Intracellular localization and processing of pp60v-src proteins expressed by two distinct temperature-sensitive mutants of Rous sarcoma virus

Suppression of v-Src transformation by andrographolide via degradation of the v-Src protein and attenuation of the Erk signaling pathway

Elevated expression and aberrant activation of the src oncogene are strongly associated with cancer initiation and progression, thereby making Src a promising molecular target for anti-cancer therapy. Through drug screening using a temperature-inducible v-Src-transformed epithelial cell line, we found that andrographolide could suppress v-Src-induced transformation and down-regulate v-Src protein expression. In addition, actin cable dissolution and E-cadherin down-regulation, features of transformed phenotype, are perturbed by andrographolide. Moreover, andrographolide promoted v-Src degradation via a ubiquitin-dependent manner. Although andrographolide treatment altered the tyrosine phosphorylation pattern in v-Src-expressing cells, it did not directly affect the kinase activity of v-Src. Both the Erk and phosphatidylinositol 3-kinase signaling pathways were strongly inhibited in andrographolide-treated v-Src cells. However, only MKK inhibitors (PD98059 and U0126) were able to cause a non-transformed morphology similar to that of andrographolide-treated v-Src cells. Moreover, overexpression of constitutively active MKK1 in v-Src cells blocked andrographolide-mediated morphological inhibition. Interestingly, andrographolide treatment could also reduce the protein level of the c-Src truncation mutant (Src531), an Src mutant originally identified from human colon cancer cells. In summary, we demonstrated that andrographolide antagonized v-Src action through promotion of v-Src protein degradation. Furthermore, attenuation of the Erk1/2 signaling pathway is essential for andrographolide-mediated inhibition of v-Src transformation. Our results demonstrate that andrographolide can act as a v-Src inhibitor and reveal a novel action mechanism of andrographolide.

PI3K activation is required for PMA-directed activation of cSrc by AFAP-110

Activation of PKCalpha will induce the cSrc binding partner AFAP-110 to colocalize with and activate cSrc. The ability of AFAP-110 to colocalize with cSrc is contingent on the integrity of the amino-terminal pleckstrin homology (PH1) domain, while the ability to activate cSrc is dependent on the integrity of its SH3 binding motif, which engages the cSrc SH3 domain. The outcome of AFAP-110-directed cSrc activation is a change in actin filament integrity and the formation of podosomes. Here, we address what cellular signals promote AFAP-110 to colocalize with and activate cSrc, in response to PKCalpha activation or PMA treatment. Because PH domain integrity in AFAP-110 is required for colocalization, and PH domains are known to interact with both protein and lipid binding partners, we sought to determine whether phosphatidylinositol 3-kinase (PI3K) activation played a role in PMA-induced colocalization between AFAP-110 and cSrc. We show that PMA treatment is able to direct activation of PI3K. Treatment of mouse embryo fibroblast with PI3K inhibitors blocked PMA-directed colocalization between AFAP-110 and cSrc and subsequent cSrc activation. PMA also was unable to induce colocalization or cSrc activation in cells that lacked the p85alpha and -beta regulatory subunits of PI3K. This signaling pathway was required for migration in a wound healing assay. Cells that were null for cSrc or the p85 regulatory subunits or expressed a dominant-negative AFAP-110 also displayed a reduction in migration. Thus PI3K activity is required for PMA-induced colocalization between AFAP-110 and cSrc and subsequent cSrc activation, and this signaling pathway promotes cell migration.

v-Src accelerates spontaneous motility via phosphoinositide 3-kinase, phospholipase C and phospholipase D, but abrogates chemotaxis in Rat-1 and MDCK cells

In Rat-1 fibroblasts, v-Src causes a profound remodelling of cortical actin cytoskeleton. This transformation includes membrane ruffling, a hallmark of the leading edge in migrating cells, and results from activation of phosphoinositide 3-kinase (PI 3-kinase), phospholipase C (PLC) and phospholipase D (PLD). We therefore reexamined whether motility is constitutively triggered by v-Src and studied whether this response is controlled by the same signalling pathway. The study was performed using Rat-1/tsLA29 and MDCK/tsLA31 cells, each harbouring a different thermosensitive v-Src kinase, active at 34 degrees C but inactivated at 40 degrees C. In both cell lines, overnight v-Src activation induced transformation and accelerated spontaneous motility by approximately twofold, as evidenced by wound-healing assay and by single-cell track, time-lapse recording in Dunn chambers. Inhibitors of PI 3-kinase, PLC and PLD selectively abrogated acceleration of motility by v-Src. Since mechanisms that co-ordinate spontaneous, as distinct from oriented, cell migration are separable, we further analysed in Dunn chambers chemotactic response of Rat-1/tsLA29 cells to PDGF and of MDCK/tsLA31 cells to EGF. In both cases, v-Src decreased the steady-state level of growth factor receptors at the cell surface twofold, and abrogated movement directionality at comparable level of occupancy as in non-transformed cells. The burst of pinocytosis in response to growth factors was also abolished by v-Src. Altogether, these results indicate that v-Src triggers motility in a PI 3-kinase-, PLC- and PLD-dependent manner, but abrogates directionality by suppressing polarised signalling downstream of growth factor receptors.

Transforming growth factor-alpha abrogates the glucocorticoid stimulation of tight junction formation and reverses the steroid-induced down-regulation of fascin in rat mammary epithelial tumor cells by a Ras-dependent pathway

In Con8 rat mammary epithelial tumor cells, the synthetic glucocorticoid dexamethasone stimulates transepithelial electrical resistance (TER), promotes the remodeling of apical junctions, and down-regulates the level of fascin, an actin-bundling protein that can bind to beta-catenin. We have previously shown that ectopic expression of fascin prevented the glucocorticoid-mediated recruitment of tight junction and adherens junction proteins to the site of cell-cell contact. Here we demonstrate that exogenous treatment or constitutive production of transforming growth factor-alpha (TGF-alpha) ablated the dexamethasone down-regulation of the fascin protein level and disrupted the dexamethasone-induced remodeling of the apical junction and stimulation of the monolayer TER. The response to TGF-alpha was polarized in that basolateral, but not apical, exposure to this growth factor coordinately reversed the steroid control of fascin production and tight junction formation. Expression of dominant negative RasN17 or treatment with the PD098059 MEK inhibitor abolished or attenuated the TGF-alpha disruptive effects on TER, junction remodeling, and fascin protein levels. Our results implicate the regulation of fascin protein levels as a target of cross-talk between the Ras-dependent growth factor signaling and glucocorticoid signaling pathways that controls tight junction dynamics in mammary epithelial tumor cells. We propose that reversing the down-regulation of fascin is critical for the ability of TGF-alpha to disrupt the glucocorticoid-induced remodeling of the apical junction that leads to tight junction formation.

The p85 subunit of phosphoinositide 3-kinase is associated with beta-catenin in the cadherin-based adhesion complex

Cell adhesion is fundamental to establishing and maintaining the discrete tissues in multicellular organisms. Adhesion must be sufficiently strong to preserve tissue architecture, whilst having the capacity to readily dissociate to permit fundamental processes, such as wound repair, to occur. However, very little is known about the signalling mechanisms involved in temporary down-regulation of cell adhesion to facilitate such processes. Cadherins are the principal mediators of cell-cell adhesion in a wide variety of tissues and species and form multi-protein complexes with cytosolic and cytoskeletal proteins to express their full adhesive capacity. In the present study we report that the p85 subunit of phosphoinositide 3-kinase (PI 3-kinase) is associated with the cadherin-based adhesion complex in human epithelial cells. The interaction of p85 with the complex is via beta-catenin. We also show that the interaction of p85 and beta-catenin is direct, involves the N-terminal Src homology domain 2 of p85 and is regulated by tyrosine phosphorylation. These data suggest that PI 3-kinase may play a role in the functional regulation of the cadherin-based adhesion complex.

A mutant form of the rho protein can restore stress fibers and adhesion plaques in v-src transformed fibroblasts

The organization of polymerized actin in the mammalian cell is regulated by several members of the rho family. Three rho proteins, cdc42, rac and rho act in a cascade to organize the intracellular actin cytoskeleton. Rho proteins are involved in the formation of actin stress fibers and adhesion plaques in fibroblasts. During transformation of mammalian cells by oncogenes the cytoskeleton is rearranged and stress fibers and adhesion plaques are disintegrated. In this paper we investigate the function of the rho protein in RR1022 rat fibroblasts transformed by the Rous sarcoma virus. Two activated mutants of the rho protein, rho G14V and rho Q63L, and a dominant negative mutant, rho N1171, were stably transfected into RR1022 cells. The resulting cell lines were analysed for the organization of polymerized actin and adhesion plaques. Cells expressing rho Q63L, but not rho wt, rho G14V or rho N1171, showed an altered morphology. These cells displayed a flat, fibroblast like shape when compared with the RR1022 ancestor cells. Immunofluorescence analyses revealed that actin stress fibers and adhesion plaques were rearranged in these cells. We conclude from these data that an active rho protein can restore elements of the actin cytoskeleton in transformed cells by overriding the tyrosine kinase phosphorylation induced by the pp60(v-src).

Glucocorticoid down-regulation of fascin protein expression is required for the steroid-induced formation of tight junctions and cell-cell interactions in rat mammary epithelial tumor cells

Glucocorticoid hormones, which are physiological regulators of mammary epithelium development, induce the formation of tight junctions in rat Con8 mammary epithelial tumor cells. We have discovered that, as part of this process, the synthetic glucocorticoid dexamethasone strongly and reversibly down-regulated the expression of fascin, an actin-bundling protein that also interacts with the adherens junction component beta-catenin. Ectopic constitutive expression of full-length mouse fascin containing a Myc epitope tag (Myc-fascin) in Con8 cells inhibited the dexamethasone stimulation of transepithelial electrical resistance, disrupted the induced localization of the tight junction protein occludin and the adherens junction protein beta-catenin to the cell periphery, and prevented the rearrangement of the actin cytoskeleton. Ectopic expression of either the carboxyl-terminal 213 amino acids of fascin, which includes the actin and beta-catenin-binding sites, or the amino-terminal 313 amino acids of fascin failed to disrupt the glucocorticoid induction of tight junction formation. Mammary tumor cells expressing the full-length Myc-fascin remained generally glucocorticoid responsive and displayed no changes in the levels or protein-protein interactions of junctional proteins or the amount of cytoskeletal associated actin filaments. However, a cell aggregation assay demonstrated that the expression of Myc-fascin abrogated the dexamethasone induction of cell-cell adhesion. Our results implicate the down-regulation of fascin as a key intermediate step that directly links glucocorticoid receptor signaling to the coordinate control of junctional complex formation and cell-cell interactions in mammary tumor epithelial cells.

Cooperation of Src homology domains in the regulated binding of phosphatidylinositol 3-kinase. A role for the Src homology 2 domain

Fibroblasts transformed by the v-Src oncoprotein exhibit elevated activity of the enzyme phosphatidylinositol 3'-kinase (PI 3-kinase), which binds to, and is activated by, a wide range of receptor tyrosine kinases as well as v-Src and transforming polyoma middle T/c-Src complexes. Here we consider the role of the v-Src homology (SH) domains, SH3 and SH2, and the tyrosine kinase catalytic domain, in the stimulation of v-Src-associated PI 3-kinase activity in response to rapid activation of the oncoprotein. As shown by others, we find that the v-Src SH3 domain tightly binds the PI 3-kinase p85 regulatory subunit in normal growing chicken embryo fibroblasts. However, we also find that in transformed cells there is additional efficient binding of PI 3-kinase to the v-Src SH2 domain in a catalytically active form. Furthermore, the binding of p85 to the SH2 domain, which is almost undetectable in quiescent cells, is rapidly stimulated upon activation of temperature-sensitive v-Src and consequent cell cycle entry, demonstrating that binding is a target for regulation. We also show that v-Src-associated PI 3-kinase differs considerably from PDGF receptor-associated enzyme by a different mode of binding, a lack of substantial allosteric activation, and a dependence on the tyrosine kinase activity of v-Src. The rapidly induced binding and activation of PI 3-kinase thus provides sensitive regulation of recruitment of PI 3-kinase to its substrates and into other signaling complexes at the cell membrane, which involves all the Src homology domains.

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

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

The ADP/ATP carrier is the 32-kilodalton receptor for an NH2-terminally myristylated src peptide but not for pp60src polypeptide

Membrane binding of pp60src is initiated via its myristylated NH2 terminus. To identify a candidate pp60src docking protein or receptor in the membrane, a radiolabelled peptide corresponding to the pp60src NH2-terminal membrane binding domain was cross-linked to fibroblast membranes and found to specifically label a 32-kDa protein. This protein was purified by appending an affinity tag to the peptide probe so that the cross-linked complex could be isolated via affinity chromatography. Microsequencing indicated that the 32-kDa protein was the mitochondrial ADP/ATP carrier (AAC). This result was further confirmed by the ability of an antibody to the AAC to immunoprecipitate the cross-linked complex, by the ability of certain inhibitors of the AAC to block cross-linking, and by membrane fractionation to show that complex formation occurred essentially exclusively in the mitochondrial fraction. While the AAC bound the myristyl-src peptide in a specific manner both in vitro and in vivo, its localization to the inner membrane of the mitochondrion precludes its being a pp60src binding protein. An analysis of pp60v-src binding in vitro was consistent with this expectation. Thus, use of a myristyl-src peptide revealed an unexpected and previously unidentified binding capacity of the AAC, most likely related to the ability of long-chain fatty acyl coenzyme As to serve as AAC inhibitors. The amphipathic nature of the pp60src NH2 terminus suggests alternative strategies for uncovering pp60src membrane binding species.

The ADP/ATP carrier is the 32-kilodalton receptor for an NH2-terminally myristylated src peptide but not for pp60src polypeptide

Membrane binding of pp60src is initiated via its myristylated NH2 terminus. To identify a candidate pp60src docking protein or receptor in the membrane, a radiolabelled peptide corresponding to the pp60src NH2-terminal membrane binding domain was cross-linked to fibroblast membranes and found to specifically label a 32-kDa protein. This protein was purified by appending an affinity tag to the peptide probe so that the cross-linked complex could be isolated via affinity chromatography. Microsequencing indicated that the 32-kDa protein was the mitochondrial ADP/ATP carrier (AAC). This result was further confirmed by the ability of an antibody to the AAC to immunoprecipitate the cross-linked complex, by the ability of certain inhibitors of the AAC to block cross-linking, and by membrane fractionation to show that complex formation occurred essentially exclusively in the mitochondrial fraction. While the AAC bound the myristyl-src peptide in a specific manner both in vitro and in vivo, its localization to the inner membrane of the mitochondrion precludes its being a pp60src binding protein. An analysis of pp60v-src binding in vitro was consistent with this expectation. Thus, use of a myristyl-src peptide revealed an unexpected and previously unidentified binding capacity of the AAC, most likely related to the ability of long-chain fatty acyl coenzyme As to serve as AAC inhibitors. The amphipathic nature of the pp60src NH2 terminus suggests alternative strategies for uncovering pp60src membrane binding species.

Loss of transformed phenotype upon senescence of Rous sarcoma virus-infected chicken neuroretinal cells

Success in obtaining permanent Rous sarcoma virus-infected chicken cell lines has been limited because of a senescence phenomenon. We show that a diminished, transformed phenotype, followed by dramatic morphological changes, precedes senescence. These changes are associated with continued expression of pp60v-src, as well as specific alterations in expression of two possible phosphorylated substrates of pp60v-src.

Lectin-induced differentiation of transformed neuroretinal cells in vitro

The orderly course of chick neuroretinal cell differentiation was disrupted in vitro by infection with a temperature-sensitive strain of the Rous sarcoma virus (LA29). The resulting cell culture LA29NR remained mitotically active at 42 degrees C, yet rapidly adopted a transformed phenotype upon activation of the pp60v-src oncogene product at 37 degrees C. As a further indication of metabolic state, LA29NR cells expressed the protooncogene product c-Fos, as shown by Western blot analysis. Highly proliferative LA29NR cells proved refractory to standard differentiation agents such as cAMP, and prostaglandin E1. In our novel approach, succinylated concanavalin A (SCA), a nontoxic derivative of the lectin concanavalin A, induced dramatic, reversible morphological changes in LA29NR cells, including neurite outgrowth and increased cell-to-cell adhesion. Fluoresceinated SCA appeared to localize to Golgi and lysosomal structures. Cellular response to SCA treatment included decreased growth rate, reversible decrease in the phosphorylation state of a 41-kDa phosphoprotein, and induction of neuron-specific enolase. The glial marker vimentin was also evident in these cultures. These data suggest that SCA is an effective differentiation agent for cells of neuroectodermal origin, permitting neuronal as well as glial phenotypic expression within these cell populations.

Inhibition of v-src-induced transformation by a GTPase-activating protein

Previous work has shown that microinjection into cells of antibodies against p21ras blocks transformation by src, suggesting that oncogenic transformation by pp60v-src is dependent on p21ras. The activity of p21ras itself is regulated by its cyclic association with GDP-GTP, where p21ras-GTP is the active form and p21ras-GDP is the inactive form. A GTPase-activating protein (GAP) mediates the inactivation of p21ras by facilitating the conversion of the active p21ras-GTP to the inactive p21ras-GDP. This predicts that overexpression of GAP would inactivate p21ras and block transformation of cells by src. In this paper, we confirm this prediction. We report that overexpression of GAP in NIH 3T3 cells blocks transformation by pp60v-src but not by v-ras. Susceptibility to transformation by v-src is restored when GAP expression is lowered to levels comparable to that in control cells. These results support the suggestion that p21ras plays a central role in the signalling pathway used by pp60v-src.

Inhibition of v-src-induced transformation by a GTPase-activating protein

Previous work has shown that microinjection into cells of antibodies against p21ras blocks transformation by src, suggesting that oncogenic transformation by pp60v-src is dependent on p21ras. The activity of p21ras itself is regulated by its cyclic association with GDP-GTP, where p21ras-GTP is the active form and p21ras-GDP is the inactive form. A GTPase-activating protein (GAP) mediates the inactivation of p21ras by facilitating the conversion of the active p21ras-GTP to the inactive p21ras-GDP. This predicts that overexpression of GAP would inactivate p21ras and block transformation of cells by src. In this paper, we confirm this prediction. We report that overexpression of GAP in NIH 3T3 cells blocks transformation by pp60v-src but not by v-ras. Susceptibility to transformation by v-src is restored when GAP expression is lowered to levels comparable to that in control cells. These results support the suggestion that p21ras plays a central role in the signalling pathway used by pp60v-src.

Human cellular src gene product: identification of the myristoylated pp60c-src and blockage of its myristoyl acylation with N-fatty acyl compounds resulted in the suppression of colony formation

A p60K protein in human colon adenocarcinoma tumor cell lines was identified as a myristoylated pp60c-src by fluorography and radioimmunoprecipitation analysis. Prevention of the myristoylation of pp60c-src was determined with N-fatty acyl glycinal compounds. Of the compounds tested, N-myristoyl glycinal diethylacetal, N-lauroyl glycinal diethylacetal, N-myristoyl glycyl glycinal diethylacetal, and N-myristoyl-4-aminobutyl-aldehyde diethylacetal strongly blocked the myristoylation, but N-decanoyl glycinal diethylacetal and N-palmitoyl glycinal diethylacetal did not. The myristoyl blocking compounds depressed colony formation, cell proliferation, and specific localization to the plasma membrane of pp60c-src. The results taken together suggest that myristoylation of the c-src oncogene product may be very important for tumorigenicity of c-src gene expressed cells.

Structural differences between repressed and derepressed forms of p60c-src

The kinase activity of p60c-src is derepressed by removal of phosphate from Tyr-527, mutation of this residue to Phe, or binding of a carboxy-terminal antibody. We have compared the structures of repressed and active p60c-src, using proteases. All forms of p60c-src are susceptible to proteolysis at the boundary between the amino-terminal region and the kinase domain, but there are several sites elsewhere that are more sensitive to trypsin digestion in repressed than in derepressed forms of p60c-src. The carboxy-terminal tail (containing Tyr-527) is more sensitive to digestion by pronase E and thermolysin when Tyr-527 is not phosphorylated. The kinase domain fragment released with trypsin has kinase activity. Relative to intact p60c-src, the kinase domain fragment shows altered substrate specificity, diminished regulation by the phosphorylated carboxy terminus, and novel phosphorylation sites. The results identify parts of p60c-src that change conformation upon kinase activation and suggest functions for the amino-terminal region.

A novel antibody against a synthetic NH2-terminal myristoyl glycyl src peptide can detect pp60v-src, the transforming protein of Rous sarcoma virus

Novel antibodies were raised against a synthetic NH2-terminal myristoyl(Myr-) tetrapeptide(N-Myr-Gly-Ser-Ser-Lys) which is characteristic of an NH2-terminal portion of pp60v-src, the transforming protein of Rous sarcoma virus. Antisera raised against N-Myr-Gly-Ser-Ser-Lys-haemocyanin reacted with 125I-albumin conjugates with N-Myr-Gly-Ser-Ser-Lys. The immunoreaction was competed for by haemocyanin as well as albumin conjugated with this N-Myr-peptide, while underivatized proteins or an NH2-terminal octapeptide (Gly-Ser-Ser-Lys-Ser-Lys-Pro-Lys) had no effect. N-Myr-Gly-Ser-Ser-Lys-(125I)tyramine was also recognized by the antibody. The reaction was competed for by N-Myr-Gly-Ser-Ser-Lys, but not by Gly-Ser-Ser-Lys-Ser-Lys-Pro-Lys. These results suggest a high affinity of the antibody for an N-Myr-peptide moiety. The major (3H)myristate-labelled protein of an apparent molecular weight of 60,000 was detected from chick embryo fibroblasts transformed by Rous sarcoma virus (tsNY68). This protein was demonstrated to possess N-Myr-Gly-Ser-Ser-Lys by the immunoprecipitation and HPLC analyses. Furthermore, the entire circumference of the transformed cells was stained by the antibody upon an immunofluorescent microscopic observation. Thus, these results taken together indicate that the haptenic antibody raised against the myristoyl peptide is useful to detect pp60v-src protein.

Structural differences between repressed and derepressed forms of p60c-src

The kinase activity of p60c-src is derepressed by removal of phosphate from Tyr-527, mutation of this residue to Phe, or binding of a carboxy-terminal antibody. We have compared the structures of repressed and active p60c-src, using proteases. All forms of p60c-src are susceptible to proteolysis at the boundary between the amino-terminal region and the kinase domain, but there are several sites elsewhere that are more sensitive to trypsin digestion in repressed than in derepressed forms of p60c-src. The carboxy-terminal tail (containing Tyr-527) is more sensitive to digestion by pronase E and thermolysin when Tyr-527 is not phosphorylated. The kinase domain fragment released with trypsin has kinase activity. Relative to intact p60c-src, the kinase domain fragment shows altered substrate specificity, diminished regulation by the phosphorylated carboxy terminus, and novel phosphorylation sites. The results identify parts of p60c-src that change conformation upon kinase activation and suggest functions for the amino-terminal region.

Deletions and insertions within an amino-terminal domain of pp60v-src inactivate transformation and modulate membrane stability

We previously showed (V. W. Raymond and J. T. Parsons, Virology 160:400-410, 1987) that variants of the Prague A strain of Rous sarcoma virus containing large deletions impinging on a region of the src gene encoding amino acid residues 143 to 169 were defective for transformation of chicken cells in culture. Here we report that introduction of small (tri-and tetrapeptide) deletions into a region of pp60v-src containing amino acid residues 155 to 175 was found to inactivate transformation. In addition, insertion of four, but not one, amino acid residues at position 161 also inhibited transformation. Biochemical analysis of the src proteins encoded by individual transformation-defective variants revealed that the structural alterations introduced into this domain had only marginal effects upon src tyrosine-specific protein kinase activity. However, the src proteins encoded by defective variants exhibited a significantly shorter half-life within the cell, although these proteins efficiently and rapidly associated with cellular membranes. Our results suggest that the structural domain encompassing residues 155 to 177 may influence the stability of pp60src in the cellular membrane, possibly via the interaction of src with a cellular membrane component(s) or substrate(s).

Identification of new actin-associated polypeptides that are modified by viral transformation and changes in cell shape

By using a monoclonal antibody we have identified a new polypeptide doublet (C4h and C4l) of Mr approximately 21 kD and pI 8 and 7, respectively, that is associated with and (at the immunofluorescence level) uniformly distributed on actin filament bundles in rat, mouse, and other vertebrate species. C4 is absent in neurones, erythrocytes, and skeletal muscle but the epitope is evolutionarily conserved as it is present in invertebrates such as molluscs and crustaceans. C4h is not found in cells such as lymphocytes and oncogenically transformed mesenchymal cells where actin stress fiber bundles are reduced in number or absent. C4l, on the other hand, is always present. C4h expression can also be blocked by switching normal nontransformed mesenchymal cells from adherent to suspension culture. Reexpression of C4h occurs 24 h after these cells are returned to normal adherent culture conditions, but can be blocked by either actinomycin D or cycloheximide, suggesting that the expression of this epitope is regulated at the transcriptional level.

A single point mutation has pleiotropic effects on pp60v-src function

The Rous sarcoma virus mutant tsLA29 encodes a pp60v-src molecule that is temperature sensitive for both tyrosine kinase activity and its ability to locate at the cell periphery. The defect in localization appears to be due to a perturbation in events following complex dissociation, since the mutant enzyme shows a rapidly reversible association with the cytoskeleton when shifted between permissive and restrictive temperatures. Although tsLA29 pp60v-src differs from the wild type at three amino acid residues, studies with chimeric proteins show that only one of the mutations, an alanine-for-proline substitution at residue 507, accounts for all the temperature-sensitive characteristics. Moreover, a single second site mutation, at residue 427, can restore the wild phenotype. Cells infected with a chimeric virus encoding only the alanine substitution at position 507 have a conspicuously fusiform morphology, suggesting that this mutation also has subtle effects on pp60v-src function that are apparently compensated for by the other mutations in native tsLA29.

Elevated phosphatidylinositol kinase activity in Rous sarcoma virus-transformed cells. Lack of evidence for enzyme translocation

Phosphatidylinositol kinase (E.C. 2.7.1.67) activity of rat fibroblasts transformed by Rous sarcoma virus (RSV) was measured and compared with immunoprecipitated protein tyrosine kinase activity associated with pp60v-src. Both enzyme activities were elevated in the particulate fractions from wild-type RSV-transformed cells and cells transformed by a temperature-sensitive mutant of RSV when grown at the permissive temperature. The presence of the non-ionic detergent Nonidet P-40 in the phosphatidylinositol kinase assays stimulated the soluble and particulate forms of the enzyme to different degrees but did not affect the relative differences between transformed and untransformed cells. Our results indicate that phosphatidylinositol kinase activity is a good correlate of RSV transformation and suggest a functional relationship between pp60v-src and phosphatidylinositol kinase.

The cytoskeletal protein vinculin is acylated by myristic acid

In non-muscle cells the mechanism by which microfilament bundles interact with the plasma membrane is unclear. Vinculin, a 130 kDa protein found in adhesion plaques, has been postulated to have a role as a membrane anchor for microfilaments and we have investigated the biochemistry of this molecule in more detail. We report that a fraction of vinculin in chick embryo fibroblasts is acylated by myristic acid. This modification was present in both membrane-bound, cytoskeletal and cytosolic vinculin and thus did not determine preferential subcellular localisation. Myristic acid was also present in vinculin from cells transformed by Rous sarcoma virus.

Analysis of p60v-src mutants carrying lesions involved in temperature sensitivity

Analysis of the src genes of three temperature-sensitive (ts) mutants of Rous sarcoma virus (tsNY68, tsNY72-4, and PA104) showed that each has two C-terminal mutations in the kinase domain required for temperature sensitivity, as assayed by morphological alteration and anchorage-independent growth. In all three mutants, one of the mutations is a valine-to-methionine change at position 461. To assess the contribution of each mutation to the biochemical properties of the src protein, we analyzed the kinase activity and the interaction with cellular proteins p50 and p90 of recombinant src gene products in which only one mutation was combined with wild-type src sequences. Chimeric src protein containing only the Met-461 mutation was indistinguishable from the wild type by all criteria examined, while the effect of the second C-terminal mutation alone varied with the defectiveness of the parental ts mutant. The second mutation alone, while not sufficient to cause ts transformation, altered p60src complex formation with cellular proteins p50 and p90 and altered the in vitro thermolability of src kinase activity. The results indicate that these biochemical properties of p60src are more sensitive to mutation than others, such as in vivo kinase activity, which require more profound structural alterations.