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

Tyrosine phosphorylation of missing in metastasis protein is implicated in platelet-derived growth factor-mediated cell shape changes

Missing in metastasis gene, or MTSS1, encodes an intracellular protein that is implicated in actin cytoskeleton reorganization and often down-regulated in certain types of tumor cells. In response to platelet-derived growth factor (PDGF), green fluorescent protein (GFP)-tagged murine Mtss1 (Mtss1-GFP) underwent redistribution from the cytoplasm to dorsal membrane ruffles along with phosphorylation at tyrosine residues in a time-dependent manner. Tyrosine phosphorylation of Mtss1-GFP was also elevated in cells where an oncogenic Src was activated but severely impaired in Src knock-out cells or cells treated with Src kinase inhibitor PP2. Mutagenesis analysis has revealed that phosphorylation occurs at multiple sites, including tyrosine residues Tyr-397 and Tyr-398. Mutation at both Tyr-397 and Tyr-398 abolished the PDGF-mediated tyrosine phosphorylation. Furthermore, recombinant Mtss1 protein was phosphorylated by recombinant Src in a manner dependent on Tyr-397 and Tyr-398. Efficient tyrosine phosphorylation of Mtss1 in response to PDGF also involves a coiled-coil domain, which is essential for a proper distribution to the cell leading edge and dorsal ruffles. Interestingly, overexpression of wild type Mtss1-GFP promoted the PDGF-induced dorsal ruffling, whereas overexpression of a mutant deficient in phosphorylation at Tyr-397 and Tyr-398 or a mutant with deletion of the coiled-coil domain impaired the formation of dorsal ruffles. These data indicate that Mtss1 represents a novel signaling pathway from PDGF receptor to the actin cytoskeleton via Src-related kinases.

The SH4-Unique-SH3-SH2 domains dictate specificity in signaling that differentiate c-Yes from c-Src

c-Src and c-Yes are highly homologous members of the Src family of non-receptor tyrosine kinases. The overall sequence similarity between c-Src and c-Yes allows them to perform many overlapping functions. However, the phenotypes of the c-src and c-yes knockout mice, and cells derived from them, are quite different, indicating functional specificity between the two proteins. Specifically, c-src-/- cells are deficient in several processes that require dynamic regulation of the actin cytoskeleton. In order to begin to understand why c-Yes is unable to compensate for c-Src signaling, we used a series of Src/Yes chimeras in which the non-catalytic functional domains of Src527F were replaced by those of c-Yes. Using chicken embryo fibroblasts as a model system, our results indicate that the c-Yes N-terminal SH4-Unique domains are sufficient to inhibit the ability of Src527F to alter cell morphology, induce actin filament rearrangements or stimulate motility or invasive potential. The data also indicate that the SH4-Unique-SH3-SH2 domains of c-Yes work cooperatively and prevent activation of signaling proteins associated with Src527F transformation, including activation of phosphatidylinositol 3-kinase, phosphorylation of c-Raf and Akt and downregulation of RhoA-GTP. These data indicate that c-Yes may not modulate signals associated with c-Src-induced changes in actin filament integrity and may explain why c-Yes fails to compensate for c-Src signaling in src-/- cells.

Distinctive regulation of v-Src-associated phosphatidylinositol 3-kinase during PC12 cell differentiation

In chicken embryo fibroblasts, the binding of v-Src to PtdIns 3-kinase requires Src homology domains, SH3, SH2 and the SH1 or kinase domain, which induces the cytoskeletal disruption associated with fibroblast transformation. In the rat phaeochromocytoma PC12 cell line, v-Src has a different effect on the cytoskeleton, inducing neurite extension rather than cytoskeletal disruption. Here we show that v-Src-induced neurite outgrowth is suppressed by the selective PtdIns 3-kinase inhibitor LY294002, suggesting that this effect of v-Src in PC12 cells also requires the activity of the lipid kinase. However, in contrast with chicken embryo fibroblasts, the association of PtdIns 3-kinase with v-Src in PC12 cells is delayed until several hours after activating the v-Src tyrosine kinase. Furthermore the v-Src-associated p85 regulatory subunit of PtdIns 3-kinase is not phosphorylated on tyrosine in PC12 cells and associates only weakly with isolated v-Src homology domains (SH3/SH2) in a Src kinase-independent manner. However, p85 and v-Src both associate with an unidentified protein (of molecular mass approx. 68 kDa; termed p68), which becomes tyrosine phosphorylated concomitantly with the association of both p85 and PtdIns 3-kinase with v-Src in PC12 cells. Thus we conclude that the mode of regulation of v-Src-associated PtdIns 3-kinase is cell-context-dependent and that p68 might act as an adaptor protein to mediate the association of p85 and v-Src in PC12 cells. The different regulation of PtdIns 3-kinase in PC12 and in chicken embryo fibroblasts in response to v-Src activity might reflect the different cytoskeletal rearrangements induced by this oncoprotein in the two cell types.

The role of the Src homology domains in morphological transformation by v-src

The Src homology (SH2 and SH3) domains of v-Src are required for transformation of Rat-2 cells and for wild-type (morphr) transformation of chicken embryo fibroblasts (CEFs). We report herein that the N-terminal domains of v-Src, when expressed in trans, cannot complement the transformation defect of a deletion mutant lacking the "unique," SH3, and SH2 regions. However, the same regions of Src can promote transformation when translocated to the C terminus of v-Src, although the transformation of CEFs is somewhat slower. We conclude that the SH3 and SH2 domains must be present in cis to the catalytic domain to promote transformation but that transformation is not dependent on the precise intramolecular location of these domains. In CEFSs and in Rat-2 cells, the expression of wild-type v-Src results in tyrosine phosphorylation of proteins that bind to the v-Src SH3 and SH2 domains in vitro; mutations in the SH2 or SH3 and SH2 domains prevent the phosphorylation of these proteins. These findings are most consistent with models in which the SH3 and SH2 domains of v-Src directly or indirectly target the catalytic domain to substrates involved in transformation. However, the N-terminal domains of v-Src can promote tyrosine phosphorylation of certain proteins, in particular p130Cas, even when expressed in the absence of the catalytic domain, indicating that the N-terminal domains of v-Src have effects that are independent of the catalytic domain.

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.

v-Src induces constitutive macropinocytosis in rat fibroblasts

The role of v-Src as regulator of fluid-phase pinocytosis was investigated in Rat-1 cells expressing a stable (Rat-1/BB16) or a thermosensitive (Rat-1/tsLA29) v-Src protein. In the second cell line, this protein is inactive when cells are cultured at 40 degrees C but recovers its tyrosine kinase activity upon transfer to 34 degrees C, resulting into a transformed phenotype. The rate of fluid-phase pinocytosis of the tracer horseradish peroxidase was 2-fold higher in v-Src-transformed fibroblasts (Rat-1/BB16, Rat-1/tsLA29 cultured at 34 degrees C) as compared to non-transformed cells (Rat-1, Rat-1/tsLA29 kept at 40 degrees C). In contrast, receptor-mediated endocytosis of transferrin was poorly affected, suggesting that structures distinct from clathrin-coated pits are involved in pinocytosis stimulation. By light and electron microscopy, transformed cells frequently contained large peroxidase-labeled pinocytic vesicles located near to membrane ruffles, demonstrating that stimulation of pinocytosis corresponds to induction of constitutive macropinocytosis. Stimulation of pinocytosis occurred more than 8 hours after transfer to the permissive temperature, whereas transfer to the non-permissive temperature partially reversed the stimulation within 2 hours. Protein synthesis inhibition for 6 hours abrogated pinocytosis stimulation in transformed cells, indicating that constitutive macropinocytosis induced by v-Src depends on continuous synthesis of a short-lived regulatory machinery.

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.