Identification and characterization of novel substrates for protein tyrosine kinases

Src: coordinating metabolism in cancer

Metabolism must be tightly regulated to fulfil the dynamic requirements of cancer cells during proliferation, migration, stemness and differentiation. Src is a node of several signals involved in many of these biological processes, and it is also an important regulator of cell metabolism. Glucose uptake, glycolysis, the pentose-phosphate pathway and oxidative phosphorylation are among the metabolic pathways that can be regulated by Src. Therefore, this oncoprotein is in an excellent position to coordinate and finely tune cell metabolism to fuel the different cancer cell activities. Here, we provide an up-to-date summary of recent progress made in determining the role of Src in glucose metabolism as well as the link of this role with cancer cell metabolic plasticity and tumour progression. We also discuss the opportunities and challenges facing this field.

SRChing for the substrates of Src

By the mid 1980's, it was clear that the transforming activity of oncogenic Src was linked to the activity of its tyrosine kinase domain and attention turned to identifying substrates, the putative next level of control in the pathway to transformation. Among the first to recognize the potential of phosphotyrosine-specific antibodies, Parsons and colleagues launched a risky shotgun-based approach that led ultimately to the cDNA cloning and functional characterization of many of today's best-known Src substrates (for example, p85-Cortactin, p110-AFAP1, p130Cas, p125FAK and p120-catenin). Two decades and over 6000 citations later, the original goals of the project may be seen as secondary to the enormous impact of these protein substrates in many areas of biology. At the request of the editors, this review is not restricted to the current status of the substrates, but reflects also on the anatomy of the project itself and some of the challenges and decisions encountered along the way.

Protein tyrosine kinases: structure, substrate specificity, and drug discovery

Protein tyrosine kinases (PTKs) play a crucial role in many cell regulatory processes. It is therefore not surprising to see that functional perturbation of PTKs results in many diseases. Despite the diverse primary structure organization of various PTKs, the catalytic or kinase domains of various PTKs as well as that of Ser/Thr kinases are generally conserved. The high resolution crystal structure of a few PTKs has been solved in the last few years. In contrast to the well-defined linear peptide substrate motifs recognized by specific Ser/Thr kinases, the identification of specific substrate motifs for PTK has been slow. It is not until recently that through the use of combinatorial peptide library methods that specific recognition motifs for specific PTKs have begun to emerge. Efficient and specific peptide substrates for some PTKs with Km at the mid microM range have been identified. Based on these peptide substrates, relatively potent (IC50 at the low microM range) and highly selective pseudosubstrate-based peptide inhibitors have been developed. There has been enormous effort in the development of PTK inhibitors for diseases such as cancer, psoriasis, and osteoporosis. Several new high-throughput PTK assay technologies have recently been described. Small molecules against specific PTK have been developed. Most of them are competitive inhibitors at the ATP binding site. Some of these inhibitors have already been in clinical trial.

The SH3 and SH2 domains are capable of directing specificity in protein interactions between the non-receptor tyrosine kinases cSrc and cYes

The c-src and c-yes proto-oncogenes encode 60 000 and 62 000 Dalton non-receptor tyrosine kinases of the Src family, pp60c-src and pp62c-yes, respectively. These kinases are over 80% homologous outside of their unique amino termini, yet several studies suggest that differences exist in the regulation, activation, and function of cSrc and cYes. The determinants of specificity in signaling between these proteins, however, remain unclear. In order to investigate the roles of the Src Homology (SH) 3 and 2 domains in mediating signaling specificity between cSrc and cYes, chimeras were created in which the SH3 and/or SH2 domains of cSrc or the fully activated variant Src527F were replaced by the corresponding domains of cYes. These constructs were used to assess the effects of the Yes SH3 and SH2 domains on the ability of Src to form stable complexes with and induce tyrosine phosphorylation of Src SH3 and SH2 domain binding partners in vivo. Both the Yes SH3 and SH2 domains were found to alter the capacity of Src to form stable associations with heterologous proteins. The Yes SH3 domain was unable to affinity absorb the Src SH3/SH2 binding partner AFAP-110 from COS-1 cell lysates, and chimeric constructs of Src527F containing the cYes SH3 domain were unable to efficiently co-immunoprecipitate with AFAP-110 from chicken embryo fibroblasts. Interactions with the Src SH2 domain binding partner pp130cas were unaffected. Additionally, only chimeras containing the cYes SH2 domain were able to co-immunoprecipitate with an unidentified 87 kDa tyrosine-phosphorylated protein. These results indicate that the SH3 and SH2 domains are capable of directing specificity in substrate binding between Src and Yes, suggesting potential mechanisms for generating specificity in signaling between these two highly related non-receptor tyrosine kinases.

Complex formation with focal adhesion kinase: A mechanism to regulate activity and subcellular localization of Src kinases

Tyrosine phosphorylation of focal adhesion kinase (FAK) creates a high-affinity binding site for the src homology 2 domain of the Src family of tyrosine kinases. Assembly of a complex between FAK and Src kinases may serve to regulate the subcellular localization and the enzymatic activity of members of the Src family of kinases. We show that simultaneous overexpression of FAK and pp60(c-src) or p59(fyn) results in the enhancement of the tyrosine phosphorylation of a limited number of cellular substrates, including paxillin. Under these conditions, tyrosine phosphorylation of paxillin is largely cell adhesion dependent. FAK mutants defective for Src binding or focal adhesion targeting fail to cooperate with pp60(c-src) or p59(fyn) to induce paxillin phosphorylation, whereas catalytically defective FAK mutants can direct paxillin phosphorylation. The negative regulatory site of pp60(c-src) is hypophosphorylated when in complex with FAK, and coexpression with FAK leads to a redistribution of pp60(c-src) from a diffuse cellular location to focal adhesions. A FAK mutant defective for Src binding does not effectively induce the translocation of pp60(c-src) to focal adhesions. These results suggest that association with FAK can alter the localization of Src kinases and that FAK functions to direct phosphorylation of cellular substrates by recruitment of Src kinases.

PSTPIP 2, a second tyrosine phosphorylated, cytoskeletal-associated protein that binds a PEST-type protein-tyrosine phosphatase

Although cytoskeletal regulation is critical to cell function during interphase and mitosis, the components of the cytoskeleton involved with its control are only beginning to be elucidated. Recently, we reported the identification of a cytoskeletal-associated protein, proline-serine-threonine phosphatase-interacting protein (PSTPIP), whose level of tyrosine phosphorylation was controlled by PEST-type protein-tyrosine phosphatases (PTPs) bound to a novel protein interaction site in the PSTPIP predicted coiled-coil domain. We also showed that the PSTPIP SH3 domain interacts with the Wiskott-Aldrich syndrome protein (WASP), a cytoskeletal regulatory protein, in a manner modulated by tyrosine phosphorylation. Here we describe the identification of PSTPIP 2, a widely expressed protein that is related to PSTPIP. PSTPIP 2 lacks an SH3 domain but contains a region predicted to bind to PEST-type PTPs, and structure-function analyses demonstrate that PSTPIP 2 interacts with the proline-rich C terminus of the PEST-type PTP hematopoietic stem cell factor in a manner similar to that previously demonstrated for PSTPIP. Confocal microscopy revealed that PSTPIP 2 colocalizes with PSTPIP in F actin-rich regions. PSTPIP 2 was found to be efficiently phosphorylated in v-Src-transfected or pervanadate-treated cells at two tyrosines conserved in PSTPIP, but in contrast to PSTPIP, tyrosine phosphorylated PSTPIP 2 was only weakly dephosphorylated in the presence of PTP HSCF. Finally, analysis of oligomer formation demonstrated that PSTPIP and PSTPIP 2 formed homo- but not heterodimers. These data suggest that a family of tyrosine phosphorylated, PEST PTP binding proteins may be implicated in cytoskeletal regulation.

Growth factor-dependent phosphorylation of the actin-binding protein cortactin is mediated by the cytoplasmic tyrosine kinase FER

Previous characterization of the nonreceptor tyrosine kinase FER identified a tight physical association with the catenin pp120 and led to the suggestion that FER may be involved in cell-cell signaling. To further understand the function of FER, we have continued our analyses of the interaction of FER with pp120 and other proteins. The majority of FER is localized to the cytoplasmic fraction where it forms a complex with the actin-binding protein cortactin. The Src homology 2 sequence of FER is required for directly binding cortactin, and phosphorylation of the FER-cortactin complex is up-regulated in cells treated with peptide growth factors. Using a dominant-negative mutant of FER, we provided evidence that FER kinase activity is required for the growth factor-dependent phosphorylation of cortactin. These data suggest that cortactin is likely to be a direct substrate of FER. Our observations provide additional support for a role of FER in mediating signaling from the cell surface, via growth factor receptors, to the cytoskeleton. The nature of the FER-cortactin interaction, and their putative enzyme-substrate relationship, support the previous proposal that one of the functions of the Src homology 2 sequences of nonreceptor tyrosine kinases is to provide a binding site for their preferred substrates.

SRC64 regulates the localization of a Tec-family kinase required for Drosophila ring canal growth

Mutation of the Src64 gene of Drosophila results in ovarian ring canal defects and reduced female fertility. We used a dosage-sensitive modifier screen to search for downstream components of the SRC64 signaling pathway. We show that mutations affecting Tec29, an essential gene encoding a member of the Tec family of protein tyrosine kinases, dominantly enhance the Src64 ring canal phenotype. Loss of Tec29 function in the female germline results in a phenotype strikingly similar to that caused by the loss of Src64 function. In each case, the ring canals are reduced in size and phosphotyrosine content. We further demonstrate that TEC29 localizes to the ring canal, and this subcellular localization requires Src64 function. These data suggest that TEC29 is a downstream target of SRC64, and that regulating TEC29 localization during ring canal growth may be a crucial SRC64 function.

Cellular functions regulated by Src family kinases

Src family protein tyrosine kinases are activated following engagement of many different classes of cellular receptors and participate in signaling pathways that control a diverse spectrum of receptor-induced biological activities. While several of these kinases have evolved to play distinct roles in specific receptor pathways, there is considerable redundancy in the functions of these kinases, both with respect to the receptor pathways that activate these kinases and the downstream effectors that mediate their biological activities. This chapter reviews the evidence implicating Src family kinases in specific receptor pathways and describes the mechanisms leading to their activation, the targets that interact with these kinases, and the biological events that they regulate.

The integrity of the SH3 binding motif of AFAP-110 is required to facilitate tyrosine phosphorylation by, and stable complex formation with, Src

The actin filament-associated protein AFAP-110 forms a stable complex with activated variants of Src in chick embryo fibroblast cells. Stable complex formation requires the integrity of the Src SH2 and SH3 domains. In addition, AFAP-110 encodes two adjacent SH3 binding motifs and six candidate SH2 binding motifs. These data indicate that both SH2 and SH3 domains may work cooperatively to facilitate Src/AFAP-110 stable complex formation. As a test for this hypothesis, we sought to understand whether one or both SH3 binding motifs in AFAP-110 modulate interactions with the Src SH3 domain and if this interaction was required to present AFAP-110 for tyrosine phosphorylation by, and stable complex formation with, Src. A proline to alanine site-directed mutation in the amino terminal SH3 binding motif (SH3bm I) was sufficient to abrogate absorption of AFAP-110 with GST-SH3STC. Co-expression of activated Src (pp60(527F)) with AFAP-110 in Cos-1 cells permit tyrosine phosphorylation of AFAP-110 and stable complex formation with pp60(527F). However, co-expression of the SH3 null-binding mutant (AFAP71A) with pp60(527F) revealed a 2.7 fold decrease in steady-state levels of tyrosine phosphorylation, compared to AFAP-110. Although a lower but detectable level of AFAP71A was phosphorylated on tyrosine, AFAP71A could not be detected in stable complex with pp60(527F), unlike AFAP-110. These data indicate that SH3 interactions facilitate presentation of AFAP-110 for tyrosine phosphorylation and are also required for stable complex formation with pp60(527F).

PSTPIP: a tyrosine phosphorylated cleavage furrow-associated protein that is a substrate for a PEST tyrosine phosphatase

We have investigated proteins which interact with the PEST-type protein tyrosine phosphatase, PTP hematopoietic stem cell fraction (HSCF), using the yeast two-hybrid system. This resulted in the identification of proline, serine, threonine phosphatase interacting protein (PSTPIP), a novel member of the actin- associated protein family that is homologous to Schizosaccharomyces pombe CDC15p, a phosphorylated protein involved with the assembly of the actin ring in the cytokinetic cleavage furrow. The binding of PTP HSCF to PSTPIP was induced by a novel interaction between the putative coiled-coil region of PSTPIP and the COOH-terminal, proline-rich region of the phosphatase. PSTPIP is tyrosine phosphorylated both endogenously and in v-Src transfected COS cells, and cotransfection of dominant-negative PTP HSCF results in hyperphosphorylation of PSTPIP. This dominant-negative effect is dependent upon the inclusion of the COOH-terminal, proline-rich PSTPIP-binding region of the phosphatase. Confocal microscopy analysis of endogenous PSTPIP revealed colocalization with the cortical actin cytoskeleton, lamellipodia, and actin-rich cytokinetic cleavage furrow. Overexpression of PSTPIP in 3T3 cells resulted in the formation of extended filopodia, consistent with a role for this protein in actin reorganization. Finally, overexpression of mammalian PSTPIP in exponentially growing S. pombe results in a dominant-negative inhibition of cytokinesis. PSTPIP is therefore a novel actin-associated protein, potentially involved with cytokinesis, whose tyrosine phosphorylation is regulated by PTP HSCF.

Csk phosphorylation and inactivation in vitro by the cAMP-dependent protein kinase

Csk is a protein tyrosine kinase that phosphorylates other protein tyrosine kinases of the Src family and down-regulates their activities. It is not known how Csk is regulated. We investigated the possibility that Csk is regulated through phosphorylation by examining if Csk can serve as an in vitro substrate for a panel of protein kinases. We found that Csk was phosphorylated by the cAMP-dependent protein kinase (PKA), but not by protein kinase C, Src, or the fibroblast growth factor receptor kinase. Csk phosphorylation in vitro by PKA is on a serine residue(s) and can reach a stoichiometry of approximately 0.6 mol phosphate per mole of enzyme. Furthermore, incubation with PKA in the presence of ATP and magnesium ion results in a time-dependent decrease in Csk kinase activity. A six-fold decrease in Csk activity (expressed as Vmax/Km ratio) was achieved due to a threefold increase in its Km and a twofold decrease in its Vmax value within 1 h of incubation with the catalytic subunit of PKA and ATP-Mg. Both phosphorylation and inactivation by PKA were blocked by a PKA-specific inhibitor. Csk mutants with a deleted SH2 or SH3 domain retained their ability to be phosphorylated and inactivated by PKA, indicating that the phosphorylation site is located within the catalytic domain. These studies suggest that the cAMP-dependent protein kinase can regulate Csk activity.

Neurite outgrowth is enhanced by anti-idiotypic monoclonal antibodies to the ganglioside GM1

Exogenously added gangliosides enhance sprouting, neurite outgrowth, and other neuronal activities; this effect may be initiated when a ganglioside binds to a membrane protein or when a ganglioside intercalates into the plasma membrane. To test whether binding to membrane proteins is sufficient for ganglioside-mediated activity, anti-idiotypic antibodies were generated that mimic the functional binding sites of the ganglioside GM1 as described by M. J. Riggott and W. D. Matthew (1996, Glycobiology, 6, 581-589). These anti-idiotypic antibodies are proteinaceous probes that model the biochemical and biological effects of gangliosides. Those anti-idiotypic ganglioside (AIG) monoclonal antibodies (mAb's) were selected based on their ability to bind a known GM1 binding protein, the beta-subunit of cholera toxin. These studies described neuronal cell surface proteins that were identified by immunocytochemistry and Western blotting using these AIG mAb's. Here we show that AIG mAb's mimic the functional properties of GM1 in that they facilitate neurite outgrowth from central and peripheral nervous system neurons in in vitro bioassays. In addition, AIG mAb binding modulates second messenger activity, suggesting that membrane protein binding alone is sufficient to invoke intracellular activation. The similarity in the pattern of protein tyrosine phosphorylation evoked by GM1 and the anti-idiotypic ganglioside antibodies suggests that the AIG mAb's modulate neurite outgrowth in a manner similar to that of GM1. Because antibodies cannot intercalate into the plasma membrane, these results suggest that the ganglioside GM1 can mediate neuronal cellular activity by binding to cell surface proteins.

The human p167 gene encodes a unique structural protein that contains centrosomin A homology and associates with a multicomponent complex

The characterization of novel cytoplasmic, structural, and enzymatic proteins has been enhanced by a panel of monoclonal antibodies specific for protein substrates of transforming and nontransforming c-Src mutants. These protein substrates have included the focal adhesion kinase (FAK), cortactin, AFAP-110, p120CAS, and p130CAS. The monoclonal antibody 4G8 was generated as part of this panel of antibodies and was used to isolate the human gene for a 167-kD polypeptide. The cDNA sequence is 5,238 nucleotides in length with a predicted open reading frame consisting of 1,382 amino acids. The polypeptide is largely hydrophilic and highly charged. The central region of p167 has 88% identity with the entire 278-amino-acid encoded sequence of the murine centrosomin A gene. The carboxyl third of p167 contains a unique cluster of 10 amino acid repeats with the consensus sequence (A/M)DDDRGPRRG. The p167 protein was found primarily in the cytoplasm of lymphocytes and is part of a multicomponent protein complex with prominent members of 167, 120, 64, 45, 40, 38, and 25 kD. Finally, we illustrate the conservation of p167 and its associated complex, and demonstrate its expression in different human tissues and cell types. The data suggest that p167 is novel and has an important cellular function as a cytoplasmic structural protein.

Processing of fluorescently labelled insulin and insulin-like growth factor-I by the rat visceral yolk sac

Insulin and the structurally related insulin-like growth factor I (IGF-I) are mitogenic peptides which have been implicated in the embryonic development of the rat. In addition to factors produced by the developing embryo itself, it is likely that maternally-derived growth factors play an important role also, with their postulated initial site of action being the extraembryonic membranes, which surround the embryo throughout gestation. We have examined the processing of these potential regulatory factors by the visceral yolk sac on the 17th day of gestation, using fluorescently-labelled ligands and fluorescence microscopy. Both insulin and IGF-I are rapidly internalized at the yolk sac surface, and appear in the tissue within discrete vesicular structures. Interestingly, in some cases when both labelled proteins are added simultaneously they do not appear to coexist within vesicles. Instead, insulin appears to remain within vesicles close to the apical surface of the yolk sac whereas IGF-I appears to penetrate the tissue more deeply, being readily transported to the internal face of the epithelium. It appears, therefore, that there is some difference in the sorting mechanisms of these related proteins, although the physiological significance of this observation is not clear.

Regulation, substrates and functions of src

Src is the best understood member of a family of 9 tyrosine kinases that regulates cellular responses to extracellular stimuli. Activated mutants of Src are oncogenic. Using Src as an example, and referring to other Src family members where appropriate, this review describes the structure of Src, the functions of the individual domains, the regulation of Src kinase activity in the cell, the selection of substrates, and the biological functions of Src. The review concentrates on developments in the last 6-7 years, and cites data resulting from the isolation and characterization of Src mutants, crystallographic studies of the structures of SH2, SH3 and tyrosine kinase domains, biochemical studies of Src kinase activity and binding properties, and the biology of transgenic and knockout mouse strains.

Different metastatic potentials of ras- and src-transformed BALB/c 3T3 A31 variant cells

The metastatic phenotype of tumor cells is thought to be induced by an aberrant signaling cascade or cascades that are different from those required for tumorigenicity. Oncogene-transfected cells with different tumorigenicities and metastatic potentials have been used to identify such pathways and responsible molecules. However, oncogenes that can induce tumorigenicity in recipient cells also frequently induce the metastatic phenotype at the same time. The difficulty in obtaining cell lines that are tumorigenic but not metastatic has hampered such studies. In this report, we transfected the activated c-Ha-ras oncogene into BALB/c 3T3 A31 variant cells and found that the transfectants were tumorigenic but they did not form metastatic lung modules in the experimental metastasis assay. The phenotype was very stable and was maintained during cultivation. On the other hand, the metastatic potentials of either the transfected cells or the original variant cells could be induced by transfection of the v-src oncogene. The src transfectants formed extensive nodules in lung when injected into the tail veins of congeneric mice. The cell motility of the metastatic src transfectants on Matrigel-coated dishes was greater than that of the ras transfectants. The src transfectants were also invasive in Matrigel when analyzed on a filter. These variant cells transformed by the ras and src oncogenes will be a useful system for identifying the signaling cascades responsible for the metastatic potential of tumors.

Integrin function: molecular hierarchies of cytoskeletal and signaling molecules

Integrin receptors play important roles in organizing the actin-containing cytoskeleton and in signal transduction from the extracellular matrix. The initial steps in integrin function can be analyzed experimentally using beads coated with ligands or anti-integrin antibodies to trigger rapid focal transmembrane responses. A hierarchy of transmembrane actions was identified in this study. Simple integrin aggregation triggered localized transmembrane accumulation of 20 signal transduction molecules, including RhoA, Rac1, Ras, Raf, MEK, ERK, and JNK. In contrast, out of eight cytoskeletal molecules tested, only tensin coaccumulated. Integrin aggregation alone was also sufficient to induce rapid activation of the JNK pathway, with kinetics of activation different from those of ERK. The tyrosine kinase inhibitors herbimycin A or genistein blocked both the accumulation of 19 out of 20 signal transduction molecules and JNK- and ERK-mediated signaling. Cytochalasin D had identical effects, whereas three other tyrosine kinase inhibitors did not. The sole exception among signaling molecules was the kinase pp125FAK which continued to coaggregate with alpha 5 beta 1 integrins even in the presence of these inhibitors. Tyrosine kinase inhibition also failed to block the ability of ligand occupancy plus integrin aggregation to trigger transmembrane accumulation of the three cytoskeletal molecules talin, alpha-actinin, and vinculin; these molecules accumulated even in the presence of cytochalasin D. However, it was necessary to fulfill all four conditions, i.e., integrin aggregation, integrin occupancy, tyrosine kinase activity, and actin cytoskeletal integrity, to achieve integrin-mediated focal accumulation of other cytoskeletal molecules including F-actin and paxillin. Integrins therefore mediate a transmembrane hierarchy of molecular responses.

The cytoplasmic tyrosine kinase FER is associated with the catenin-like substrate pp120 and is activated by growth factors

The FER gene encodes a cytoplasmic tyrosine kinase with a single SH2 domain and an extensive amino terminus. In order to understand the cellular function of the FER kinase, we analyzed the effect of growth factor stimulation on the phosphorylation and activity of FER. Stimulation of A431 cells and 3T3 fibroblasts with epidermal growth factor or platelet-derived growth factor results in the phosphorylation of FER and two associated polypeptides. The associated polypeptides were shown to be the epidermal growth factor receptor or the platelet-derived growth factor receptor and a previously identified target, pp120. Since pp120 had previously been shown to interact with components of the cadherin-catenin complex, these results implicate FER in the regulation of cell-cell interactions. The physical association of FER with pp120 was found to be constitutive and was mediated by a 400-amino-acid sequence in the amino terminus of FER. Analyses of that sequence revealed that it has the ability to form coiled coils and that it oligomerizes in vitro. The identification of a coiled coil sequence in the FER kinase and the demonstration that the sequence mediates association with a potential substrate suggest a novel mechanism for signal transduction by cytoplasmic tyrosine kinases.

Specific and redundant roles of Src and Fyn in organizing the cytoskeleton

Mouse embryos lacking Csk, a negative regulator of Src family kinases, exhibit defects in neurulation and die at mid-gestation. To determine the role of activated Src family kinases in the csk- phenotype, we have introduced mutations in the src and fyn genes into the csk- mutant background. Genetic analysis reveals that src, but not fyn, is partly epistatic to the csk gene. Biochemical analysis indicates that several cytoskeletal proteins are hyperphosphorylated on tyrosine residues in csk- cells. Regulation of cortactin and tensin hyperphosphorylation is Src-dependent, whereas focal adhesion kinase and paxillin hyperphosphorylation is partly dependent on both Src and Fyn. Furthermore, the src- mutation can restore the normal distribution of cortactin and partly correct filamentous actin organization in csk-cells. Thus, Src family kinases have both specific and overlapping functions in regulation of the cytoskeleton. The disturbance of these functions may be a molecular basis for the phenotype exhibited by csk- mutants.

Identification and characterization of a novel synthetic peptide substrate specific for Src-family protein tyrosine kinases

Using a random combinatorial peptide library method [Wu, J., Ma, Q. N. & Lam, K. S. (1994) Biochemistry 33, 14825-14833] a novel peptide, YIYGSFK, was identified as a substrate for p60c-src protein tyrosine kinase. Mass spectrometric analysis showed that tyrosine-3 from the N-terminus was the phosphorylation site. Kinetic studies showed that the Km of YIYGSFK for p60c-src was 55 microM, about 6.4-fold lower than a peptide derived from p34cdc2 [cdc2(6-20), KVEKIGEGTYGVVYK], which had been reported to be a specific and efficient substrate for the Src-family protein tyrosine kinases. Comparison of the specificity of YIYGSFK and cdc2(6-20) as a substrate for various Src-family and non-Src-family protein tyrosine kinases suggests that YIYGSFK is a much more specific and efficient substrate for the Src-family protein tyrosine kinases.

Role of tyrosine kinase and protein kinase C in the steroidogenic actions of angiotensin II, alpha-melanocyte-stimulating hormone and corticotropin in the rat adrenal cortex

The role of protein kinases in the steroidogenic actions of alpha-melanocyte-stimulating hormone (alpha-MSH), angiotensin II (AngII) and corticotropin (ACTH) in the rat adrenal zona glomerulosa was examined. Ro31-8220, a potent selective inhibitor of protein kinase C (PKC), inhibited both AngII- and alpha-MSH-stimulated aldosterone secretion but had no effect on aldosterone secretion in response to ACTH. The effect of Ro31-8220 on PKC activity was measured in subcellular fractions. Basal PKC activity was higher in cytosol than in membrane or nuclear fractions. Incubation of the zona glomerulosa with either alpha-MSH or AngII resulted in significant increases in PKC activity in the nuclear and cytosolic fractions and decreases in the membrane fraction. These effects were all inhibited by Ro31-8220. ACTH caused a significant increase in nuclear PKC activity only, and this was inhibited by Ro31-8220 without any significant effect on the steroidogenic response to ACTH, suggesting that PKC translocation in response to ACTH may be involved in another aspect of adrenal cellular function. Tyrosine phosphorylation has not previously been considered to be an important component of the response of adrenocortical cells to peptide hormones. Both AngII and alpha-MSH were found to activate tyrosine kinase, but ACTH had no effect, observations that have not been previously reported. Tyrphostin 23, a specific antagonist of tyrosine kinases, inhibited aldosterone secretion in response to AngII and alpha-MSH, but not ACTH. These data confirm the importance of PKC in the adrenocortical response to AngII and alpha-MSH, and, furthermore, indicate that tyrosine kinase may play a critical role in the steroidogenic actions of AngII and alpha-MSH in the rat adrenal zona glomerulosa.

Beta-catenin: a common target for the regulation of cell adhesion by Wnt-1 and Src signaling pathways

Beta-catenin is a cytosolic protein originally identified through its association with the cadherin class of cell-adhesion proteins. However, recent studies have demonstrated that there are cadherin-independent pools of beta-catenin and that beta-catenin binds at least one other protein, the product of the tumor-suppressor gene APC. Furthermore, beta-catenin is the target of two signal transduction pathways mediated by the proto-oncogenes src and wnt-1. This raises the possibility that beta-catenin plays a pivotal role in balancing cellular responses to both adhesive and proliferative signals.

Identification and sequence analysis of cDNAs encoding a 110-kilodalton actin filament-associated pp60src substrate

Transformation of chicken embryo cells by oncogenic forms of pp60src (e.g., pp60v-src or pp60527F) is linked with a concomitant increase in the steady-state levels of tyrosine-phosphorylated cellular proteins. Activated forms of the Src protein-tyrosine kinase stably associate with tyrosine-phosphorylated proteins, including a protein of 110 kDa, pp110. Previous reports have established that stable complex formation between pp110 and pp60src requires the structural integrity of the Src SH2 and SH3 domains, whereas tyrosine phosphorylation of pp110 requires only the structural integrity of the SH3 domain. In normal chicken embryo cells, pp110 colocalizes with actin stress filaments, and in Src-transformed cells, pp110 is found associated with podosomes (rosettes). Here, we report the identification and characterization of cDNAs encoding pp110. The predicted open reading frame encodes a polypeptide of 635 amino acids which exhibits little sequence similarity with other protein sequences present in the available sequence data bases. Thus, pp110 is a distinctive cytoskeleton-associated protein. On the basis of its association with actin stress filaments, we propose the term AFAP-110, for actin filament-associated protein of 110 kDa. In vitro analysis of AFAP-110 binding to bacterium-encoded glutathione S-transferase (GST) fusion proteins revealed that AFAP-110 present in normal cell extracts binds efficiently to Src SH3/SH2-containing fusion proteins, less efficiently to Src SH3-containing proteins, and poorly to SH2-containing fusion proteins. In contrast, AFAP-110 in Src-transformed cell extracts bound to GST-SH3/SH2 and GST-SH2 fusion proteins. Analysis of AFAP-110 cDNA sequences revealed the presence of sequence motifs predicted to bind to SH2 and SH3 domains, respectively. We suggest that AFAP-110 may represent a cellular protein capable of interacting with SH3-containing proteins and, upon tyrosine phosphorylation, binds tightly to SH2-containing proteins, such as pp60src or pp59fyn. The potential roles of AFAP-110 as an SH3/SH2 cytoskeletal binding protein are discussed.

Identification and sequence analysis of cDNAs encoding a 110-kilodalton actin filament-associated pp60src substrate

Transformation of chicken embryo cells by oncogenic forms of pp60src (e.g., pp60v-src or pp60527F) is linked with a concomitant increase in the steady-state levels of tyrosine-phosphorylated cellular proteins. Activated forms of the Src protein-tyrosine kinase stably associate with tyrosine-phosphorylated proteins, including a protein of 110 kDa, pp110. Previous reports have established that stable complex formation between pp110 and pp60src requires the structural integrity of the Src SH2 and SH3 domains, whereas tyrosine phosphorylation of pp110 requires only the structural integrity of the SH3 domain. In normal chicken embryo cells, pp110 colocalizes with actin stress filaments, and in Src-transformed cells, pp110 is found associated with podosomes (rosettes). Here, we report the identification and characterization of cDNAs encoding pp110. The predicted open reading frame encodes a polypeptide of 635 amino acids which exhibits little sequence similarity with other protein sequences present in the available sequence data bases. Thus, pp110 is a distinctive cytoskeleton-associated protein. On the basis of its association with actin stress filaments, we propose the term AFAP-110, for actin filament-associated protein of 110 kDa. In vitro analysis of AFAP-110 binding to bacterium-encoded glutathione S-transferase (GST) fusion proteins revealed that AFAP-110 present in normal cell extracts binds efficiently to Src SH3/SH2-containing fusion proteins, less efficiently to Src SH3-containing proteins, and poorly to SH2-containing fusion proteins. In contrast, AFAP-110 in Src-transformed cell extracts bound to GST-SH3/SH2 and GST-SH2 fusion proteins. Analysis of AFAP-110 cDNA sequences revealed the presence of sequence motifs predicted to bind to SH2 and SH3 domains, respectively. We suggest that AFAP-110 may represent a cellular protein capable of interacting with SH3-containing proteins and, upon tyrosine phosphorylation, binds tightly to SH2-containing proteins, such as pp60src or pp59fyn. The potential roles of AFAP-110 as an SH3/SH2 cytoskeletal binding protein are discussed.