A novel ligand for SH3 domains. The Nck adaptor protein binds to a serine/threonine kinase via an SH3 domain

The P21-Activated Kinase 1 and 2 As Potential Therapeutic Targets for the Management of Cardiovascular Disease

Group I p21-activated kinases (Paks) are members of the serine/threonine protein kinase family. Paks are encoded by three genes (Pak 1 - 3) and are involved in the regulation of various biological processes. Pak1 and Pak2 are key members, sharing 91% sequence identity in their kinase domains. Recent studies have shown that Pak1/2 protect the heart from various types of stresses. Activated Pak1/2 participate in the maintenance of cellular homeostasis and metabolism, thus enhancing the adaptation and resilience of cardiomyocytes to stress. The structure, activation and function of Pak1/2 as well as their protective roles against the occurrence of cardiovascular disease are described in this review. The values of Pak1/2 as therapeutic targets are also discussed.

Nck adapter proteins: functional versatility in T cells

Nck is a ubiquitously expressed adapter protein that is almost exclusively built of one SH2 domain and three SH3 domains. The two isoproteins of Nck are functionally redundant in many aspects and differ in only few amino acids that are mostly located in the linker regions between the interaction modules. Nck proteins connect receptor and non-receptor tyrosine kinases to the machinery of actin reorganisation. Thereby, Nck regulates activation-dependent processes during cell polarisation and migration and plays a crucial role in the signal transduction of a variety of receptors including for instance PDGF-, HGF-, VEGF- and Ephrin receptors. In most cases, the SH2 domain mediates binding to the phosphorylated receptor or associated phosphoproteins, while SH3 domain interactions lead to the formation of larger protein complexes. In T lymphocytes, Nck plays a pivotal role in the T cell receptor (TCR)-induced reorganisation of the actin cytoskeleton and the formation of the immunological synapse. However, in this context, two different mechanisms and adapter complexes are discussed. In the first scenario, dependent on an activation-induced conformational change in the CD3epsilon subunits, a direct binding of Nck to components of the TCR/CD3 complex was shown. In the second scenario, Nck is recruited to the TCR complex via phosphorylated Slp76, another central constituent of the membrane proximal activation complex. Over the past years, a large number of putative Nck interactors have been identified in different cellular systems that point to diverse additional functions of the adapter protein, e.g. in the control of gene expression and proliferation.

Regulation of SPIN90 Phosphorylation and Interaction with Nck by ERK and Cell Adhesion

SPIN90 is a widely expressed Nck-binding protein that contains one Src homology 3 (SH3) domain, three Pro-rich motifs, and a serine/threonine-rich region, and is known to participate in sarcomere assembly during cardiac myocyte differentiation. We used in vitro binding assays and yeast two-hybrid screening analysis to identify Nck, betaPIX, Wiscott-Aldrich syndrome protein (WASP), and ERK1 as SPIN90-binding proteins. It appears that betaPIX, WASP, and SPIN90 form a complex that interacts with Nck in a manner dependent upon cell adhesion to extracellular matrix. The betaPIX.WASP.SPIN90.Nck interaction was abolished in suspended and cytochalasin D-treated cells, but was recovered when cells were replated on fibronectin-coated dishes. The SPIN90.betaPIX.WASP complex was stable, even in suspended cells, suggesting SPIN90 serves as an adaptor molecule to recruit other proteins to Nck at focal adhesions. In addition, we found that overexpression of the SPIN90 SH3 domain or Pro-rich region, respectively, abolished SPIN90.Nck and SPIN90.betaPIX interactions, resulting in detachment of cells from extracellular matrix. SPIN90 was phosphorylated by ERK1, which was, itself, activated by cell adhesion and platelet-derived growth factor. Such phosphorylation of SPIN90 likely promotes the interaction of the SPIN90.betaPIX.WASP complex and Nck. It thus appears that the interaction of the betaPIX.WASP.SPIN90 complex with Nck is crucial for stable cell adhesion and can be dynamically modulated by SPIN90 phosphorylation that is dependent on cell adhesion and ERK activation.

Stimulus-specific requirements for MAP3 kinases in activating the JNK pathway

Mitogen-activated protein kinases (MAPKs) are activated by numerous ligands typically through a protein kinase cascade minimally composed of the MAPK in series with a MAP2 kinase (MAP2K) and a MAP3K. This arrangement is thought to confer specificity and appropriate kinetic properties on the activation of MAPKs in response to physiological stimuli. Surprisingly, more than a dozen MAP3Ks have been identified that activate the c-Jun N-terminal kinases (JNKs) when overexpressed, but there is no clear understanding of which kinases actually mediate JNK activation by ligands. Here, we use double-stranded RNA-mediated interference of gene expression to reveal the explicit participation of discrete MAP3Ks in controlling JNK activity by multiple stimuli. Maximal activation of JNK by lipopolysaccharide requires the MAP3K TAK1. On the other hand, sorbitol requires expression of four MAP3Ks to cause maximal JNK activation. Thus, we demonstrate that specific stimuli use different mechanisms to recruit distinct MAP3Ks to regulate the JNK pathway.

The Nck family of adapter proteins: regulators of actin cytoskeleton

SH2/SH3 domain-containing adapter proteins, such as the Nck family, play a major role in regulating tyrosine kinase signalling. They serve to recruit proline-rich effector molecules to tyrosine-phosphorylated kinases or their substrates. Initially, it was not clear why cells from nematodes to vertebrates contain redundant and closely related SH2/SH3 adapters, such as Grb2, Crk and Nck. Recent evidence suggests that their biological roles are clearly different, whereas, for example, Grb2 connects activated receptor tyrosine kinases to Sos and Ras, leading to cell proliferation. The proteins of Nck family are implicated in organisation of actin cytoskeleton, cell movement or axon guidance in flies. In this review, the author attempts to summarise signalling pathways in which Nck plays a critical role.

Actin-based motility of intracellular microbial pathogens

A diverse group of intracellular microorganisms, including Listeria monocytogenes, Shigella spp., Rickettsia spp., and vaccinia virus, utilize actin-based motility to move within and spread between mammalian host cells. These organisms have in common a pathogenic life cycle that involves a stage within the cytoplasm of mammalian host cells. Within the cytoplasm of host cells, these organisms activate components of the cellular actin assembly machinery to induce the formation of actin tails on the microbial surface. The assembly of these actin tails provides force that propels the organisms through the cell cytoplasm to the cell periphery or into adjacent cells. Each of these organisms utilizes preexisting mammalian pathways of actin rearrangement to induce its own actin-based motility. Particularly remarkable is that while all of these microbes use the same or overlapping pathways, each intercepts the pathway at a different step. In addition, the microbial molecules involved are each distinctly different from the others. Taken together, these observations suggest that each of these microbes separately and convergently evolved a mechanism to utilize the cellular actin assembly machinery. The current understanding of the molecular mechanisms of microbial actin-based motility is the subject of this review.

The Nck-interacting kinase (NIK) phosphorylates the Na+-H+ exchanger NHE1 and regulates NHE1 activation by platelet-derived growth factor

NIK, a recently identified Nck-interacting kinase, acts upstream of the MEK kinase MEKK1 to activate the c-Jun N-terminal kinase JNK. We now show that NIK binds to and divergently activates the plasma membrane Na(+)-H(+) exchanger NHE1. In a genetic screen, NHE1 interacted with NIK at a site N-terminal (amino acids 407-502) to the Nck-binding domain, and this site is critical for its association with NHE1 in vivo. NIK also phosphorylates NHE1; however, the phosphorylation sites, which are distal to amino acid 638, are distinct from the NIK-binding site on NHE1 (amino acids 538-638). Expression of wild-type, but not a kinase-inactive, NIK in fibroblasts increased NHE1 phosphorylation and activity. The kinase domain of NIK, however, was not sufficient for this response in vivo. Full phosphorylation and activation of NHE1 required both the kinase and the NHE1-binding domains of NIK, suggesting that the NHE1-binding site functions as a targeting signal. The functional significance of an interaction between NIK and NHE1 was confirmed by the ability of a kinase-inactive NIK to selectively inhibit activation of NHE1 by platelet-derived growth factor but not by thrombin. Moreover, although NIK activates JNK through a mechanism dependent on MEKK1, it phosphorylated and activated NHE1 independently of MEKK1. These findings indicate that NIK acts downstream of platelet-derived growth factor receptors to phosphorylate and activate NHE1 divergently of its activation of JNK.

Mitogen-activated protein (MAP) kinase pathways: regulation and physiological functions

Mitogen-activated protein (MAP) kinases comprise a family of ubiquitous proline-directed, protein-serine/threonine kinases, which participate in signal transduction pathways that control intracellular events including acute responses to hormones and major developmental changes in organisms. MAP kinases lie in protein kinase cascades. This review discusses the regulation and functions of mammalian MAP kinases. Nonenzymatic mechanisms that impact MAP kinase functions and findings from gene disruption studies are highlighted. Particular emphasis is on ERK1/2.

Interaction of paxillin with p21-activated Kinase (PAK). Association of paxillin alpha with the kinase-inactive and the Cdc42-activated forms of PAK3

p21-activated kinases (PAKs) are implicated in integrin signalings, and have been proposed to associate with paxillin indirectly. We show here that paxillin can bind directly to PAK3. We examined several representative focal adhesion proteins, and found that paxillin is the sole protein that associates with PAK3. PAK3 associated with the alpha and beta isoforms of paxillin, but not with gamma. We also show that paxillin alpha associated with both the kinase-inactive and the Cdc42-activated forms of PAK3 in vivo, without affecting the activation states of the kinase. A number of different functions have been ascribed to PAKs; and PAKs can bind directly to growth factor signaling-adaptor molecule, Nck, and a guanine nucleotide exchanger, betaPIX. Our results revealed that paxillin alpha can compete with Nck and betaPIX in the binding of PAK3. Moreover, paxillin alpha can be phosphorylated by PAK3 at serine. Therefore, paxillin alpha, but not gamma, appears to be capable of linking both the kinase-inactive and activated forms of PAK3 to integrins independent of Nck and betaPIX, as Nck links PAK1 to growth factor receptors. Our results also revealed that paxillin is involved in highly complexed protein-protein interactions in integrin signaling.

Membrane-targeting of signalling molecules by SH2/SH3 domain-containing adaptor proteins

SH2/SH3 domain-containing adaptor proteins play a critical role in regulating tyrosine kinase signalling pathways. The major function of these adaptors, such as Grb2, Nck, and Crk, is to recruit proline-rich effector molecules to tyrosine-phosphorylated kinases or their substrates. In recent years dozens of novel proteins have emerged that are capable of associating with the SH2 and the SH3 domains of adaptors. In this review, the author attempts to summarise these novel binding partners of Grb2, Nck, and Crk, and to discuss current controversies regarding function and regulation of protein multicomplexes held together by SH2/SH3 adaptor molecules at the plasma membrane.

Association of Nck with tyrosine-phosphorylated SLP-76 in activated T lymphocytes

The Nck adaptor protein links tyrosine kinases or their substrates to proteins containing proline-rich motifs. Here we show that in activated T cells two tyrosine phosphoproteins of 75 and 120 kDa are co-immunoprecipitated with polyclonal antibodies against Nck. Analysis of Nck immunoprecipitates with various candidate antibodies revealed that the 75-kDa tyrosine phosphoprotein is the SH2 domain-containing leukocyte protein referred to as SLP-76. In vitro experiments show that the interaction between Nck and SLP-76 is mediated via the Nck SH2 domain. Using specific phosphopeptides corresponding to the major tyrosine phosphorylation sites of SLP-76, it was found that Y113 and Y128 phosphopeptides could compete binding of SLP-76 to the SH2 domain of Nck. In addition, the 120-kDa tyrosine phosphoprotein was recognized by an antibody raised against Cbl, a proto-oncogene product that has been previously found to be associated with Nck. These results suggest that the Nck adaptor protein interacts with key signaling molecules and may play an important role in activation of T lymphocytes.

Requirement of multiple SH3 domains of Nck for ligand binding

The Nck adaptor protein comprises a single C-terminal SH2 domain and three SH3 domains. The domain structure of Nck suggests that Nck links tyrosine kinase substrates to proteins containing proline-rich motifs. Here we show that Bcr/Abl tyrosine kinase, and three tyrosine phosphorylated proteins (115, 120 and 155 kDa) are co-immunoprecipitated with antibody against Nck from lysates of the human leukaemia cell line K562. By means of affinity purification with the Nck-binding phosphopeptide EPGPY(P)AQPSV, we could also detect the association of endogenous Nck with the proto-oncogene product Cbl. An investigation of the nature of interactions revealed that Bcr/Abl, Cbl, and the 155-kDa tyrosine phosphotyrosine bind exclusively to the SH3 domains of Nck. In addition, none of the single SH3 domains of Nck expressed as glutathione-S-transferase (GST) fusion proteins is able to interact with the proline-rich ligands. However, combined first and second SH3 domains have the capacity to bind Bcr/Abl, Chl and p155. Mutations of conserved tryptophan to Lysine in either of the combined first and second SH3 domains completely abolish ligand binding. These data suggest that cooperation exists among the SH3 domains of Nck for a high-affinity binding of proteins containing proline-rich motifs.

Identification of Grb4/Nckbeta, a src homology 2 and 3 domain-containing adapter protein having similar binding and biological properties to Nck

Adapter proteins made up of Src homology (SH) domains mediate multiple cellular signaling events initiated by receptor protein tyrosine kinases. Here we report that Grb4 is an adapter protein closely related to but distinct from Nck that is made up of three SH3 domains and one SH2 domain. Northern analysis indicated that both genes are expressed in multiple tissues. Both Nck and Grb4 proteins could associate with receptor tyrosine kinases and the SH3-binding proteins PAK, Sos1, and PRK2, and they synergized with v-Abl and Sos to induce gene expression via the transcription factor Elk-1. Although neither protein was transforming on its own, both Nck and Grb4 cooperated with v-Abl to transform NIH 3T3 cells and influenced the morphology and anchorage-dependent growth of wild type Ras-transformed cells. Nck and Grb4 therefore appear to be functionally redundant.

Nck-2, a novel Src homology2/3-containing adaptor protein that interacts with the LIM-only protein PINCH and components of growth factor receptor kinase-signaling pathways

Many of the protein-protein interactions that are essential for eukaryotic intracellular signal transduction are mediated by protein binding modules including SH2, SH3, and LIM domains. Nck is a SH3- and SH2-containing adaptor protein implicated in coordinating various signaling pathways, including those of growth factor receptors and cell adhesion receptors. We report here the identification, cloning, and characterization of a widely expressed, Nck-related adaptor protein termed Nck-2. Nck-2 comprises primarily three N-terminal SH3 domains and one C-terminal SH2 domain. We show that Nck-2 interacts with PINCH, a LIM-only protein implicated in integrin-linked kinase signaling. The PINCH-Nck-2 interaction is mediated by the fourth LIM domain of PINCH and the third SH3 domain of Nck-2. Furthermore, we show that Nck-2 is capable of recognizing several key components of growth factor receptor kinase-signaling pathways including EGF receptors, PDGF receptor-beta, and IRS-1. The association of Nck-2 with EGF receptors was regulated by EGF stimulation and involved largely the SH2 domain of Nck-2, although the SH3 domains of Nck-2 also contributed to the complex formation. The association of Nck-2 with PDGF receptor-beta was dependent on PDGF activation and was mediated solely by the SH2 domain of Nck-2. Additionally, we have detected a stable association between Nck-2 and IRS-1 that was mediated primarily via the second and third SH3 domain of Nck-2. Thus, Nck-2 associates with PINCH and components of different growth factor receptor-signaling pathways via distinct mechanisms. Finally, we provide evidence indicating that a fraction of the Nck-2 and/or Nck-1 proteins are associated with the cytoskeleton. These results identify a novel Nck-related SH2- and SH3-domain-containing protein and suggest that it may function as an adaptor protein connecting the growth factor receptor-signaling pathways with the integrin-signaling pathways.

Regulation of PAK activation and the T cell cytoskeleton by the linker protein SLP-76

Tyrosine phosphorylation of linker proteins enables the T cell antigen receptor (TCR)-associated protein tyrosine kinases to phosphorylate and regulate effector molecules that generate second messengers. We demonstrate here that the SLP-76 linker protein interacts with both nck, an adaptor protein, and Vav, a guanine nucleotide exchange factor for Rho-family GTPases. The assembly of this tri-molecular complex permits the activated Rho-family GTPases to regulate target effectors that interact through nck. In turn, assembly of this complex mediates the enzymatic activation of the p21-activated protein kinase 1 and facilitates actin polymerization. Hence, phosphorylation of linker proteins not only bridges the TCR-associated PTK, ZAP-70, with downstream effector proteins, but also provides a scaffold to integrate distinct signaling complexes to regulate T cell function.

Identification of Nck family genes, chromosomal localization, expression, and signaling specificity

Already a dozen molecules share binding to the Src homology (SH) 3 domains of human Nck, an SH3-SH3-SH3-SH2 adapter protein. We reason that there may be multiple gene members of Nck to accommodate the large binding repertoires. Here we report identification of novel human and mouse Nck genes and rename them as the Nckalpha and Nckbeta genes (including the human Nckalpha, human Nckbeta, mouse Nckalpha, and mouse Nckbeta genes). Nckalpha and Nckbeta share 68% amino acid identity, whereas the two Nckalpha and two Nckbeta across the species show 96% identity to each other. The human Nckbeta gene is mapped to 2q12, whereas the human Nckalpha gene has previously been mapped at 3q21. Antibodies specifically against Nckalpha and Nckbeta detect Nckalpha and Nckbeta with an identical molecular mass in the same cells of various origins. Ectopically expressed Nckbeta, but not its SH2 domain mutant, strongly inhibits epidermal growth factor- and platelet-derived growth factor-stimulated DNA synthesis. Consistently, epidermal growth factor receptor and platelet-derived growth factor receptor preferentially interact with Nckbeta over Nckalpha in vitro. This study indicates that Nck is a multiple gene family and that each gene may have its own signaling specificity. Because previous anti-Nck (human Nckalpha) antibodies cross-react with Nckbeta, reassessment of those studies with specific Nck genes would be necessary.

The Wiskott-Aldrich syndrome protein-interacting protein (WIP) binds to the adaptor protein Nck

Nck is a ubiquitous adaptor molecule composed of three Src homology 3 (SH3) domains followed by a single SH2 domain. Nck links, via its SH2 domain, tyrosine-phosphorylated receptors to effector proteins that contain SH3-binding proline-rich sequences. In this report, we demonstrate that recombinant Nck precipitates endogenous WIP, a novel proline-rich protein that interacts with the Wiskott-Aldrich syndrome protein (WASP), from BJAB cell lysates. Nck binds through its second SH3 domain to WIP, and Nck binds to WIP at a site (amino acids 321-415) that differs from the WASP-binding site (amino acids 416-488). WIP has been shown to associate with the actin polymerization regulatory protein profilin and to induce actin polymerization and cytoskeletal reorganization in lymphoid cells. We demonstrate the presence of profilin in Nck precipitates suggesting that Nck may couple extracellular signals to the cytoskeleton via its interaction with WIP and profilin.

Signaling by Eph receptors and their ephrin ligands

What are the mechanisms by which Eph receptor tyrosine kinases guide axons and migrating cells? Recent results show that Eph receptors bind some of the well-known effector molecules that are implicated in the organization of the actin cytoskeleton; moreover, biochemical and genetic evidence suggests a signaling role for transmembrane ephrin ligands in axon pathfinding.

Wiskott-Aldrich syndrome protein is associated with the adapter protein Grb2 and the epidermal growth factor receptor in living cells

Src homology domains [i.e., Src homology domain 2 (SH2) and Src homology domain 3 (SH3)] play a critical role in linking receptor tyrosine kinases to downstream signaling networks. A well-defined function of the SH3-SH2-SH3 adapter Grb2 is to link receptor tyrosine kinases, such as the epidermal growth factor receptor (EGFR), to the p21ras-signaling pathway. Grb2 has also been implicated to play a role in growth factor-regulated actin assembly and receptor endocytosis, although the underlying mechanisms remain unclear. In this study, we show that Grb2 interacts through its SH3 domains with the human Wiskott-Aldrich syndrome protein (WASp), which plays a role in regulation of the actin cytoskeleton. We find that WASp is expressed in a variety of cell types and is exclusively cytoplasmic. Although the N-terminal SH3 domain of Grb2 binds significantly stronger than the C-terminal SH3 domain to WASp, full-length Grb2 shows the strongest binding. Both phosphorylation of WASp and its interaction with Grb2, as well as with another adapter protein Nck, remain constitutive in serum-starved or epidermal growth factor-stimulated cells. WASp coimmunoprecipitates with the activated EGFR after epidermal growth factor stimulation. Purified glutathione S-transferase-full-length-Grb2 fusion protein, but not the individual domains of Grb2, enhances the association of WASp with the EGFR, suggesting that Grb2 mediates the association of WASp with EGFR. This study suggests that Grb2 translocates WASp from the cytoplasm to the plasma membrane and the Grb2-WASp complex may play a role in linking receptor tyrosine kinases to the actin cytoskeleton.

NIK is a new Ste20-related kinase that binds NCK and MEKK1 and activates the SAPK/JNK cascade via a conserved regulatory domain

Nck, an adaptor protein composed of one SH2 and three SH3 domains, is a common target for a variety of cell surface receptors. We have identified a novel mammalian serine/threonine kinase that interacts with the SH3 domains of Nck, termed Nck Interacting Kinase (NIK). This kinase is most homologous to the Sterile 20 (Ste20) family of protein kinases. Of the members of this family, GCK and MSST1 are most similar to NIK in that they bind neither Cdc42 nor Rac and contain an N-terminal kinase domain with a putative C-terminal regulatory domain. Transient overexpression of NIK specifically activates the stress-activated protein kinase (SAPK) pathway. Both the kinase domain and C-terminal regulatory region of NIK are required for full activation of SAPK. NIK likely functions upstream of MEKK1 to activate this pathway; a dominant-negative MEK kinase 1 (MEKK1) blocks activation of SAPK by NIK. MEKK1 and NIK also associate in cells and this interaction is mediated by regulatory domains on both proteins. Two other members of this kinase family, GCK and HPK1, contain C-terminal regulatory domains with homology to that of NIK. These findings indicate that the C-terminal domain of these proteins encodes a new protein domain family and suggests that this domain couples these kinases to the SAPK pathway, possibly by interacting with MEKK1 or related kinases.

Interaction of Nck-associated protein 1 with activated GTP-binding protein Rac

Bacterially expressed glutathione S-transferase fusion proteins containing Rac1 were used to identify binding proteins of this Rho family GTPase present in a bovine brain extract. Five proteins of 85, 110, 125, 140 and 170 kDa were detected, all of which were associated exclusively with guanosine 5'-[gamma-thio]triphosphate-bound Rac1, not with GDP-bound Rac1. The 85 and 110 kDa proteins were identified as the regulatory and catalytic subunits respectively of phosphatidylinositol 3-kinase. Several lines of evidence suggested that the 125 kDa protein is identical with Nck-associated protein 1 (Nap1). The mobilities of the 125 kDa protein and Nap1 on SDS/PAGE were indistinguishable, and the 125 kDa protein was depleted from brain extract by preincubation with the Src homology 3 domain of Nck to which Nap1 binds. Furthermore, antibodies to Nap1 reacted with the 125 kDa protein. Nap1 was co-immunoprecipitated with a constitutively active form of Rac expressed in Chinese hamster ovary cells. The observation that complex formation between activated Rac and PAK, but not that between Rac and Nap1, could be reproduced in vitro with recombinant proteins indicates that the interaction of Nap1 with Rac is indirect. The 140 kDa Rac-binding protein is a potential candidate for a link that connects Nap1 to Rac. The multimolecular complex comprising Rac, Nap1 and probably the 140 kDa protein might mediate some of the biological effects transmitted by the multipotent GTPase.

Activation of Pak by membrane localization mediated by an SH3 domain from the adaptor protein Nck

BACKGROUND

The adaptor protein Nck consists of three Src homology 3 (SH3) domains followed by one SH2 domain. Like the Grb2 adaptor protein, which is known to couple receptor tyrosine kinases to the small GTPase Ras, Nck is presumed to bind to tyrosine-phosphorylated proteins using its SH2 domain and to downstream effector proteins using its SH3 domain. Little is known, however, about the specific biological function of Nck. The Pak family of serine/threonine kinases are known to be activated by binding to the GTP-bound form of Cdc42 or Rac1, which are small GTPases of the Rho family that are involved in regulating the organization of the actin cytoskeleton.

RESULTS

We present evidence that Nck can mediate the relocalization and subsequent activation of the Pak1 kinases. We show that Nck associates in vivo with Pak using the second of its three SH3 domains, and that localization of this individual Nck SH3 domain, or of Pak kinase itself, to the membrane results in activation of Pak and stimulation of downstream mitogen activated protein kinase cascades. Activation of downstream signaling by the membrane-localized Nck SH3 domain is blocked by a kinase-inactive mutant form of Pak1.

CONCLUSION

These results demonstrate that localization of Pak1 to the membrane in the absence of other signals is sufficient for its activation, and imply that the Nck adaptor protein could function to link changes in tyrosine phosphorylation of cellular proteins to the Cdc42/Pak signaling pathway.

A casein kinase I activity is constitutively associated with Nck

Nck is a 47-kDa cytosolic protein devoid of intrinsic catalytic activity and consisting of Src homology 2 and 3 (SH2 and SH3) domains organized as follows: SH3-SH3-SH3-SH2. Nck is believed to act as an adaptor protein mediating signal transduction initiated by receptor tyrosine kinases (RTKs). Through its SH2 domain, Nck recognizes a specific phosphotyrosine residue on RTKs or on protein substrates of RTKs like insulin receptor substrate-1, the major substrate of the insulin receptor, and through its SH3 domains it interacts with poorly characterized effector molecules. To identify novel proteins that might interact with Nck, we have used the amino-terminal segment of Nck encompassing its three SH3 domains in the yeast two-hybrid system. Among the polypeptides that associate with Nck, we have identified the gamma2 isoform of the serine/threonine casein kinase I (CKI-gamma2). In transformed rat hepatocytes overexpressing the insulin receptor (HTC-IR cells), serine/threonine protein kinase activity coimmunoprecipitates with Nck, an interaction mediated mainly by the third SH3 domain of Nck. This kinase activity is not apparently modulated by insulin, nor is it sensitive to staurosporine or heparin, and it does not use GTP as a phosphate donor. However the kinase activity coimmunoprecipitated with Nck is completely abolished by N-(2-aminoethyl)-5-chloroisoquinoline-8-sulfonamide, a specific inhibitor of casein kinase I. In an in vitro renaturation gel kinase assay, a protein kinase of 70-75 kDa was detected associated with the SH3 domains of Nck. Far Western analysis demonstrated that the SH3 domains of Nck bound directly to a cytosolic protein of 70-75 kDa. A rabbit polyclonal antibody raised against the C-terminal region of CKI-gamma2 protein kinase immunoprecipitated a single specific protein of 70-75 kDa from HTC-IR cell lysates and detected CKI-gamma2 among the proteins coimmunoprecipitated with Nck. These results support an in vivo interaction between Nck and CKI-gamma2 and suggest that CKI-gamma2 could be involved in signaling pathways downstream of RTKs.

The SH3 domain of the tight junction protein ZO-1 binds to a serine protein kinase that phosphorylates a region C-terminal to this domain

ZO-1 is a tight junction phosphoprotein partially homologous to a tumor suppressor in Drosophila. The homologous region contains an SH3 domain with an unidentified function. Using fusion proteins containing the SH3 domain and various N- and C-terminal sequences, we tested for association of a kinase with this protein domain in extracts of MDCK cells. We show that the SH3 domain of ZO-1 binds a serine protein kinase that phosphorylates a region immediately C-terminal to the SH3 domain. This kinase associates specifically with the SH3 domain of ZO-1 and appears to be also associated with junctional complexes extracted from MDCK cells.

Isolation of a NCK-associated kinase, PRK2, an SH3-binding protein and potential effector of Rho protein signaling

The NCK adapter protein is comprised of three consecutive Src homology 3 (SH3) protein-protein interaction domains and a C-terminal SH2 domain. Although the association of NCK with activated receptor protein-tyrosine kinases, via its SH2 domain, implicates NCK as a mediator of growth factor-induced signal transduction, little is known about the pathway(s) downstream of NCK recruitment. To identify potential downstream effectors of NCK we screened a bacterial expression library to isolate proteins that bind its SH3 domains. Two molecules were isolated, the Wiskott-Aldrich syndrome protein (WASP, a putative CDC42 effector) and a serine/threonine protein kinase (PRK2, closely related to the putative Rho effector PKN). Using interspecific backcross analysis the Prk2 gene was mapped to mouse chromosome 3. Unlike WASP, which bound the SH3 domains of several signaling proteins, PRK2 specifically bound to the middle SH3 domain of NCK and (weakly) that of phospholipase Cgamma. PRK2 also specifically bound to Rho in a GTP-dependent manner and cooperated with Rho family proteins to induce transcriptional activation via the serum response factor. These data suggest that PRK2 may coordinately mediate signal transduction from activated receptor protein-tyrosine kinases and Rho and that NCK may function as an adapter to connect receptor-mediated events to Rho protein signaling.

Interaction of the Nck adapter protein with p21-activated kinase (PAK1)

The p21-activated kinases (PAKs) link G protein-coupled receptors and growth factor receptors (S. Dharmawardhane, R. H. Daniels, and G. M. Bokoch, submitted for publication) to activation of MAP kinase cascades and to cytoskeletal reorganization (M. A. Sells, U. G. Knaus, D. Ambrose, S. Bagrodia, G. M. Bokoch, and J. Chernoff, submitted for publication). The proteins that interact with PAK to mediate its cellular effects and to couple it to upstream receptors are unknown. We describe here a specific interaction of the Nck adapter molecule with PAK1 both in vitro and in vivo. PAK1 and Nck associate in COS-7 and Swiss 3T3 cells constitutively, but this interaction is strengthened upon platelet-derived growth factor receptor stimulation. We show that Nck binds to PAK1 through its second Src homology 3 (SH3) domain, while PAK1 interacts with Nck via the first proline-rich SH3 binding motif at its amino terminus. The interaction of active PAK1 with Nck leads to the phosphorylation of Nck at multiple sites. Association of Nck with PAK1 may serve to link this important regulatory kinase to cell activation by growth factor receptors.

The SH3 domain of Src tyrosyl protein kinase interacts with the N-terminal splice region of the PDE4A cAMP-specific phosphodiesterase RPDE-6 (RNPDE4A5)

The PDE4A (type IV) cAMP-specific, rolipram-inhibited phosphodiesterase RPDE-6 (RNPDE4A5), when transiently expressed in COS7 cells, could be complexed with the v-Src-SH3 domain expressed as a glutathione S-transferase (GST) fusion protein. RPDE-6 did not interact with GST itself. This complex was not disrupted by treatment with high NaCl concentration together with Triton X-100. Interaction was apparently determined by the N-terminal splice region of RPDE-6, as the PDE4A splice variant RPDE-39, which differs from RPDE-6 at the extreme N-terminus, failed to associate with v-Src-SH3; met26RD1 (where RD1 is rat 'dunc-like' PDE), which has the N-terminal splice region deleted, failed to associate with v-Src-SH3, and the association of RPDE-6 and v-Src-SH3 was blocked by a fusion protein formed from the N-terminal splice region. RDPE-6 showed binding to GST fusion proteins of both the intact Src kinase and an SH2-SH3 construct but did not bind to the Src-SH2 domain or to the adaptor protein Grb-2. RPDE-6 could be co-immunoprecipitated from cytosol extracts of transfected cells by using anti-Src antiserum. RPDE-6 exhibited selectivity in binding to the SH3 domains of c-Abl, Crk, Csk, Lck, Lyn, Fyn and v-Src, with binding to the SH3 regions of the Src-related tyrosyl kinases Lyn and Fyn being the most effective. The binding of RPDE-6 to the SH3 domains of Crk, Csk and Lck led to a marked reduction in PDE activity, but no change was apparent in complexes with other species. Endogenous RPDE-6 from brain, but not endogenous RPDE-39 from testis, bound to the Src-SH3 domain. We suggest that the PDE4A splice variant RPDE-6 has a propensity for interaction with selective SH3 domains, in particular those from Src and the Src-related tyrosyl kinases Lyn and Fyn. This interaction seems to be governed by alternative splicing of the PDE4A gene, because RPDE-39, a splice variant that lacks the proline-rich N-terminal splice region of RPDE-6, does not interact with these SH3 domains. It is proposed that the binding site on RPDE-6 for SH3 domains lies within the unique first 102 residues of its N-terminal splice domain, where two motifs representing Class I SH3 binding sites with selectivity for Src kinase SH3 domains can be identified and one motif for a putative Class II SH3 binding site.

Association between mitogen-activated protein kinase and the zeta chain of the T cell receptor (TcR) with the SH2,3 domain of p56lck. Differential regulation by TcR cross-linking

A number of protein-tyrosine kinases have been shown to be important in T cell activation. One such kinase, Lck, has been demonstrated genetically to be essential for T cell receptor (TcR) signaling, and the SH2 and SH3 (src homology 2 and 3) domains of Lck have been shown to be indispensable for T cell activation. We have sought substrates with which the SH2,3 domain would interact following T cell activation, using fusion proteins containing the Lck SH2 and SH3 domains linked to glutathione S-transferase. We demonstrate that the SH2,3 region interacts specifically and directly with numerous tyrosine-phosphorylated molecules following TcR cross-linking, including constitutively with mitogen-activated protein kinase (MAPK)/extracellular-regulated kinase and inducibly with the zeta chain of the TcR. The interaction with MAPK/extracellular-regulated kinase was via the SH3 domain. The interaction with the tyrosine-phosphorylated zeta chain, while phosphotyrosine-dependent, required both the SH3 and SH2 domains. These interactions were specific as molecules known to be tyrosine-phosphorylated following TcR cross-linking, phospholipase C-gamma1 and Fyn, were not bound. Thus, we suggest that during TcR signaling, Lck interacts with numerous molecules, including MAPK and TcR-zeta, via its SH2,3 domain. The interaction with MAPK would place Lck in a position to be involved in the complex resulting in the activation of MAPK. In addition, the binding of Lck to the tyrosine-phosphorylated zeta chain of the TcR would serve to strengthen the interaction of the associated CD4 and the TcR complex, leading to increased avidity for the antigen-major histocompatibility protein complex.

Wiskott-Aldrich syndrome protein physically associates with Nck through Src homology 3 domains

In the second of a series of experiments designed to identify p47nck-Src homology 3 (SH3)-binding molecules, we report the cloning of SAKAP II (Src A box Nck-associated protein II) from an HL60 cDNA expression library. This molecule has been identified as a cDNA encoding the protein product of WASP, which is mutated in Wiskott-Aldrich syndrome patients. Studies in vivo and in vitro demonstrated a highly specific interaction between the SH3 domains of p47nck and Wiskott-Aldrich syndrome protein. Furthermore, anti-Wiskott-Aldrich syndrome protein antibodies recognized a protein of 66 kDa by Western blot (immunoblot) analysis. In vitro translation studies identified the 66-kDa protein as the protein product of WASP, and subcellular fractionation experiments showed that p66WASP is mainly present in the cytosol fraction, although significant amounts are also present in membrane and nuclear fractions. The main p47nck region implicated in the association with p66WASP was found to be the carboxy-terminal SH3 domain.

Identification of a mouse p21Cdc42/Rac activated kinase

We have isolated a novel member of the mammalian PAK (p21 activated kinase) and yeast Ste20 serine/threonine kinase family from a mouse fibroblast cDNA library, designated mPAK-3. Expression of mPAK-3 in Saccharomyces cerevisiae partially restores mating function in ste20 null cells. Like other PAKs, mPAK-3 contains a putative Cdc42Hs/Rac binding sequence and when transiently expressed in COS cells, full-length mPAK-3 binds activated (GTP gamma S (guanosine 5'-3-O-(thio-triphosphate)-bound) glutathione S-transferase (GST)-Cdc42Hs and GST-Rac1 but not GST-RhoA. As expected for a putative target molecule, mPAK-3 does not bind to an effector domain mutant of Cdc42Hs. Furthermore, activated His-tagged Cdc42Hs and His-tagged Rac stimulate mPAK-3 autophosphorylation and phosphorylation of myelin basic protein by mPAK-3 in vitro. Interestingly, the amino-terminal region of mPAK-3 contains potential SH3-binding sites and we find that mPAK-3, expressed in vitro and in vivo, shows highly specific binding to the SH3 domain of phospholipase C-gamma and at least one SH3 domain in the adapter protein Nck. These results raise the possibility of an additional level of regulation of the PAK family in vivo.