src SH3-binding proteins were phosphorylated in Src-transformed cells. The phosphorylation of these proteins was not detected in cells transformed by a mutant variant of Src lacking the SH3 domain, while there was little change in tyrosine phosphorylation of other Src-induced phosphoproteins. In addition, the coprecipitation of v-Src with two tyrosyl-phosphorylated proteins with M(r)s of 62,000 and 130,000 was inhibited by incubation with a Src SH3 peptide ligand, suggesting that the binding of these substrate proteins is dependent on interactions with the SH3 domain. These results strongly suggest a role for the Src SH3 domain in the recruitment of substrates to this protein tyrosine kinase, either through direct interaction with the SH3 domain or indirectly through interactions with proteins that bind to the SH3 domain.

Mutations in v-Src SH3 and catalytic domains that jointly confer temperature-sensitive transformation with minimal temperature-dependent changes in cellular tyrosine phosphorylation

We have analyzed two functionally significant amino acid alterations encoded by the temperature-sensitive (ts) v-src mutant of Rous sarcoma virus, LA32. The G-to-V change at residue 300 in the catalytic domain nonconditionally impairs morphological transformation, in vitro kinase activity, in vivo tyrosine phosphorylation, and the cytoskeletal association of v-Src while rendering anchorage- and serum-independent growth ts. The R-to-P mutation in the SH3 domain subtly enhances morphological transformation but has no phenotype if the catalytic domain is inactivated. In the presence of the G-300-to-V mutation, this SH3 domain lesion does not affect v-Src in vitro kinase activity and cytoskeletal association, but it nonconditionally enhances cellular tyrosine phosphorylation and restores morphological transformation at the permissive temperature only. This ability to induce a ts transformed morphology, in concert with nonconditional elevations of cellular phosphotyrosine, suggest that a subset of v-Src targets that are crucial to transformation may be affected in ts fashion by the SH3 mutation. Consistent with this, we find that the R-107-to-P mutation confers ts activity and tyrosine phosphorylation on the SH3-binding enzyme phosphatidylinositol 3'-kinase. Thus, both the SH3 and catalytic domain mutations in LA32 have some ts attributes and they cooperate in determining the mutant's behavior. The ts SH3 mutation is unique and offers the potential for deeper understanding of the function of this domain.

Identification of Src, Fyn, and Lyn SH3-binding proteins: implications for a function of SH3 domains

Src homology 3 (SH3) domains mediate protein-protein interactions necessary for the coupling of cellular proteins involved in intracellular signal transduction. We previously established solution-binding conditions that allow affinity isolation of Src SH3-binding proteins from cellular extracts (Z. Weng, J. A. Taylor, C. E. Turner, J. S. Brugge, and C. Seidel-Dugan, J. Biol. Chem. 268:14956-14963, 1993). In this report, we identified three of these proteins: Shc, a signaling protein that couples membrane tyrosine kinases with Ras; p62, a protein which can bind to p21rasGAP; and heterogeneous nuclear ribonucleoprotein K, a pre-mRNA-binding protein. All of these proteins contain proline-rich peptide motifs that could serve as SH3 domain ligands, and the binding of these proteins to the Src SH3 domain was inhibited with a proline-rich Src SH3 peptide ligand. These three proteins, as well as most of the other Src SH3 ligands, also bound to the SH3 domains of the closely related protein tyrosine kinases Fyn and Lyn. However, Src- and Lyn-specific SH3-binding proteins were also detected, suggesting subtle differences in the binding specificity of the SH3 domains from these related proteins. Several Src SH3-binding proteins were phosphorylated in Src-transformed cells. The phosphorylation of these proteins was not detected in cells transformed by a mutant variant of Src lacking the SH3 domain, while there was little change in tyrosine phosphorylation of other Src-induced phosphoproteins. In addition, the coprecipitation of v-Src with two tyrosyl-phosphorylated proteins with M(r)s of 62,000 and 130,000 was inhibited by incubation with a Src SH3 peptide ligand, suggesting that the binding of these substrate proteins is dependent on interactions with the SH3 domain. These results strongly suggest a role for the Src SH3 domain in the recruitment of substrates to this protein tyrosine kinase, either through direct interaction with the SH3 domain or indirectly through interactions with proteins that bind to the SH3 domain.

Interactions of p59fyn and ZAP-70 with T-cell receptor activation motifs: defining the nature of a signalling motif

The tyrosine-based activation motif is a 20- to 25-amino-acid sequence contained in the cytoplasmic domains of many hematopoietic receptors which is sufficient by itself to reconstitute signalling. This motif is characterized by two YXXL/I sequences separated by approximately 10 residues. The molecular basis of signalling by this motif is unknown. Here we demonstrate that the tyrosine-based activation motif is required and sufficient for association with the tyrosine kinases p59fyn and ZAP-70, suggesting that association with these kinases is a general feature of this motif. Focusing on the single activation motif present in epsilon, we analyzed which residues of the motif were critical for binding of p59fyn and ZAP-70. Surprisingly, we found that no single mutation of any residue of epsilon resulted in the loss of p59fyn association. In contrast, single mutations at five residues of the epsilon activating motif abrogated ZAP-70 binding. Both of the tyrosines and the leucine or isoleucine residues that follow them were critical. The spacing between the tyrosines was also important, as deletion of two residues disrupted binding of ZAP-70, although p59fyn binding was not disrupted. Most of the defined features of the tyrosine activation motif are therefore requirements for ZAP-70 binding. Interestingly, the interaction of ZAP-70 with the motif was dependent on the presence of both ZAP-70 SH2 domains and both of the tyrosine residues in the motif, suggesting that ZAP-70 interacts with two phosphotyrosine residues and that the binding of the two SH2 domains is cooperative. In addition, we demonstrate that the interaction between the tyrosine activation motif is direct and requires prior tyrosine phosphorylation of the motif. We propose that the activation of cells by the tyrosine activating motif occurs in four discrete steps: binding of p59fyn, phosphorylation of the motif, binding of ZAP-70, and activation of ZAP-70 kinase activity.

Suppression by delta-9-tetrahydrocannabinol of lipopolysaccharide-induced and intrinsic tyrosine phosphorylation and protein expression in mouse peritoneal macrophages

Lipopolysaccharide (LPS, 100 ng/mL)-induced tyrosine phosphorylation of four proteins (p41, p42, p77, and p82) in mouse resident peritoneal macrophages was observed using a monoclonal anti-phosphotyrosine antibody PY20 immunoblotting method. Macrophages pretreated for 3 hr with 1 microgram delta-9-tetrahydrocannabinol (THC)/mL had decreased tyrosine phosphorylation of p77 and p82 after incubation with LPS for 30 min. Simultaneous treatment of macrophages with THC (10 micrograms/mL) plus LPS for 30 min had a similar effect on p77 and p82 tyrosine phosphorylation. When the THC pretreatment protocol was combined with the simultaneous treatment protocol, 0.5 and 5 micrograms THC/mL, respectively, completely blocked LPS-induced p77 and p82 tyrosine phosphorylation. However, neither simultaneous treatment with THC nor pre- and simultaneous treatment had any effect on LPS-induced tyrosine phosphorylation of p41 and p42 in macrophages. Pretreatment with 1 microgram THC/mL followed by simultaneous treatment with 10 micrograms THC/mL induced a p43 protein that showed tyrosine phosphorylation in place of p41 and p42. Further analysis of THC effects on macrophages revealed an increase in tyrosine phosphorylation as an immediate early even after THC treatment. Prolonged treatment of macrophages with THC resulted in a broad suppression of tyrosine phosphorylation and some cellular protein expression. Three cellular proteins (p65, p70, and p72) seemed most susceptible to inhibition by THC. The data suggest that suppression of tyrosine phosphorylation by THC in macrophages may be one of the mechanisms associated with inhibition of cell function, including the suppression of tumor necrosis factor-alpha release from macrophages.

Role of Src homology 3 domains in assembly and activation of the phagocyte NADPH oxidase

The phagocyte NADPH oxidase, dormant in resting cells, is activated during phagocytosis to produce superoxide, a precursor of microbicidal oxidants. The activated oxidase is a complex of membrane-integrated cytochrome b558, composed of 91-kDa (gp91phox) and 22-kDa (p22phox) subunits, and two cytosolic factors (p47phox and p67phox), each containing two Src homology 3 (SH3) domains. Here we show that the region of the tandem SH3 domains of p47phox (p47-SH3) expressed as a glutathione S-transferase fusion protein inhibits the superoxide production in a cell-free system, indicating involvement of the domains in the activation. Furthermore, we find that arachidonic acid and sodium dodecyl sulfate, activators of the oxidase in vitro, cause exposure of p47-SH3, which has probably been masked by the C-terminal region of this protein in a resting state. The unmasking of p47-SH3 appears to play a crucial role in the assembly of the oxidase components, because p47-SH3 binds to both p22phox and p67phox but fails to interact with a mutant p22phox carrying a Pro-156-->Gln substitution in a proline-rich region, which has been found in a patient with chronic granulomatous disease. Based on the observations, we propose a signal-transducing mechanism whereby normally inaccessible SH3 domains become exposed upon activation to interact with their target proteins.

Nucleotide sequence analysis of an 11.7 kb fragment of yeast chromosome II including BEM1, a new gene of the WD-40 repeat family and a new member of the KRE2/MNT1 family

This paper reports the DNA sequence and analysis of an 11.7 kb segment localized on the right arm of Saccharomyces cerevisiae chromosome II. This fragment contains one incomplete and five long and non-overlapping open reading frames (ORFs) designated from centromere to telomere-proximal side as: YBR1406, 1409, 1410, 1411, 1412 and 1413. YBR1406 corresponds to the 5' end to PG11 encoding phosphoglucoisomerase. YBR1410 encodes a polypeptide of 798 amino acids whose C terminus contains five repeats (WD-40 repeat) similar to those found in the beta-subunits of G proteins and different yeast proteins such as Tup1, Prp4 and Cdc4. The higher similarity score is obtained with dTAFII80, a component of the RNA polymerase II transcriptional complex TFIID. YBR1411 encodes a polypeptide of 464 amino acids which belongs to the family of alpha-mannosyltransferases: KRE2/MNT1, KTR1, KTR2, YUR1 and the product of previously sequenced ORF YBR1445. YBR1412 corresponds to BEM1. The two ORFs, YBR1409 and YBR1413, which do not exhibit significant similarity with any known coding sequences, define new genes.

JAK protein tyrosine kinases: their role in cytokine signalling

Protein tyrosine kinases (PTKs) are integral components of the cellular machinery that mediates the transduction and/or processing of many extra- and intracellular signals. Members of the JAK family of intracellular PTKs (JAK1, JAK2 and TYK2) are characterized by the possession of a PTK-related domain and five additional homology domains, in addition to a classical PTK domain. An important breakthrough in the understanding of JAK kinases function(s) has come from the recent observations that many cytokine receptors compensate for their lack of a PTK domain by utilizing members of the JAK family for signal transduction.

Interactions of p59fyn and ZAP-70 with T-cell receptor activation motifs: defining the nature of a signalling motif

The tyrosine-based activation motif is a 20- to 25-amino-acid sequence contained in the cytoplasmic domains of many hematopoietic receptors which is sufficient by itself to reconstitute signalling. This motif is characterized by two YXXL/I sequences separated by approximately 10 residues. The molecular basis of signalling by this motif is unknown. Here we demonstrate that the tyrosine-based activation motif is required and sufficient for association with the tyrosine kinases p59fyn and ZAP-70, suggesting that association with these kinases is a general feature of this motif. Focusing on the single activation motif present in epsilon, we analyzed which residues of the motif were critical for binding of p59fyn and ZAP-70. Surprisingly, we found that no single mutation of any residue of epsilon resulted in the loss of p59fyn association. In contrast, single mutations at five residues of the epsilon activating motif abrogated ZAP-70 binding. Both of the tyrosines and the leucine or isoleucine residues that follow them were critical. The spacing between the tyrosines was also important, as deletion of two residues disrupted binding of ZAP-70, although p59fyn binding was not disrupted. Most of the defined features of the tyrosine activation motif are therefore requirements for ZAP-70 binding. Interestingly, the interaction of ZAP-70 with the motif was dependent on the presence of both ZAP-70 SH2 domains and both of the tyrosine residues in the motif, suggesting that ZAP-70 interacts with two phosphotyrosine residues and that the binding of the two SH2 domains is cooperative. In addition, we demonstrate that the interaction between the tyrosine activation motif is direct and requires prior tyrosine phosphorylation of the motif. We propose that the activation of cells by the tyrosine activating motif occurs in four discrete steps: binding of p59fyn, phosphorylation of the motif, binding of ZAP-70, and activation of ZAP-70 kinase activity.

Mutagenic analysis of the roles of SH2 and SH3 domains in regulation of the Abl tyrosine kinase

We have used in vitro mutagenesis to examine in detail the roles of two modular protein domains, SH2 and SH3, in the regulation of the Abl tyrosine kinase. As previously shown, the SH3 domain suppresses an intrinsic transforming activity of the normally nontransforming c-Abl product in vivo. We show here that this inhibitory activity is extremely position sensitive, because mutants in which the position of the SH3 domain within the protein is subtly altered are fully transforming. In contrast to the case in vivo, the SH3 domain has no effect on the in vitro kinase activity of the purified protein. These results are consistent with a model in which the SH3 domain binds a cellular inhibitory factor, which in turn must physically interact with other parts of the kinase. Unlike the SH3 domain, the SH2 domain is required for transforming activity of activated Abl alleles. We demonstrate that SH2 domains from other proteins (Ras-GTPase-activating protein, Src, p85 phosphatidylinositol 3-kinase subunit, and Crk) can complement the absence of the Abl SH2 domain and that mutants with heterologous SH2 domains induce altered patterns of tyrosine-phosphorylated proteins in vivo. The positive function of the SH2 domain is relatively position independent, and the effect of multiple SH2 domains appears to be additive. These results suggest a novel mechanism for regulation of tyrosine kinases in which the SH2 domain binds to, and thereby enhances the phosphorylation of, a subset of proteins phosphorylated by the catalytic domain. Our data also suggest that the roles of the SH2 and SH3 domains in the regulation of Abl are different in several respects from the roles proposed for these domains in the closely related Src family of tyrosine kinases.

c-Abl kinase regulates the protein binding activity of c-Crk

c-Crk is a proto-oncogene product composed largely of Src homology (SH) 2 and 3 domains. We have identified a kinase activity, which binds to the first Crk SH3 domain and phosphorylates c-Crk on tyrosine 221 (Y221), as c-Abl. c-Abl has a strong preference for c-Crk, when compared with common tyrosine kinase substrates. The phosphorylation of c-Crk Y221 creates a binding site for the Crk SH2 domain. Bacterially expressed c-Crk protein lacks phosphorylation on Y221 and can bind specifically to several proteins, while mammalian c-Crk, which is phosphorylated on tyrosine, remains uncomplexed. The protein binding activity of c-Crk is therefore likely regulated by a mechanism similar to that of the Src family kinases. v-Crk is truncated before c-Crk Y221 and forms constitutive complexes with c-Abl and other proteins. Our results suggest that c-Abl regulates c-Crk function and that it could be involved in v-Crk transformation.

NH2-terminal sequence of macrophage-expressed natural resistance-associated macrophage protein (Nramp) encodes a proline/serine-rich putative Src homology 3-binding domain

The Lsh/Ity/Bcg locus on mouse chromosome 1 regulates macrophage (m phi) priming/activation for antimicrobial activity against intracellular pathogens. A candidate Bcg gene, designated natural resistance-associated m phi protein (Nramp), recently isolated from a pre-B cell cDNA library encodes a polytopic integral membrane protein with structural features common to prokaryotic and eukaryotic transporters. In the present study, an activated m phi cDNA library yielded new Nramp clones that differ in the 5' region from the published pre-B cell-derived clone sequence, resulting in addition of 64 amino acids at the NH2 terminus of the predicted protein. This new domain is rich in proline, serine, and basic amino acids, and includes three protein kinase C phosphorylation sites and a putative Src homology 3 binding domain. RNAs containing this domain are the only form found in the m phi. Hence, the protein encoded by this RNA is the candidate molecule mediating natural resistance to intra-m phi pathogens.

Mutagenic analysis of the roles of SH2 and SH3 domains in regulation of the Abl tyrosine kinase

We have used in vitro mutagenesis to examine in detail the roles of two modular protein domains, SH2 and SH3, in the regulation of the Abl tyrosine kinase. As previously shown, the SH3 domain suppresses an intrinsic transforming activity of the normally nontransforming c-Abl product in vivo. We show here that this inhibitory activity is extremely position sensitive, because mutants in which the position of the SH3 domain within the protein is subtly altered are fully transforming. In contrast to the case in vivo, the SH3 domain has no effect on the in vitro kinase activity of the purified protein. These results are consistent with a model in which the SH3 domain binds a cellular inhibitory factor, which in turn must physically interact with other parts of the kinase. Unlike the SH3 domain, the SH2 domain is required for transforming activity of activated Abl alleles. We demonstrate that SH2 domains from other proteins (Ras-GTPase-activating protein, Src, p85 phosphatidylinositol 3-kinase subunit, and Crk) can complement the absence of the Abl SH2 domain and that mutants with heterologous SH2 domains induce altered patterns of tyrosine-phosphorylated proteins in vivo. The positive function of the SH2 domain is relatively position independent, and the effect of multiple SH2 domains appears to be additive. These results suggest a novel mechanism for regulation of tyrosine kinases in which the SH2 domain binds to, and thereby enhances the phosphorylation of, a subset of proteins phosphorylated by the catalytic domain. Our data also suggest that the roles of the SH2 and SH3 domains in the regulation of Abl are different in several respects from the roles proposed for these domains in the closely related Src family of tyrosine kinases.

C3G, a guanine nucleotide-releasing protein expressed ubiquitously, binds to the Src homology 3 domains of CRK and GRB2/ASH proteins

CRK protein, together with GRB2/ASH and Nck proteins, belongs to the adaptor-type Src homology (SH)2-containing molecules, which transduce signals from tyrosine kinases. Here another guanine nucleotide-releasing protein (GNRP), C3G, has been identified as a CRK SH3-binding protein. The nucleotide sequence of a 4.1-kb C3G cDNA contains a 3.2-kb open reading frame encoding a 121-kDa protein, and antibodies against C3G have been shown to detect a protein of 130-140 kDa. The carboxyl terminus of C3G has a peptide sequence homologous to GNRPs for Ras, and the expression of this carboxyl terminus region suppresses the loss of CDC25 function in the yeast Saccharomyces cerevisiae. The C3G protein expressed in Escherichia coli binds to CRK and GRB2/ASH proteins. Mutational analysis of C3G assigns the SH3 binding region to a 50-amino acid region containing a proline-rich sequence. The mRNAs of both the C3G and CRK proteins are expressed ubiquitously in human adult and fetal tissues. The results of these studies suggest that the complex of CRK and C3G, or GRB2/ASH and C3G, may transduce the signals from tyrosine kinases to Ras in a number of different tissues.

The actin binding site in the tail domain of Dictyostelium myosin IC (myoC) resides within the glycine- and proline-rich sequence (tail homology region 2)

The majority of protozoan myosins I possess tail domains composed of three distinct and conserved regions of sequence, referred to as tail homology regions 1, 2 and 3 (TH.1, TH.2 and TH.3). While the N-terminal approximately half of the tail (corresponding to TH.1) has been implicated in membrane binding, all or some portion of the C-terminal approximately half of the tail (corresponding to TH.2 plus TH.3) has been implicated in binding to F-actin in a nucleotide-insensitive fashion. Here we show, using fusion proteins containing portions of the Dictyostelium myosin IC (myoC) tail domain and F-actin sedimentation assays, that the ability of the myoC tail to bind to actin resides entirely within the glycine- and proline-rich TH.2 domain. The src-like TH.3 domain does not bind to actin, nor does it augment the binding properties of the TH.2 domain. In addition to defining more precisely the location of the actin binding site in the tail domain of a protozoan myosin I, these results have implications for the function of the src-like TH.3 domain in myosins I and other proteins.

Abl protein-tyrosine kinase selects the Crk adapter as a substrate using SH3-binding sites

To understand the normal and oncogenic functions of the protein-tyrosine kinase Abl, the yeast two-hybrid system has been used for identifying proteins that interact with it. One interacting protein is Crk-I, an SH3/SH2-containing adapter protein that was originally identified as the oncogenic element in the avian sarcoma virus CT10. Direct interaction between the Crk-I SH3 and Abl at novel, approximately 10 amino acid sites just carboxy-terminal to the Abl kinase domain occurs in vitro and in mammalian cells. There is a nearby site specific for binding another adapter, Nck, and these sites also bind Grb-2. When bound to Abl, Crk-I was phosphorylated on tyrosine. Thus, the SH3-binding sites on Abl serve as substrate recognition sites for the relatively nonspecific kinase of Abl. In Crk-I-transformed cells, Crk-I associates with endogenous c-Abl and is phosphorylated on tyrosine. The association of Crk and Abl suggests that Abl could play a role in v-Crk and Crk-I transformation and that normal Abl function may be partly mediated through bound adapter molecules.

Induction of early-response genes KC and JE by mycobacterial lipoarabinomannans: regulation of KC expression in murine macrophages by Lsh/Ity/Bcg (candidate Nramp)

The murine chromosome 1 gene Lsh/Ity/Bcg (candidate Nramp) regulates macrophage activation for antimicrobial activity against Salmonella typhimurium, Leishmania donovani, and Mycobacterium spp. To determine early events in the activation pathway, the ability of mycobacterial lipoarabinomannan (LAM) to induce early gene (KC and JE) expression in macrophages from susceptible (S) C57BL/10ScSn (Lshs) and congenic resistant (R) B10.L-Lshr mice was investigated. Stimulation with 1.8 microgram of arabinofuranosyl-terminated LAM (AraLAM) per ml resulted in similar kinetics for KC or JE expression in S and R macrophages. However, whereas JE/glyceraldehyde 3-phosphate dehydrogenase (GAPDH) mRNA ratios remained equivalent, R macrophages consistently showed enhanced KC/GAPDH ratios within 30 to 40 min of stimulation compared with S macrophages. Significant differences in KC/GAPDH ratios were observed throughout the peak period (0.5 to 6 h) of the KC response and with doses of AraLAM ranging from 0.01 to 2.5 micrograms/ml. Heavily mannosylated LAM from virulent Mycobacterium tuberculosis Erdman, in doses of up to 2.5 micrograms/ml, failed to stimulate KC or JE in S or R macrophages. Gamma interferon alone (25 U/ml) stimulated equivalent JE expression in S and R macrophages and synergized with AraLAM to enhance JE in both. In contrast, AraLAM-induced KC expression was inhibited in the presence of gamma interferon. Agonist/inhibitor studies were undertaken to determine the signal transduction pathways mediating KC expression. The protein kinase C (PKC) inhibitor Calphostin C (200 nM) inhibited AraLAM-induced KC by 34% +/- 4% in S macrophages and 43% +/- 5% in R macrophages; the cyclic AMP-dependent PKA inhibitor KT5720 (2 microM) inhibited AraLAM-induced KC by 33% +/- 4% (S) and 25% +/- 5% (R). A role for Ca2+ was indicated because ionophore alone stimulated KC expression and synergized with AraLAM to give a dramatically enhanced response. Induction of KC was also inhibited by (i) blocking constitutive nitric oxide (NO) production by preincubation of macrophages with NG-monomethyl-L-arginine (400 microM) (48% +/- 8% [S] and 40% +/- 11% [R]) and (ii) incubation of macrophages with the cyclic GMP-dependent kinase inhibitor KT5823 (4 microM) (65% +/- 4% [S] and 72% +/- 6% [R]). The manner in which these PKC-, PKA-, and Ca(2+)-dependent, NO-mediated cyclic GMP-dependent kinase signal transduction pathways may relate to function of the candidate Lsh/Ity/Bcg gene Nramp is discussed.

p56lck interacts via its src homology 2 domain with the ZAP-70 kinase

p56lck, a member of the src family of protein tyrosine kinases, is an essential component in T cell receptor (TCR) signal transduction. p56lck contains a src homology 2 (SH2) domain found in a number of proteins involved in intracellular signaling. SH2 domains have been implicated in protein-protein interactions by binding to sequences in target proteins containing phosphorylated tyrosine. Using an in vitro assay, we have studied specific binding of tyrosine-phosphorylated proteins to a recombinant p56lck SH2 domain. In nonactivated Jurkat cells, two tyrosine-phosphorylated proteins were detected. Stimulation with anti-CD3 monoclonal antibodies induced the binding of seven additional tyrosine-phosphorylated proteins to the SH2 domain of p56lck. We have identified the zeta-associated tyrosine kinase, ZAP-70, as one of these proteins. Evidence suggests that binding of ZAP-70 to p56lck SH2 is direct and not mediated by zeta. The significance of this interaction was further investigated in vivo. p56lck could be coprecipitated with the zeta/ZAP-70 complex and conversely, ZAP-70 was detected in p56lck immunoprecipitates of activated Jurkat cells. The physical association of p56lck and ZAP-70 during activation supports the recently proposed functional cooperation of these two tyrosine kinases in TCR signaling.

Induction of nitric oxide release by MRC OX-44 (anti-CD53) through a protein kinase C-dependent pathway in rat macrophages

Many membrane proteins are implicated in the control of cell function by triggering specific signaling pathways. There is a new family of membrane proteins, defined by its structural motifs, which includes several lymphoid antigens, but lacks a function. To study its biological role, we determined which signaling pathways are affected by the CD53 antigen, a prototypic member of this family, in rat macrophages. Activation of CD53 by cross-linking results in an increase in inositol phosphates and diacylglycerol and in Ca2+ mobilization, which are insensitive to pertussis or cholera toxins. There is a translocation of protein kinase C to the membrane accompanied by nitric oxide (NO) release in macrophages. This effect is the result of the expression of the inducible nitric oxide synthase (iNOS), which is dependent on protein kinase C and protein synthesis. These results have linked a new receptor with a specific pathway of NO induction and thus have opened up a novel aspect of NO regulation in cell biology.

The Ras signal transduction pathway

Considerable progress has been made over the past year in elucidating the mechanisms by which extracellular signals are transduced via cell surface receptors to trigger changes in gene expression which determine the growth and differentiated state of a cell. In particular, Ras proteins have been implicated as key intermediates that mediate the signal from upstream tyrosine kinases to a downstream cascade of serine/threonine kinases, which then activate nuclear factors that control gene expression and protein synthesis. How Ras proteins function is regulated in this role as a molecular switch, and how the signal is transmitted between the various components of the pathway, are now being determined. Finally, the Rho family of Ras-related proteins, which regulate the actin cytoskeleton, have also been implicated as mediators of oncogenic Ras transformation. The brisk pace at which the key components of Ras-mediated signal transduction pathways are being identified hold great promise that new targets for therapeutic intervention in cancer may now be identified.

SH2/SH3 signaling proteins

SH2 and SH3 domains are small protein modules that mediate protein-protein interactions in signal transduction pathways that are activated by protein tyrosine kinases. SH2 domains bind to short phosphotyrosine-containing sequences in growth factor receptors and other phosphoproteins. SH3 domains bind to target proteins through sequences containing proline and hydrophobic amino acids. SH2 and SH3 domain containing proteins, such as Grb2 and phospholipase C gamma, utilize these modules in order to link receptor and cytoplasmic protein tyrosine kinases to the Ras signaling pathway and to phosphatidylinositol hydrolysis, respectively. The three-dimensional structures of several SH2 and SH3 domains have been determined by NMR and X-ray crystallography, and the molecular basis of their specificity is beginning to be unveiled.

The insulin receptor: structure, function, and signaling

The insulin receptor is a member of the ligand-activated receptor and tyrosine kinase family of transmembrane signaling proteins that collectively are fundamentally important regulators of cell differentiation, growth, and metabolism. The insulin receptor has a number of unique physiological and biochemical properties that distinguish it from other members of this large well-studied receptor family. The main physiological role of the insulin receptor appears to be metabolic regulation, whereas all other receptor tyrosine kinases are engaged in regulating cell growth and/or differentiation. Receptor tyrosine kinases are allosterically regulated by their cognate ligands and function as dimers. In all cases but the insulin receptor (and 2 closely related receptors), these dimers are noncovalent, but insulin receptors are covalently maintained as functional dimers by disulfide bonds. The initial response to the ligand is receptor autophosphorylation for all receptor tyrosine kinases. In most cases, this results in receptor association of effector molecules that have unique recognition domains for phosphotyrosine residues and whose binding to these results in a biological response. For the insulin receptor, this does not occur; rather, it phosphorylates a large substrate protein that, in turn, engages effector molecules. Possible reasons for these differences are discussed in this review. The chemistry of insulin is very well characterized because of possible therapeutic interventions in diabetes using insulin derivatives. This has allowed the synthesis of many insulin derivatives, and we review our recent exploitation of one such derivative to understand the biochemistry of the interaction of this ligand with the receptor and to dissect the complicated steps of ligand-induced insulin receptor autophosphorylation. We note possible future directions in the study of the insulin receptor and its intracellular signaling pathway(s).

Phospholipases and protein kinases during phagocyte activation

Phospholipases and protein kinases are critical for the intracellular transmission and amplification of signals induced by extracellular ligands. Chemotactic activation of phagocytes through G protein coupled receptors leads to inflammatory responses of the immune cells. Downstream of G proteins, phospholipases generate precursors for eicosanoid synthesis and are involved in the functional responses. Recently, the molecular characterization of specific enzymes of the signalling cascades has gained much attention in research.

A new function for a phosphotyrosine phosphatase: linking GRB2-Sos to a receptor tyrosine kinase

Autophosphorylated growth factor receptors provide binding sites for the src homology 2 domains of intracellular signaling molecules. In response to epidermal growth factor (EGF), the activated EGF receptor binds to a complex containing the signaling protein GRB2 and the Ras guanine nucleotide-releasing factor Sos, leading to activation of the Ras signaling pathway. We have investigated whether the platelet-derived growth factor (PDGF) receptor binds GRB2-Sos. In contrast with the EGF receptor, the GRB2 does not bind to the PDGF receptor directly. Instead, PDGF stimulation induces the formation of a complex containing GRB2; 70-, 80-, and 110-kDa tyrosine-phosphorylated proteins; and the PDGF receptor. Moreover, GRB2 binds directly to the 70-kDa protein but not to the PDGF receptor. Using a panel of PDGF beta-receptor mutants with altered tyrosine phosphorylation sites, we identified Tyr-1009 in the PDGF receptor as required for GRB2 binding. Binding is inhibited by a phosphopeptide containing a YXNX motif. The protein tyrosine phosphatase Syp/PTP1D/SHPTP2/PTP2C is approximately 70 kDa, binds to the PDGF receptor via Tyr-1009, and contains several YXNX sequences. We found that the 70-kDa protein that binds to the PDGF receptor and to GRB2 comigrates with Syp and is recognized by anti-Syp antibodies. Furthermore, both GRB2 and Sos coimmunoprecipitate with Syp from lysates of PDGF-stimulated cells, and GRB2 binds directly to tyrosine-phosphorylated Syp in vitro. These results indicate that GRB2 interacts with different growth factor receptors by different mechanisms and the cytoplasmic phosphotyrosine phosphatase Syp acts as an adapter between the PDGF receptor and the GRB2-Sos complex.

Binding of the Src SH2 domain to phosphopeptides is determined by residues in both the SH2 domain and the phosphopeptides

Src homology 2 (SH2) domains are found in a variety of signaling proteins and bind phosphotyrosine-containing peptide sequences. To explore the binding properties of the SH2 domain of the Src protein kinase, we used immobilized phosphopeptides to bind purified glutathione S-transferase-Src SH2 fusion proteins. With this assay, as well as a free-peptide competition assay, we have estimated the affinities of the Src SH2 domain for various phosphopeptides relative to a Src SH2-phosphopeptide interaction whose Kd has been determined previously (YEEI-P; Kd = 4 nM). Two Src-derived phosphopeptides, one containing the regulatory C-terminal Tyr-527 and another containing the autophosphorylation site Tyr-416, bind the Src SH2 domain in a specific though low-affinity manner (with about 10(4)-lower affinity than the YEEI-P peptide). A platelet-derived growth factor receptor (PDGF-R) phosphopeptide containing Tyr-857 does not bind appreciably to the Src SH2 domain, suggesting it is not the PDGF-R binding site for Src as previously reported. However, another PDGF-R-derived phosphopeptide containing Tyr-751 does bind the Src SH2 domain (with an affinity approximately 2 orders of magnitude lower than that of YEEI-P). All of the phosphopeptides which bind to the Src SH2 domain contain a glutamic acid at position -3 or -4 with respect to phosphotyrosine; changing this residue to alanine greatly diminishes binding. We have also tested Src SH2 mutants for their binding properties and have interpreted our results in light of the recent crystal structure solution for the Src SH2 domain. Mutations in various conserved and nonconserved residues (R155A, R155K, N198E, H201R, and H201L) cause slight reductions in binding, while two mutations cause severe reductions. The W148E mutant domain, which alters the invariant tryptophan that marks the N-terminal border of the SH2 domain, binds poorly to phosphopeptides. Inclusion of the SH3 domain in the fusion protein partially restores the binding by the W148E mutant. A change in the invariant arginine that coordinates twice with phosphotyrosine in the peptide (R175L) results in a nearly complete loss of binding. The R175L mutant does display high affinity for the PDGF-R peptide containing Tyr-751, via an interaction that is at least partly phosphotyrosine independent. We have used this interaction to show that the R175L mutation also disrupts the intramolecular interaction between the Src SH2 domain and the phosphorylated C terminus within the context of the entire Src protein; thus, the binding properties observed for mutant domains in an in vitro assay appear to mimic those that occur in vivo.

A new function for a phosphotyrosine phosphatase: linking GRB2-Sos to a receptor tyrosine kinase

Autophosphorylated growth factor receptors provide binding sites for the src homology 2 domains of intracellular signaling molecules. In response to epidermal growth factor (EGF), the activated EGF receptor binds to a complex containing the signaling protein GRB2 and the Ras guanine nucleotide-releasing factor Sos, leading to activation of the Ras signaling pathway. We have investigated whether the platelet-derived growth factor (PDGF) receptor binds GRB2-Sos. In contrast with the EGF receptor, the GRB2 does not bind to the PDGF receptor directly. Instead, PDGF stimulation induces the formation of a complex containing GRB2; 70-, 80-, and 110-kDa tyrosine-phosphorylated proteins; and the PDGF receptor. Moreover, GRB2 binds directly to the 70-kDa protein but not to the PDGF receptor. Using a panel of PDGF beta-receptor mutants with altered tyrosine phosphorylation sites, we identified Tyr-1009 in the PDGF receptor as required for GRB2 binding. Binding is inhibited by a phosphopeptide containing a YXNX motif. The protein tyrosine phosphatase Syp/PTP1D/SHPTP2/PTP2C is approximately 70 kDa, binds to the PDGF receptor via Tyr-1009, and contains several YXNX sequences. We found that the 70-kDa protein that binds to the PDGF receptor and to GRB2 comigrates with Syp and is recognized by anti-Syp antibodies. Furthermore, both GRB2 and Sos coimmunoprecipitate with Syp from lysates of PDGF-stimulated cells, and GRB2 binds directly to tyrosine-phosphorylated Syp in vitro. These results indicate that GRB2 interacts with different growth factor receptors by different mechanisms and the cytoplasmic phosphotyrosine phosphatase Syp acts as an adapter between the PDGF receptor and the GRB2-Sos complex.