The protein encoded by the transforming gene of avian sarcoma virus (pp60src) and a homologous protein in normal cells (pp60proto-src) are associated with the plasma membrane

Seeking a better understanding of the non-receptor tyrosine kinase, SRMS

SRMS (Src-Related kinase lacking C-terminal regulatory tyrosine and N-terminal Myristoylation Sites) is a non-receptor tyrosine kinase first reported in a 1994 screen for genes regulating murine neural precursor cells. SRMS, pronounced "Shrims", lacks the C-terminal regulatory tyrosine critical for the regulation of the enzymatic activity of Src-family kinases (SFKs). Another remarkable characteristic of SRMS is its localization into distinct SRMS cytoplasmic punctae (SCPs) or GREL (Goel Raghuveera-Erique Lukong) bodies, a pattern not observed in the SFKs. This unique subcellular localization of SRMS could dictate its cellular targets, proteome, and potentially, substrates. However, the function of SRMS is still relatively unknown. Further, how is its activity regulated and by what cellular targets? Studies have emerged highlighting the potential role of SRMS in autophagy and in regulating the activation of BRK/PTK6. Potential novel cellular substrates have also been identified, including DOK1, vimentin, Sam68, FBKP51, and OTUB1. Recent studies have also demonstrated the potential role of the kinase in various cancers, including gastric and colorectal cancers and platinum resistance in ovarian cancer. This review discusses the advancements made in SRMS-related biology to date and the path to understanding the cellular and physiological significance of the kinase.

Regulation of Src trafficking and activation by the endocytic regulatory proteins MICAL-L1 and EHD1

Localization of the non-receptor tyrosine kinase Src to the cell periphery is required for its activation and to mediate focal adhesion turnover, cell spreading and migration. Inactive Src localizes to a perinuclear compartment and the movement of Src to the plasma membrane is mediated by endocytic transport. However, the precise pathways and regulatory proteins that are responsible for SRC transport are incompletely understood. Here, we demonstrate that Src partially colocalizes with the endocytic regulatory protein MICAL-L1 (molecule interacting with CasL-like protein 1) in mammalian cells. Furthermore, MICAL-L1 is required for growth-factor- and integrin-induced Src activation and transport to the cell periphery in HeLa cells and human fibroblasts. Accordingly, MICAL-L1 depletion impairs focal adhesion turnover, cell spreading and cell migration. Interestingly, we find that the MICAL-L1 interaction partner EHD1 (EH domain-containing protein 1) is also required for Src activation and transport. Moreover, the MICAL-L1-mediated recruitment of EHD1 to Src-containing recycling endosomes is required for the release of Src from the perinuclear endocytic recycling compartment in response to growth factor stimulation. Our study sheds new light on the mechanism by which Src is transported to the plasma membrane and activated, and provides a new function for MICAL-L1 and EHD1 in the regulation of intracellular non-receptor tyrosine kinases.

Fatty acyl chain-dependent but charge-independent association of the SH4 domain of Lck with lipid membranes

The SH4 domain of Src family of nonreceptor protein tyrosine kinases represents the extreme N-terminal 1-16 amino acid region which mediates membrane association of these proteins and facilitates their functions. The SH4 domains among Src members lack well-defined sequence consensus and vary in the net charge. However, they readily anchor to the cytoplasmic face of the plasma membrane upon fatty acid acylation. Here, we report the membrane association of differentially acylated SH4 domain of Lck kinase, which has net negative charge at physiological pH. Our results suggest that despite the net negative charge, the SH4 domain of Lck associates with membranes upon fatty acid acylation. While myristoylation at the N-terminus is sufficient for providing membrane anchorage, multiple acylation determines orientation of the peptide chain with respect to the lipid bilayer. Hence, fatty acylation serves more than just a lipid anchor. It has an important role in regulating the spatial orientation of the peptide domain with respect to the lipid bilayer, which could be important for the interaction of the other domains of these kinases with their partners.

Recruitment of Nox4 to a plasma membrane scaffold is required for localized reactive oxygen species generation and sustained Src activation in response to insulin-like growth factor-I

Nox4-derived ROS is increased in response to hyperglycemia and is required for IGF-I-stimulated Src activation. This study was undertaken to determine the mechanism by which Nox4 mediates sustained Src activation. IGF-I stimulated sustained Src activation, which occurred primarily on the SHPS-1 scaffold protein. In vitro oxidation experiments indicated that Nox4-derived ROS was able to oxidize Src when they are in close proximity, and Src oxidation leads to its activation. Therefore we hypothesized that Nox4 recruitment to the plasma membrane scaffold SHPS-1 allowed localized ROS generation to mediate sustained Src oxidation and activation. To determine the mechanism of Nox4 recruitment, we analyzed the role of Grb2, a component of the SHPS-1 signaling complex. We determined that Nox4 Tyr-491 was phosphorylated after IGF-I stimulation and was responsible for Nox4 binding to the SH2 domain of Grb2. Overexpression of a Nox4 mutant, Y491F, prevented Nox4/Grb2 association. Importantly, it also prevented Nox4 recruitment to SHPS-1. The role of Grb2 was confirmed using a Pyk2 Y881F mutant, which blocked Grb2 recruitment to SHPS-1. Cells expressing this mutant had impaired Nox4 recruitment to SHPS-1. IGF-I-stimulated downstream signaling and biological actions were also significantly impaired in Nox4 Y491F-overexpressing cells. Disruption of Nox4 recruitment to SHPS-1 in aorta from diabetic mice inhibited IGF-I-stimulated Src oxidation and activation as well as cell proliferation. These findings provide insight into the mechanism by which localized Nox4-derived ROS regulates the sustained activity of a tyrosine kinase that is critical for mediating signal transduction and biological actions.

Regulation of SRC family kinases in human cancers

The nonreceptor protein tyrosine kinase Src plays a crucial role in the signal transduction pathways involved in cell division, motility, adhesion, and survival in both normal and cancer cells. Although the Src family kinases (SFKs) are activated in various types of cancers, the exact mechanisms through which they contribute to the progression of individual tumors remain to be defined. The activation of Src in human cancers may occur through a variety of mechanisms that include domain interaction and structural remodeling in response to various activators or upstream kinases and phosphatastes. Because of Src's prominent roles in invasion and tumor progression, epithelial-to-mesenchymal transition, angiogenesis, and the development of metastasis, Src is a promising target for cancer therapy. Several small molecule inhibitors of Src are currently being investigated in clinical trials. In this article, we will summarize the mechanisms regulating Src kinase activity in normal and cancer cells and discuss the status of Src inhibitor development against various types of cancers.

Acute ligand-independent Src activation mimics low EGF-induced EGFR surface signalling and redistribution into recycling endosomes

Src, a non-receptor tyrosine kinase, is a key signal transduction partner of epidermal growth factor (EGF) receptor (EGFR). In human breast cancer, EGFR and Src are frequently over-expressed and/or over-activated. Although reciprocal activation is documented, mechanisms underlying Src:EGFR interactions are incompletely understood. We here exploited ts/v-Src thermo-activation in MDCK monolayers to test whether acute Src activation impacts on signalling and trafficking of non-liganded EGFR. We found that thermo-activation caused rapid Src recruitment to the plasma membrane, concomitant association with EGFR, and its phosphorylation at Y845 and Y1173 predominantly at the cell surface. Like low EGF concentrations, activated Src (i) decreased EGF surface binding without affecting the total EGFR pool; (ii) triggered EGFR endocytosis via clathrin-coated vesicles; (iii) and led to its sequestration in perinuclear/recycling endosomes with avoidance of multivesicular bodies and lysosomal degradation. Combined Src activation and EGF were synergistic for EGFR-Y845 and -Y1173 phosphorylation at some endosomes. We conclude that acute effects of Src in MDCK cells may mimic those of low EGF on EGFR activation and redistribution. Src:EGFR interactions may be sufficient to trigger EGFR activation and might contribute to its local signalling, without requiring either soluble extracellular signal or receptor over-expression.

Microtubule-mediated Src tyrosine kinase trafficking in neuronal growth cones

Src family tyrosine kinases are important signaling enzymes in the neuronal growth cone, and they have been implicated in axon guidance; however, the detailed localization, trafficking, and cellular functions of Src kinases in live growth cones are unclear. Here, we cloned two novel Aplysia Src kinases, termed Src1 and Src2, and we show their association with both the plasma membrane and the microtubule cytoskeleton in the growth cone by live cell imaging, immunocytochemistry, and cell fractionation. Activated Src2 is enriched in filopodia tips. Interestingly, Src2-enhanced green fluorescent protein-positive endocytic vesicles and tubulovesicular structures undergo microtubule-mediated movements that are bidirectional in the central domain and mainly retrograde in the peripheral domain. To further test the role of microtubules in Src trafficking in the growth cone, microtubules were depleted with either nocodazole or vinblastine treatment, resulting in an increase in Src2 plasma membrane levels in all growth cone domains. Our data suggest that microtubules regulate the steady-state level of active Src at the plasma membrane by mediating retrograde recycling of endocytosed Src. Expression of constitutively active Src2 results in longer filopodia that protrude from smaller growth cones, implicating Src2 in controlling the size of filopodia and lamellipodia.

Endosomal trafficking of Src tyrosine kinase

Endosomal trafficking is an essential cellular process involved in the transport of proteins such as integrins, hormone receptors, growth factor receptors, receptor tyrosine kinases, and lipids (e.g. sphingomyelin). Regulation of this process is highly complex and involves Arf GAPs, SNAREs, Rab proteins, Rho GTPases and the actin cytoskeleton. In this article, we focus on the intracellular targeting of the Src family of non-receptor tyrosine kinases (nRTKs), and the role of endosomes in the delivery of nRTKs to the plasma membrane. Furthermore, we discuss the role of the actin cytoskeleton in this process and consider how endosome-regulated intracellular trafficking affects cell signalling.

PTP1B contributes to the oncogenic properties of colon cancer cells through Src activation

Src-specific activity has been reported to be elevated in a high percentage of colon cancer cell lines and tumors, but the underlying mechanisms are largely unknown. In this study, we report that, in the seven cancer cell lines tested, Src-specific activity was elevated (5.2- to 18.7-fold) relative to normal colon cells (FHC). This activation of Src correlated with reduced phosphorylation at Y530 of Src, whereas there was no significant change in the level of phosphorylation at Y419. The membrane tyrosine phosphatase activity for a Src family-specific phosphopeptide substrate FCP (Fyn COOH-terminal peptide phosphorylated by Csk) was greatly increased in the cancer cells and was attributed to PTP1B in most of the cell lines. Membrane PTP1B protein levels were also greatly increased. Overexpression of PTP1B increased Src specific activity in colon cancer cells by reducing phosphorylation at Y530 of Src. It also increased anchorage-independent cell growth and this increase was blocked by the Src inhibitor PP2 and Src small interfering RNA (siRNA). Down-regulating PTP1B activity by PTP1B inhibitor CinnGEL 2Me or knocking down PTP1B using siRNA also reduced Src kinase activity and colony formation ability of colon cancer cells. PTP1B siRNA reduced tumor growth in nonobese diabetic/severe combined immunodeficient mice. This study suggests that (a) PTP1B can act as an important activator of Src in colon cancer cells via dephosphorylation at Y530 of Src and (b) elevated levels of PTP1B can increase tumorigenicity of colon cancer cells by activating Src.

The membrane targeting and spatial activation of Src, Yes and Fyn is influenced by palmitoylation and distinct RhoB/RhoD endosome requirements

Src activation is a tightly regulated process which requires RhoB endosome-associated actin assembly and transit to the cell periphery. We show here that although two other ubiquitous Src family kinases (SFKs) Yes and Fyn also require intact actin filaments for peripheral membrane targeting, they display distinct spatial activation and endosomal requirements. Unlike Src, both Yes and Fyn are constitutively membrane-localized to some extent, and Fyn is present in RhoD-positive endosomes whereas Yes does not visibly colocalize with either of the endosomal markers RhoB or RhoD. By modulating amino acid acceptor sites for palmitoylation in Src, Yes and Fyn, we show that Src S3C/S6C, which is palmitoylated (unlike wild-type Src) behaves in a manner more similar to Fyn, by predominantly colocalizing with RhoD endosomes, and the targeting of both Fyn and Src S3C/S6C is inhibited by siRNA-mediated knockdown of RhoD. Moreover, Fyn C3S/C6S, which is no longer palmitoylated, behaves much more like Src by colocalizing with RhoB endosomes and by requiring RhoB for activation and membrane translocation. These data imply that distinct modes of spatial activation and membrane delivery, at least partly under the control of specific acylation attachment sequences and endosome sub-type requirements, define distinct properties of the three ubiquitously expressed SFKs.

Central roles of Mg2+ and MgATP2- in the regulation of protein synthesis and cell proliferation: significance for neoplastic transformation

Growth factors are polypeptides that combine with specific membrane receptors on animal cells to stimulate proliferation, but they also stimulate glucose transport, uridine phosphorylation, intermediary metabolism, protein synthesis, and other processes of the coordinate response. There are a variety of nonspecific surface action treatments which stimulate the same set of reactions as the growth factors do, of which protein synthesis is most directly related to the onset of DNA synthesis. Mg(2+) is required for a very wide range of cellular reactions, including all phosphoryl transfers, and its deprivation inhibits all components of the coordinate response that have so far been tested. Growth factors raise the level of free Mg(2+) closer to the optimum for the initiation of protein synthesis. The resulting increase in protein synthesis accelerates progression through G1 to the onset of DNA synthesis and mitosis. None of the other 3 major cellular cations are similarly involved in growth regulation, although internal pH may play an auxiliary role. Almost 10(5) externally bound divalent cations are displaced from membranes for every attached insulin molecule, implying a conformational membrane change that releases enough Mg(2+) from the internal surface of the plasma membrane to account for the increase in free cytosolic Mg(2+). It is proposed that mTOR, the central control point for protein synthesis of the PI 3-K kinase cascade stimulated by insulin, is regulated by MgATP(2-) which varies directly with cytosolic Mg(2+). Other elements of the coordinate response to growth factors such as the increased transport of glucose and phosphorylation of uridine are also dependent upon an increase of Mg(2+). Deprivation of Mg(2+) in neoplastically transformed cultures normalizes their appearance and growth behavior and raises their abnormally low Ca(2+) concentration. Tight packing of the transformed cells at very high saturation density confers the same normalizing effects, which are retained for a few days after subculture at low density. The results suggest that the activity of Mg(2+) within the cell is a central regulator of normal cell growth, and the loss of its membrane-mediated control can account for the neoplastic phenotype.

SRC gene expression in human cancer: the role of transcriptional activation

Human pp60c-Src (or c-Src) is a 60 kDa nonreceptor tyrosine kinase encoded by the SRC gene and is the cellular homologue to the potent transforming v-Src viral oncogene. c-Src functions at the hub of a vast array of signal transduction cascades that influence cellular proliferation, differentiation, motility, and survival. c-Src activation has been documented in upwards of 50% of tumors derived from the colon, liver, lung, breast, and pancreas. Therefore, a major focus has been to understand the mechanisms of c-Src activation in human cancer. Early studies concentrated on post-translational mechanisms that lead to increased c-Src kinase activity, which often correlated with overexpression of c-Src protein. More recently, the discovery of an activating SRC mutation in a small subset of advanced colon tumors has been reported. In addition, elevated SRC transcription has been identified as yet another mechanism contributing significantly to c-Src activation in a subset of human colon cancer cell lines. Interestingly, histone deacetylase (HDAC) inhibitors, agents with well-documented anti-cancer activity, repress SRC transcription in a wide variety of human cancer cell lines. Analysis of the mechanisms behind HDAC inhibitor mediated repression could be utilized in the future to specifically inhibit SRC gene expression in human cancer.

C-terminal Src kinase (CSK) modulates insulin-like growth factor-I signaling through Src in 3T3-L1 differentiation

IGF-I stimulates both proliferation and differentiation of adipocyte-precursor cells, preadipocytes in vivo and in vitro. We have previously shown that IGF-I stimulates proliferation of 3T3-L1 preadipocytes through activation of MAPK and MAPK activation by IGF-I is mediated through the Src family of nonreceptor tyrosine kinases. In addition, we have shown that when 3T3-L1 cells reach growth arrest and are stimulated to differentiate, IGF-I can no longer activate the MAPK pathway. We hypothesized that the loss of IGF-I signaling to MAPK in differentiating 3T3-L1 cells is due to loss of IGF-I activation of Src family kinases. We measured c-Src kinase activity in cell lysates from proliferating, growth-arrested and differentiating 3T3-L1 cells. Src activity increased 2- to 4-fold in IGF-I-stimulated proliferating cells; however, IGF-I had a marginal affect on Src activity in growth-arrested cells and inhibited Src activity localized at the membrane in differentiating cells. C-terminal Src kinase (CSK), a ubiquitously expressed nonreceptor tyrosine kinase, negatively regulates the Src family kinases by phosphorylation of the Src C-terminal tyrosine. IGF-I decreased phosphorylation of the Src C-terminal tyrosine in proliferating cells and increased phosphorylation of this site in differentiating cells. IGF-I stimulated CSK kinase activity 2-fold in differentiating 3T3-L1 cells. An association between CSK and c-Src was detected by immunoprecipitation following IGF-I stimulation of differentiating but not proliferating 3T3-L1 cells. These results suggest that the loss of IGF-I downstream mitogenic signaling in differentiating 3T3-L1 cells is due to a change in IGF-I activation of c-Src and CSK may mediate the inactivation of c-Src by IGF-I in 3T3-L1 adipogenesis.

Helicobacter pylori CagA protein activates serum response element-driven transcription independently of tyrosine phosphorylation

BACKGROUND & AIMS

Infection with Helicobacter pylori possessing the cag pathogenicity island (PAI) is associated with severe gastritis and gastric cancer. CagA protein, one of the products of cag PAI, is translocated into epithelial cells, where cytoskeletal rearrangements occur as a result of CagA tyrosine (Tyr) phosphorylation. Here we identify a new role for CagA protein as an activator of host cell signaling.

METHODS

We transfected CagA into epithelial cells and analyzed its effect on transcription by reporter assays. The mechanism of reporter activation was assessed by electrophoretic mobility shift assays (EMSA) and immunoblots. Responsible regions of CagA for reporter activation were determined by truncation and mutagenesis of cagA gene.

RESULTS

In HeLa cells, expression of CagA increased serum response element (SRE)-driven and serum response factor (SRF)-driven transcription by 40-fold and 3.3-fold, respectively, but did not affect nuclear factor kappaB- or AP-1-driven transcription. CagA-mediated SRE activation was also observed in gastric cell lines. Immunoblotting and EMSA revealed that transfection of CagA enhanced phosphorylation of and DNA binding by Elk1. Furthermore, involvement of Ras and MEK in CagA-mediated Elk1 phosphorylation was observed. SRE activation was dependent on several regions within the C-terminal portion of CagA (CagA(873-1002)), and independent of Tyr phosphorylation.

CONCLUSIONS

The C-terminal portion of CagA enhances SRE-driven transcription by activating an upstream signaling cascade without requiring CagA Tyr phosphorylation. This result suggests that translocated CagA regulates 2 distinct cellular responses: phosphorylation-dependent cytoskeletal rearrangement and phosphorylation-independent transcriptional activation.

Phosphorylation of Hrs downstream of the epidermal growth factor receptor

The hepatocyte growth factor-regulated tyrosine kinase substrate Hrs is an early endosomal protein that is thought to play a regulatory role in the trafficking of growth factor/receptor complexes through early endosomes. Stimulation of cells with epidermal growth factor (EGF) rapidly leads to phosphorylation of Hrs, raising the question whether the receptor tyrosine kinase phosphorylates Hrs directly. Here, we present evidence that a downstream kinase, rather than the active receptor kinase is responsible. We show that the nonreceptor tyrosine kinase Src is able to phosphorylate Hrs in vitro and in vivo, but that Hrs is nevertheless phosphorylated in Src-, Yes- and Fyn-negative cells. Moreover, we show that only 10-20% of Hrs is phosphorylated following EGF stimulation, and that phosphorylation occurs at multiple tyrosines located in different parts of Hrs. These results suggest that Hrs is a substrate for several kinases downstream of the EGF receptor.

v-Src's hold over actin and cell adhesions

The oncoprotein v-Src and its cellular homologue (c-Src) are tyrosine kinases that modulate the actin cytoskeleton and cell adhesions. Through the concerted action of their protein-interaction and kinase domains, they are targeted to cell matrix integrin adhesions or cadherin-dependent junctions between epithelial cells, where they phosphorylate substrates that induce adhesion turnover and actin re-modelling. Recent experiments have defined some of the key targets and effector pathways that mediate the pleiotropic oncogenic effects of v-Src.

c-Src/Lyn kinases activate Helicobacter pylori CagA through tyrosine phosphorylation of the EPIYA motifs

The human pathogen Helicobacter pylori colonizes the mucous layer of the stomach. During parasitic infection, freely swimming bacteria adhere to the gastric epithelial cells and trigger intracellular signalling pathways. This process requires the translocation of the effector protein CagA into the host cell through a specialized type IV secretion system encoded in the cag pathogenicity island. Following transfer, CagA is phosphorylated on tyrosine residues by a host cell kinase. Here, we describe how the tyrosine phosphorylation of CagA is restricted to a previously identified repeated sequence called D1. This sequence is located in the C-terminal half of the protein and contains the five-amino-acid motif EPIYA, which is amplified by duplications in a large fraction of clinical isolates. Tyrosine phosphorylation of CagA is essential for the activation process that leads to dramatic changes in the morphology of cells growing in culture. In addition, we observed that two members of the src kinases family, c-Src and Lyn, account for most of the CagA-specific kinase activity in host cell lysates. Thus, CagA translocation followed by tyrosine phosphorylation at the EPIYA motifs promotes a growth factor-like response with intense cytoskeletal rearrangements, cell elongation effects and increased cellular motility.

Differential modulation of cellular death and survival pathways by conjugated bile acids

BACKGROUND

The liver-derived McNtcp.24 cells transport bile acids and show distinctive responses to the two classes of conjugated bile acids. Whereas taurine-conjugated bile acids are non-toxic, glycine-conjugated bile acids efficiently induce apoptosis. The aim of this study was to determine if the differential sensitivity is limited to cells that normally transport bile acids and if bile acid binding proteins could reduce bile acid-mediated apoptosis. The apical sodium/bile acid co-transporter (asbt) was expressed in Chinese hamster ovary (CHO) cells to establish active bile acid transport in a non-liver-derived cell model (CHO.asbt). A high-affinity bile acid binder was expressed in McNtcp.24 cells.

RESULTS

The tolerance of McNtcp.24 cells to taurine-conjugated bile acids was associated with the stimulation of phosphatidylinositol 3-kinase (PI3K) activity. Treatment of CHO.asbt cells with taurine- and glycine-conjugated bile acids resulted in apoptosis. Unlike in McNtcp.24 cells, PI3K activity was not increased in CHO.asbt cells treated with taurine-conjugated bile acids. High level expression of a bile acid binder did not attenuate bile acid-induced cytotoxicity in McNtcp.24 cells.

CONCLUSION

The data suggest that McNtcp.24 cells possess a mechanism that can elaborate distinctive responses to the different classes of bile acids. Additionally, activation of a signaling pathway involving PI3K appears to be the dominant mechanism responsible for the tolerance of McNtcp.24 cells to taurine-conjugated bile acids.

Selected glimpses into the activation and function of Src kinase

Since the discovery of the v-src and c-src genes and their products, much progress has been made in the elucidation of the structure, regulation, localization, and function of the Src protein. Src is a non-receptor protein tyrosine kinase that transduces signals that are involved in the control of a variety of cellular processes such as proliferation, differentiation, motility, and adhesion. Src is normally maintained in an inactive state, but can be activated transiently during cellular events such as mitosis, or constitutively by abnormal events such as mutation (i.e. v-Src and some human cancers). Activation of Src occurs as a result of disruption of the negative regulatory processes that normally suppress Src activity, and understanding the various mechanisms behind Src activation has been a target of intense study. Src associates with cellular membranes, in particular the plasma membrane, and endosomal membranes. Studies indicate that the different subcellular localizations of Src could be important for the regulation of specific cellular processes such as mitogenesis, cytoskeletal organization, and/or membrane trafficking. This review will discuss the history behind the discovery and initial characterization of Src and the regulatory mechanisms of Src activation, in particular, regulation by modification of the carboxy-terminal regulatory tyrosine by phosphatases and kinases. Its focus will then turn to the different subcellular localizations of Src and the possible roles of nuclear and perinuclear targets of Src. Finally, a brief section will review some of our present knowledge regarding Src involvement in human cancers.

Reconstitution of membranes simulating "glycosignaling domain" and their susceptibility to lyso-GM3

GM3 ganglioside at the surface of mouse melanoma B16 cells is clustered and organized with signal transducer molecules c-Src, Rho A, and focal adhesion kinase (FAK) to form a membrane unit separable from caveolae, which are enriched in cholesterol and caveolin but do not contain GM3 or the above three signal transducers. The GM3-enriched membrane units are involved in GM3-dependent cell adhesion coupled with activation of c-Src, Rho A, and FAK and are termed the "glycosphingolipid signaling domain" or the "glycosignaling domain" (GSD). In order to assess the essential components that display GSD function, membranes with properties similar to those of GSD were reconstituted using GM3, sphingomyelin, and c-Src, with or without other lipid components. The reconstituted membrane thus prepared displayed GM3-dependent adhesion to plates coated with Gg3 or anti-GM3 antibody, resulting in enhanced c-Src phosphorylation (c-Src phosphorylation response). This response in reconstituted membrane depends on GM3 concentration and was not observed when GM3 was absent or replaced with other gangliosides GM1 or GD1a, or with LacCer. The GM3-dependent c-Src phosphorylation response was enhanced when cholesterol and phosphatidylcholine were added. Although GM3, sphingomyelin, and c-Src are essential for GSD function, a small quantity of cholesterol and phosphatidylcholine may act as an auxiliary factor to stabilize membrane. GSD function in terms of GM3-dependent adhesion and signaling was blocked in the presence of lyso-GM3 or its analogue but not psychosine, lactosyl-sphingosine, or lyso-phosphatidylcholine. Such susceptibility of reconstituted GSD to lyso-GM3 and other lyso compounds is the same as GSD of original B16 cells. Thus, functional organization of the reconstituted membrane closely simulates that of GSD in B16 cells, which is based on clustered GM3 organized with c-Src as the essential components.

cSrc is a major cytosolic tyrosine kinase in vascular tissue

We are interested in identifying, in vascular tissue, nonreceptor tyrosine kinases that may be responsible for the contractile actions of G-protein-coupled agonists such as angiotensin II. By using a series of chromatographic steps, including ion exchange, hydrophobic, and affinity chromatography, we have isolated a major fraction of tyrosine kinase activity from the cytosolic fraction of porcine aorta tissue. According to (i) its immunologic cross-reactivity with the monoclonal anti-cSrc antibody, m327, and with the N-terminally directed monoclonal cSrc2-17 antibody, (ii) its inhibition by the C-terminal cSrc kinase, CSK, and (iii) its specificity for phosphorylating tyrosine 15 in the cdc2(6-20) peptide kinase substrate, we conclude that the kinase we have isolated represents porcine cSrc. A substantial proportion of the enzyme (>70%) was recovered in the cytoplasmic fraction from aorta tissue. The profile of inhibition of the human and porcine cSrc enzymes by a spectrum of tyrosine kinase inhibitors (PP1>> AG82 > AG490 congruent to genistein > AG10) was compared with the profile of inhibition of angiotensin II mediated contraction in a porcine coronary vascular preparation (AG10 >> genistein [Formula: see text] AG82 [Formula: see text] AG490; PP1inactive). The different inhibitory profiles indicated that cSrc does not represent the vascular tyrosine kinase responsible for the contractile actions of angiotensin II. We suggest, nonetheless, that cSrc plays a key role for other actions of angiotensin II in intact vascular tissue, such as the regulation of mitogen-activated protein kinase activity and gene transcription.Key words: tyrosine kinase, smooth muscle, cSrc.

Low molecular weight protein-tyrosine phosphatase tyrosine phosphorylation by c-Src during platelet-derived growth factor-induced mitogenesis correlates with its subcellular targeting

The low molecular weight phosphotyrosine phosphatase (LMW-PTP) is an enzyme that is involved in the early events of platelet-derived growth factor (PDGF) receptor signal transduction. Our previous results have shown that LMW-PTP is able to specifically bind and dephosphorylate activated PDGF receptor, thus modulating PDGF-induced mitogenesis. In particular LMW-PTP is involved in pathways that regulate the transcription of the immediately early genes myc and fos in response to growth factor stimulation. In this study we have established that, in nontransformed NIH3T3 cells, LMW-PTP exists constitutively in cytosolic and cytoskeleton-associated localization and that, after PDGF stimulation, c-Src is able to bind and to phosphorylate LMW-PTP only in the cytoskeleton-associated fraction. As a consequence of its tyrosine phosphorylation, LMW-PTP significantly increases its catalytic activity. After PDGF stimulation these two LMW-PTP pools act on distinct substrates, contributing in different manners to the PDGF receptor signaling. The cytoplasmic LMW-PTP fraction exerts its well known action on activated PDGF receptor. On the other hand we have now demonstrated that the cytoskeleton-associated LMW-PTP acts specifically on a few not yet identified proteins that become tyrosine-phosphorylated in response to the PDGF receptor activation. Finally, these two LMW-PTP pools markedly differ in the timing of the processes in which they are involved. The cytoplasmic LMW-PTP pool exerts its action within a few minutes from PDGF receptor activation (short term action), while tyrosine phosphorylation of cytoskeleton-associated LMW-PTP lasts for more than 40 min (long term action). In conclusion LMW-PTP is a striking example of an enzyme that exerts different functions and undergoes different regulation in consequence of its subcellular localization.

Regulation of matrix metalloproteinase-9 and inhibition of tumor invasion by the membrane-anchored glycoprotein RECK

A human fibroblast cDNA expression library was screened for cDNA clones giving rise to flat colonies when transfected into v-Ki-ras-transformed NIH 3T3 cells. One such gene, RECK, encodes a membrane-anchored glycoprotein of about 110 kDa with multiple epidermal growth factor-like repeats and serine-protease inhibitor-like domains. While RECK mRNA is expressed in various human tissues and untransformed cells, it is undetectable in tumor-derived cell lines and oncogenically transformed cells. Restored expression of RECK in malignant cells resulted in suppression of invasive activity with concomitant decrease in the secretion of matrix metalloproteinase-9 (MMP-9), a key enzyme involved in tumor invasion and metastasis. Moreover, purified RECK protein was found to bind to, and inhibit the proteolytic activity of, MMP-9. Thus, RECK may link oncogenic signals to tumor invasion and metastasis.

The catalytic activity of Src is dispensable for translocation to focal adhesions but controls the turnover of these structures during cell motility

The Src family of protein tyrosine kinases is involved in transducing signals at sites of cellular adhesion. In particular, the v-Src oncoprotein resides in cellular focal adhesions, where it induces tyrosine phosphorylation of pp125FAK and focal adhesion loss during transformation. v-Src is translocated to cellular focal adhesions by an actin-dependent process. Here we have used mutant v-Src proteins that are temperature-dependent for translocation, but with secondary mutations that render them constitutively kinase-inactive or myristylation-defective, to show that neither v-Src kinase activity nor a myristyl group are required to induce association of v-Src with actin stress fibres and redistribution to sites of focal adhesions at the stress fibre termini. Moreover, switching the constitutively kinase-inactive or myristylation-defective temperature-sensitive v-Src proteins to the permissive temperature resulted in concomitant association with tyrosine-phosphorylated focal adhesion kinase (pp125FAK) and redistribution of both to focal adhesions. However, both catalytic activity and myristylation-mediated membrane association are required to induce dissociation of pp125FAK from v-Src, later degradation of pp125FAK and focal adhesion turnover during transformation and cell motility. These observations provide strong evidence that the role of the tyrosine kinase activity of the Src family at sites of cellular focal adhesions is to regulate the turnover of these structures during cell motility.

Translocation of Src kinase to the cell periphery is mediated by the actin cytoskeleton under the control of the Rho family of small G proteins

We have isolated Swiss 3T3 subclones that are resistant to the mitogenic and morphological transforming effects of v-Src as a consequence of aberrant translocation of the oncoprotein under low serum conditions. In chicken embryo and NIH 3T3 fibroblasts under similar conditions, v-Src rapidly translocates from the perinuclear region to the focal adhesions upon activation of the tyrosine kinase, resulting in downstream activation of activator protein-1 and mitogen-activated protein kinase, which are required for the mitogenic and transforming activity of the oncoprotein. Since serum deprivation induces cytoskeletal disorganization in Swiss 3T3, we examined whether regulators of the cytoskeleton play a role in the translocation of v-Src, and also c-Src, in response to biological stimuli. Actin stress fibers and translocation of active v-Src to focal adhesions in quiescent Swiss 3T3 cells were restored by microinjection of activated Rho A and by serum. Double labeling with anti-Src and phalloidin demonstrated that v-Src localized along the reformed actin filaments in a pattern that would be consistent with trafficking in complexes along the stress fibers to focal adhesions. Furthermore, treatment with the actin-disrupting drug cytochalasin D, but not the microtubule-disrupting drug nocodazole, prevented v-Src translocation. In addition to v-Src, we observed that PDGF-induced, Rac-mediated membrane ruffling was accompanied by translocation of c-Src from the cytoplasm to the plasma membrane, an effect that was also blocked by cytochalasin D. Thus, we conclude that translocation of Src from its site of synthesis to its site of action at the cell membrane requires an intact cytoskeletal network and that the small G proteins of the Rho family may specify the peripheral localization in focal adhesions or along the membrane, mediated by their effects on the cytoskeleton.

The role of Src family kinases in the normal and neoplastic gastrointestinal tract

Src family kinases are a group of non-receptor tyrosine kinases that mediate signal transduction pathways involved in the growth and differentiation of normal tissues. Considerable evidence exists for a role of these proteins in neoplastic progression in various organ systems including the nervous, hematopoietic and skeletal systems. In addition, the role of the Src kinase family has been characterized for colon cancer, but only limited progress has been made in delineating the role of Src kinases in the normal gastrointestinal (GI) tract and extracolonic GI cancers. In this review, we provide an up-to-date assessment of the Src family kinases in the normal and neoplastic GI tract.

Myristoylation and differential palmitoylation of the HCK protein-tyrosine kinases govern their attachment to membranes and association with caveolae

The human proto-oncogene HCK encodes two versions of a protein-tyrosine kinase, with molecular weights of 59,000 (p59hck) and 61,000 (p61hck). The two proteins arise from a single mRNA by alternative initiations of translation. In this study, we explored the functions of these proteins by determining their locations within cells and by characterizing lipid modifications required for the proteins to reach those locations. We found that p59hck is entirely associated with cellular membranes, including the organelles known as caveolae; in contrast, only a portion of p61hck is situated on membranes, and none is detectable in preparations of caveolae. These distinctions can be attributed to differential modification of the two HCK proteins with fatty acids. Both proteins are at least in part myristoylated, p59hck more so than p61hck. In addition, however, p59hck is palmitoylated on cysteine 3 in the protein. Palmitoylation of the protein requires prior myristoylation and, in turn, is required for targeting to caveolae. These findings are in accord with recent reports for other members of the SRC family of protein-tyrosine kinases. Taken together, the results suggest that HCK and several of its relatives may participate in the functions of caveolae, which apparently include the transduction of signals across the plasma membrane to the interior of the cell.

Phosphorylation of the growth arrest-specific protein Gas2 is coupled to actin rearrangements during Go-->G1 transition in NIH 3T3 cells

Growth arrest-specific (Gas2) protein has been shown to be a component of the microfilament system, that is highly expressed in growth arrested mouse and human fibroblasts and is hyperphosphorylated upon serum stimulation of quiescent cells. (Brancolini, C., S. Bottega, and C. Schneider. 1992. J. Cell Biol. 117:1251-1261). In this study we demonstrate that the kinetics of Gas2 phosphorylation, during Go-->G1 transition, as induced by addition of 20% FCS to serum starved NIH 3T3 cells, is temporally coupled to the reorganization of actin cytoskeleton. To better dissect the relationship between Gas2 phosphorylation and the modification of the microfilament architecture we used specific stimuli for both membrane ruffling (PDGF and PMA) and stress fiber formation (L-alpha-lysophosphatidic acid LPA) (Ridley, A. J., and A. Hall. 1992. Cell. 70:389-399). All of them, similarly to 20% FCS, are able to downregulate Gas2 biosynthesis. PDGF and PMA induce Gas2 hyperphosphorylation that is temporally coupled with the appearance of membrane ruffling where Gas2 localizes. On the other hand LPA, a specific stimulus for stress fiber formation, fails to induce a detectable Gas2 hyperphosphorylation. Thus, Gas2 hyperphosphorylation is specifically correlated with the formation of membrane ruffling possibly implying a role of Gas2 in this process.

Modulation of pp60v-src and pp60c-src expression in Rous sarcoma virus-transformed hamster fibroblasts transfected with activated N-ras

Three phenotypically different hamster cell lines transformed with Rous sarcoma virus (RSV) were transfected with plasmid DNA containing an activated N-ras oncogene, and nine clones expressing various levels of p21N-ras were characterized. We examined the effects of p21N-ras on expression and kinase activity of resident src proteins by using a variety of assays that allowed us to discriminate between viral and cellular src proteins. In eight clones with a 10- to 20-fold increase in p21N-ras levels relative to the endogenous protein, we observed a marked reduction in the synthesis and kinase activity of p60v-src. This decrease correlated with transcriptional downregulation of RSV genomic and v-src subgenomic mRNAs. In the same cells, we found a concomitant accumulation of p60c-src and, accordingly, an increase in its protein kinase activity without an apparent increase in c-src mRNA levels. Therefore, modulation of viral and cellular src proteins in cells overexpressing p21N-ras appeared to result from two distinct effects: a downregulation of long terminal repeat-driven transcription and a more complex interaction with cellular effectors that control the stability of p60c-src. Such modulation also seemed to depend on the levels of p21N-ras and, possibly, on host-cell factors, since it was not observed in the third cell line, in which the relative increase in p21N-ras was only 2.5-fold to fivefold.

Osteoclasts express high levels of pp60c-src in association with intracellular membranes

Deletion of the c-src gene in transgenic mice by homologous recombination leads to osteopetrosis, a skeletal defect characterized by markedly deficient bone resorption (Soriano, P., C. Montgomery, R. Geske, and A. Bradley. 1991. Cell. 64:693-702), demonstrating a critical functional role of pp60c-src in osteoclast activity. Since decreased bone resorption could result from a defect either within the osteoclast or within other cells present in its environment, indirectly affecting osteoclast functions, we determined which cell(s) in bone expressed high levels of pp60c-src Measuring pp60c-src protein and kinase activities in osteoclasts and immunolocalizing pp60c-src in bone, we find that expression of pp60c-src is nearly as high in osteoclasts as in brain and platelets. In contrast, other bone cells contain only very low levels of the protein. In addition, expression of the c-src gene product increases when bone marrow cells are induced to express an osteoclast-like phenotype by 1,25-dihydroxy-vitamin D3, further suggesting that high expression of pp60c-src is part of the osteoclast phenotype. Three other src-like kinases, c-fyn, c-yes, and c-lyn, are also expressed in osteoclasts at ratios to pp60c-src similar to what is found in platelets. These src-related proteins do not, however, compensate for the absence of pp60c-src in the src- mice, thereby suggesting that pp60c-src may have a specific function in osteoclasts. Although further work is necessary to elucidate what the critical role of pp60c-src in osteoclasts is, our observation that the protein is associated mostly with the membranes of intracellular organelles suggests the possibility that this role might be at least in part related to the targeting or fusion of membrane vesicles.

Lysine residues form an integral component of a novel NH2-terminal membrane targeting motif for myristylated pp60v-src

Association of pp60v-src with the plasma membrane is fundamental to generation of the transformed phenotype. Although myristylation of pp60v-src is required for interaction with a membrane-bound receptor, the importance of NH2-terminal amino acids in receptor binding has not yet been uncoupled from their role in signaling myristylation. Using chimeric src proteins, peptides identical or related to the NH2 terminus of src, and site-directed mutagenesis, we demonstrate that NH2-terminal lysines in conjunction with myristate are essential for membrane localization. Subsequent to NH2-terminal interaction with the "src receptor," internal regions of the src protein also participate in membrane binding. This novel NH2-terminal motif and internal contact mechanism may direct other members of the src family of tyrosine kinases to their membrane receptors.

Cell transformation and activation of pp60c-src by overexpression of a protein tyrosine phosphatase

The kinase activity of pp60c-src is specifically and transiently increased during mitosis and repressed during interphase. Loss of cell-cycle control of pp60c-src occurs on mutation of Tyr527 to Phe or when pp60c-src is associated with polyoma middle-T-antigen, and these conditions result in cell transformation or tumorigenesis. In both cases, pp60c-src has elevated kinase activity which is maintained throughout the cell cycle and accompanied by dephosphorylation of the carboxy-terminal negative regulatory Tyr527 site, or mimicry of Tyr527 dephosphorylation in the case of the mutant. Here we report that overexpression of the receptor-like protein tyrosine phosphatase PTP alpha results in persistent activation of pp60c-src kinase, with concomitant cell transformation and tumorigenesis. In PTP alpha-overexpressing cells, the pp60c-src kinase activation is accompanied by dephosphorylation at Tyr527, and direct dephosphorylation of this site by purified PTP alpha occurs in vitro. Our results suggest that PTP alpha is involved in the regulation of cell proliferation, exerting at least some of its effects through pp60c-src kinase, and has oncogenic capability when overexpressed.

Association of p60c-src with endosomal membranes in mammalian fibroblasts

We have examined the subcellular localization of p60c-src in mammalian fibroblasts. Analysis of indirect immunofluorescence by three-dimensional optical sectioning microscopy revealed a granular cytoplasmic staining that co-localized with the microtubule organizing center. Immunofluorescence experiments with antibodies against a number of membrane markers demonstrated a striking co-localization between p60c-src and the cation-dependent mannose-6-phosphate receptor (CI-MPR), a marker that identifies endosomes. Both p60c-src and the CI-MPR were found to cluster at the spindle poles throughout mitosis. In addition, treatment of interphase and mitotic cells with brefeldin A resulted in a clustering of p60c-src and CI-MPR at a peri-centriolar position. Biochemical fractionation of cellular membranes showed that a major proportion of p60c-src co-enriched with endocytic membranes. Treatment of membranes containing HRP to alter their apparent density also altered the density of p60c-src-containing membranes. Similar density shift experiments with total cellular membranes revealed that the majority of membrane-associated p60c-src in the cell is associated with endosomes, while very little is associated with plasma membranes. These results support a role for p60c-src in the regulation of endosomal membranes and protein trafficking.

Specific association of the proto-oncogene product pp60c-src with an intracellular organelle, the PC12 synaptic vesicle

The protein product of the proto-oncogene c-src is a membrane-associated tyrosine kinase of unknown function. Identification of pp60c-src target membranes may elucidate the function of the c-src protein. The available evidence indicates that pp60c-src associates with distinct membranes within single cell types and has different distributions in different cell types. Our experiments demonstrate targeting of pp60c-src to an isolatable and biochemically identified membrane fraction in the neuroendocrine cell line PC12. The c-src protein was found to be specifically associated with synaptic vesicles since: (a) the pp60c-src immunofluorescent pattern overlapped with a synaptic vesicle marker, synaptophysin; (b) a significant proportion (44%) of the pp60c-src from PC12 but not fibroblast postnuclear supernatants was recovered in a small vesicle fraction; (c) an anti-synaptophysin cytoplasmic domain antibody immunodepleted all of the pp60c-src vesicles in this fraction, and (d) pp60c-src copurified during a 100-fold purification of PC12 synaptic vesicles. These results suggest a role for the c-src protein in the regulation of synaptic vesicle function.

Two isoforms of murine hck, generated by utilization of alternative translational initiation codons, exhibit different patterns of subcellular localization

Mammalian hck, a member of the src family of tyrosine kinases, is expressed predominantly in cells of the myeloid and B-lymphoid lineages. Using mutational analysis, we have investigated the molecular basis of two immunoreactive forms of murine hck of 56 and 59 kDa found in numerous hemopoietic cell types. Our results indicate that translation of murine p59hck initiates from a CTG codon located 21 codons 5' of an ATG that is utilized to generate p56hck. We provide evidence that two human hck isoforms are generated by the same mechanism. Subcellular fractionation studies reveal that while p59hck and p56hck are associated with membranes of various murine B-lymphoid and myeloid cell lines, p59hck alone is also located in the cytosol. In contrast to membrane-associated p59hck, which is metabolically labeled with [3H]myristic acid and exhibits amphiphilic properties in Triton X-114 detergent, cytosolic p59hck is hydrophilic, suggesting that it is not acylated. Possible mechanisms are proposed to account for these observations.

Two isoforms of murine hck, generated by utilization of alternative translational initiation codons, exhibit different patterns of subcellular localization

Mammalian hck, a member of the src family of tyrosine kinases, is expressed predominantly in cells of the myeloid and B-lymphoid lineages. Using mutational analysis, we have investigated the molecular basis of two immunoreactive forms of murine hck of 56 and 59 kDa found in numerous hemopoietic cell types. Our results indicate that translation of murine p59hck initiates from a CTG codon located 21 codons 5' of an ATG that is utilized to generate p56hck. We provide evidence that two human hck isoforms are generated by the same mechanism. Subcellular fractionation studies reveal that while p59hck and p56hck are associated with membranes of various murine B-lymphoid and myeloid cell lines, p59hck alone is also located in the cytosol. In contrast to membrane-associated p59hck, which is metabolically labeled with [3H]myristic acid and exhibits amphiphilic properties in Triton X-114 detergent, cytosolic p59hck is hydrophilic, suggesting that it is not acylated. Possible mechanisms are proposed to account for these observations.

Analysis of cDNAs of the proto-oncogene c-src: heterogeneity in 5' exons and possible mechanism for the genesis of the 3' end of v-src

To further characterize the gene structure of the proto-oncogene c-src and the mechanism for the genesis of the v-src sequence in Rous sarcoma virus, we have analyzed genomic and cDNA copies of the chicken c-src gene. From a cDNA library of chicken embryo fibroblasts, we isolated and sequenced several overlapping cDNA clones covering the full length of the 4-kb c-src mRNA. The cDNA sequence contains a 1.84-kb sequence downstream from the 1.6-kb pp60c-src coding region. An open reading frame of 217 amino acids, called sdr (src downstream region), was found 105 nucleotides from the termination codon for pp60c-src. Within the 3' noncoding region, a 39-bp sequence corresponding to the 3' end of the RSV v-src was detected 660 bases downstream of the pp60c-src termination codon. The presence of this sequence in the c-src mRNA exon supports a model involving an RNA intermediate during transduction of the c-src sequence. The 5' region of the c-src cDNA was determined by analyzing several cDNA clones generated by conventional cloning methods and by polymerase chain reaction. Sequences of these chicken embryo fibroblast clones plus two c-src cDNA clones isolated from a brain cDNA library show that there is considerable heterogeneity in sequences upstream from the c-src coding sequence. Within this region, which contains at least 300 nucleotides upstream of the translational initiation site in exon 2, there exist at least two exons in each cDNA which fall into five cDNA classes. Four unique 5' exon sequences, designated exons UE1, UE2, UEX, and UEY, were observed. All of them are spliced to the previously characterized c-src exons 1 and 2 with the exception of type 2 cDNA. In type 2, the exon 1 is spliced to a novel downstream exon, designated exon 1a, which maps in the region of the c-src DNA defined previously as intron 1. Exon UE1 is rich in G+C content and is mapped at 7.8 kb upstream from exon 1. This exon is also present in the two cDNA clones from the brain cDNA library. Exon UE2 is located at 8.5 kb upstream from exon 1. The precise locations of exons UEX and UEY have not been determined, but both are more than 12 kb upstream from exon 1. The existence and exon arrangements of these 5' cDNAs were further confirmed by RNase protection assays and polymerase chain reactions using specific primers. Our findings indicate that the heterogeneity in the 5' sequences of the c-src mRNAs results from differential splicing and perhaps use of distinct initiation sites. All of these RNAs have the potential of coding for pp60c-src, since their 5' exons are all eventually joined to exon 2.

Analysis of cDNAs of the proto-oncogene c-src: heterogeneity in 5' exons and possible mechanism for the genesis of the 3' end of v-src

To further characterize the gene structure of the proto-oncogene c-src and the mechanism for the genesis of the v-src sequence in Rous sarcoma virus, we have analyzed genomic and cDNA copies of the chicken c-src gene. From a cDNA library of chicken embryo fibroblasts, we isolated and sequenced several overlapping cDNA clones covering the full length of the 4-kb c-src mRNA. The cDNA sequence contains a 1.84-kb sequence downstream from the 1.6-kb pp60c-src coding region. An open reading frame of 217 amino acids, called sdr (src downstream region), was found 105 nucleotides from the termination codon for pp60c-src. Within the 3' noncoding region, a 39-bp sequence corresponding to the 3' end of the RSV v-src was detected 660 bases downstream of the pp60c-src termination codon. The presence of this sequence in the c-src mRNA exon supports a model involving an RNA intermediate during transduction of the c-src sequence. The 5' region of the c-src cDNA was determined by analyzing several cDNA clones generated by conventional cloning methods and by polymerase chain reaction. Sequences of these chicken embryo fibroblast clones plus two c-src cDNA clones isolated from a brain cDNA library show that there is considerable heterogeneity in sequences upstream from the c-src coding sequence. Within this region, which contains at least 300 nucleotides upstream of the translational initiation site in exon 2, there exist at least two exons in each cDNA which fall into five cDNA classes. Four unique 5' exon sequences, designated exons UE1, UE2, UEX, and UEY, were observed. All of them are spliced to the previously characterized c-src exons 1 and 2 with the exception of type 2 cDNA. In type 2, the exon 1 is spliced to a novel downstream exon, designated exon 1a, which maps in the region of the c-src DNA defined previously as intron 1. Exon UE1 is rich in G+C content and is mapped at 7.8 kb upstream from exon 1. This exon is also present in the two cDNA clones from the brain cDNA library. Exon UE2 is located at 8.5 kb upstream from exon 1. The precise locations of exons UEX and UEY have not been determined, but both are more than 12 kb upstream from exon 1. The existence and exon arrangements of these 5' cDNAs were further confirmed by RNase protection assays and polymerase chain reactions using specific primers. Our findings indicate that the heterogeneity in the 5' sequences of the c-src mRNAs results from differential splicing and perhaps use of distinct initiation sites. All of these RNAs have the potential of coding for pp60c-src, since their 5' exons are all eventually joined to exon 2.

Immunolocalization of the cellular src protein in interphase and mitotic NIH c-src overexpresser cells

The mouse mAb, mAb 327, that recognizes specifically both pp60v-src and pp60c-src in a wide variety of cells, has been used to determine precisely the various locations of pp60c-src in NIH c-src overexpresser cells, using the technique of immunofluorescence microscopy. In interphase cells, the protein exhibits two main distributions: one that appears uniform and in association with the cell surface and the other that is patchy and juxtanuclear and coincides with the centrosomes. The juxtanuclear aggregation of pp60c-src-containing patches depends on microtubules and does not seem to occur within the Golgi apparatus and the rough ER. At the G2-to-M-phase transition, a drastic change in the localization patterns of pp60c-src takes place. We also report experiments in which the NIH c-src overexpresser cells were exposed to Con A for various times to induce a redistribution of the cell surface Con A receptors. We show that, at each stage of the Con A-mediated endocytotic process, the Con A-receptor complexes redistribute into structures to which pp60c-src appears also to be associated: at first, into patches that form at the cell surface level and then, into a cap that stands at the cell center in a juxtanuclear position and that coincides with the Golgi apparatus. During this capping process, pp60c-src-containing vesicles continue to accumulate in a centriolar spot, as in interphase, Con A-untreated cells, from which Con A is excluded. The significance of the intracellular locations of pp60c-src to the possible functions of the protein is discussed. Also, the distribution patterns of the cellular protein in the NIH c-src overexpresser cells are compared with those of pp60v-src in RSV-transformed cells. The differences observed are discussed in relation with the differences in transforming capacities of the two proteins. Finally, the possible physiological significance of the association between pp60c-src and the structures generated after the binding of Con A to its surface receptors is addressed.

An alternative non-tyrosine protein kinase product of the c-src gene in chicken skeletal muscle

While the c-src locus is expressed as a 4.0-kilobase (kb) mRNA coding for pp60c-src in various chicken tissues, including embryonic muscle, it is expressed as a novel 3.0-kb mRNA in adult skeletal muscle. We have analyzed the primary structure of this alternatively transcribed and spliced c-src mRNA. The sequence revealed three open reading frames, with the previously defined c-src exons 1 through 5 or 6 comprising the third, on the 3' untranslated region of this 3-kb mRNA. The exons coding for the tyrosine kinase domain of pp60c-src were excluded. On the 5' side, 2 kb of sequence upstream from the previously defined exon 1 of the c-src gene was included in this mRNA. The start site for the 3-kb mRNA probably lies downstream of that for the 4-kb mRNA. The first reading frame of the 3.0-kb mRNA, called sur (for src upstream region), encoded a 24-kilodalton (kDa) protein product rich in cysteine and proline residues. In vitro analysis indicated that the 24-kDa sur protein was membrane associated. Antibodies to sur protein detected in vivo a 24-kDa muscle-specific protein which was developmentally regulated and corresponded to the switch from the 4-kb to the 3-kb c-src mRNA. A striking kinetic pattern of appearance of sur protein and disappearance of pp60c-src suggests that the expression of these two proteins is inversely related.

An alternative non-tyrosine protein kinase product of the c-src gene in chicken skeletal muscle

While the c-src locus is expressed as a 4.0-kilobase (kb) mRNA coding for pp60c-src in various chicken tissues, including embryonic muscle, it is expressed as a novel 3.0-kb mRNA in adult skeletal muscle. We have analyzed the primary structure of this alternatively transcribed and spliced c-src mRNA. The sequence revealed three open reading frames, with the previously defined c-src exons 1 through 5 or 6 comprising the third, on the 3' untranslated region of this 3-kb mRNA. The exons coding for the tyrosine kinase domain of pp60c-src were excluded. On the 5' side, 2 kb of sequence upstream from the previously defined exon 1 of the c-src gene was included in this mRNA. The start site for the 3-kb mRNA probably lies downstream of that for the 4-kb mRNA. The first reading frame of the 3.0-kb mRNA, called sur (for src upstream region), encoded a 24-kilodalton (kDa) protein product rich in cysteine and proline residues. In vitro analysis indicated that the 24-kDa sur protein was membrane associated. Antibodies to sur protein detected in vivo a 24-kDa muscle-specific protein which was developmentally regulated and corresponded to the switch from the 4-kb to the 3-kb c-src mRNA. A striking kinetic pattern of appearance of sur protein and disappearance of pp60c-src suggests that the expression of these two proteins is inversely related.

Down regulation by p60v-src of genes specifically expressed and developmentally regulated in postmitotic quail neuroretina cells

The avian neuroretina (NR) is composed of photoreceptors and different neurons that are derived from proliferating precursor cells. Neuronal differentiation takes place after terminal mitosis. We have previously shown that differentiating NR cells can be induced to proliferate by infection with Rous sarcoma virus (RSV) and that cell multiplication requires expression of a functional v-src gene. We speculated that the quiescence of NR cells could be determined by specific genes. Cell proliferation could then result from the negative regulation of these genes by the v-src protein. By differential hybridization of a cDNA library, we isolated eight clones corresponding to genes expressed in postmitotic NR cells from 13-day-old quail embryos, transcriptional levels of which are significantly reduced in NR cells induced to proliferate by tsNY68, an RSV mutant with temperature-sensitive mitogenic activity. Partial sequencing analysis indicated that one RNA encoded the calmodulin gene, whereas the other seven showed no similarity to known sequences. By using v-src mutants that induce NR cell proliferation in the absence of transformation, we showed that transcription of six genes was negatively regulated by the v-src protein and that of four genes was correlated with NR cell quiescence. We also report that a subset of genes are specifically transcribed in neural cells and developmentally regulated in the NR. These results indicate that the v-src protein regulates expression of genes likely to play a role in the control of neural cell growth or differentiation.

Identification of a 32K plasma membrane protein that binds to the myristylated amino-terminal sequence of p60v-src

The transforming protein of Rous sarcoma virus, p60v-src, is a myristylated membrane-bound phosphoprotein. Interaction of p60v-src with the plasma membrane is essential for transforming activity, and is mediated by association with a membrane-bound Src receptor protein. Evidence for the existence of an Src receptor is based on the ability of a myristylated peptide containing the N-terminal Src sequence to inhibit binding of p60v-src to plasma membranes in vitro: binding of p60v-src to a plasma membrane receptor is therefore mediated by N-terminal Src sequences. Here we report that a myristyl-Src peptide, but not the corresponding non-myristylated peptide, can be specifically crosslinked to a plasma membrane protein of relative molecular mass 32,000 (Mr32K). The 32K protein represents an Src-binding protein in the plasma membrane that is likely to be a component of the myristyl-Src receptor, and which could be involved in cellular transformation.

Down regulation by p60v-src of genes specifically expressed and developmentally regulated in postmitotic quail neuroretina cells

The avian neuroretina (NR) is composed of photoreceptors and different neurons that are derived from proliferating precursor cells. Neuronal differentiation takes place after terminal mitosis. We have previously shown that differentiating NR cells can be induced to proliferate by infection with Rous sarcoma virus (RSV) and that cell multiplication requires expression of a functional v-src gene. We speculated that the quiescence of NR cells could be determined by specific genes. Cell proliferation could then result from the negative regulation of these genes by the v-src protein. By differential hybridization of a cDNA library, we isolated eight clones corresponding to genes expressed in postmitotic NR cells from 13-day-old quail embryos, transcriptional levels of which are significantly reduced in NR cells induced to proliferate by tsNY68, an RSV mutant with temperature-sensitive mitogenic activity. Partial sequencing analysis indicated that one RNA encoded the calmodulin gene, whereas the other seven showed no similarity to known sequences. By using v-src mutants that induce NR cell proliferation in the absence of transformation, we showed that transcription of six genes was negatively regulated by the v-src protein and that of four genes was correlated with NR cell quiescence. We also report that a subset of genes are specifically transcribed in neural cells and developmentally regulated in the NR. These results indicate that the v-src protein regulates expression of genes likely to play a role in the control of neural cell growth or differentiation.

The src protein contains multiple domains for specific attachment to membranes

The proteins encoded by the oncogene v-src and its cellular counterpart c-src (designated generically here as pp60src) are tightly associated with both plasma membranes and intracellular membranes. This association is due in part to the amino-terminal myristylation of pp60src, but several lines of evidence suggest that amino-terminal portions of the protein itself are also involved. We now report that pp60src contains at least three domains which, in conjunction with myristylation, are capable of mediating attachment to membranes and determining subcellular localization. We identified these domains by fusing various portions of pp60src to pyruvate kinase, which is normally a cytoplasmic protein. Amino acids 1 to 14 of pp60src are sufficient to mediate both myristylation and the attachment of pyruvate kinase to cytoplasmic granules. In contrast, amino acids 38 to 111 mediate association with the plasma membrane and perinuclear membranes, whereas amino acids 204 to 259 mediate association primarily with perinuclear membranes. We conclude that pp60src contains independent domains that target the protein to distinctive subcellular locations and thus may facilitate diverse biological functions of the protein.

The src protein contains multiple domains for specific attachment to membranes

The proteins encoded by the oncogene v-src and its cellular counterpart c-src (designated generically here as pp60src) are tightly associated with both plasma membranes and intracellular membranes. This association is due in part to the amino-terminal myristylation of pp60src, but several lines of evidence suggest that amino-terminal portions of the protein itself are also involved. We now report that pp60src contains at least three domains which, in conjunction with myristylation, are capable of mediating attachment to membranes and determining subcellular localization. We identified these domains by fusing various portions of pp60src to pyruvate kinase, which is normally a cytoplasmic protein. Amino acids 1 to 14 of pp60src are sufficient to mediate both myristylation and the attachment of pyruvate kinase to cytoplasmic granules. In contrast, amino acids 38 to 111 mediate association with the plasma membrane and perinuclear membranes, whereas amino acids 204 to 259 mediate association primarily with perinuclear membranes. We conclude that pp60src contains independent domains that target the protein to distinctive subcellular locations and thus may facilitate diverse biological functions of the protein.