Three-dimensional structure of the tyrosine kinase c-Src

SH3-mediated Hck tyrosine kinase activation and fibroblast transformation by the Nef protein of HIV-1

Tyrosine kinases of the Src family are regulated via their Src homology 2 (SH2) and SH3 domains. The Nef protein of human immunodeficiency virus-1 (HIV-1) has previously been shown to bind with high affinity and specificity in vitro to the SH3 domain of Hck, a Src family member expressed primarily in myeloid cells. However, the effect of Nef on Hck activity in living cells is unknown. Here we show that Rat-2 fibroblasts co-expressing Hck and Nef rapidly developed transformed foci, whereas control cells expressing either protein alone did not. Nef formed a stable complex with Hck and stimulated its tyrosine kinase activity in vivo. Mutagenesis of the Nef proline-rich motif essential for SH3 binding completely blocked complex formation, kinase activation, and transformation, indicating that the Nef SH3-binding function is required for its effects on Hck. These results provide direct evidence that SH3 engagement is sufficient to activate a Src family kinase in vivo and suggest that Hck may be activated by Nef in HIV-infected macrophages.

A crystal milestone: the structure of regulated Src

The viral and cellular forms of the Src protein tyrosine kinases take a prototypic role in oncology and signal transduction research, by virtue of being holders of an impressive number of 'firsts'. Our understanding of the biochemistry and physiology of Src has therefore always been used as a reference for our general advancement in the field of protein phosphorylation and growth control. The recent solution of the crystal structure of two members of the Src family represents a milestone in these disciplines and, as usual, provides a general lookout post for developments to come.

Focal adhesion kinase overexpression enhances ras-dependent integrin signaling to ERK2/mitogen-activated protein kinase through interactions with and activation of c-Src

Cell adhesion to extracellular matrix proteins such as fibronectin (FN) triggers a number of intracellular signaling events including the increased tyrosine phosphorylation of the cytoplasmic focal adhesion protein-tyrosine kinase (PTK) and also the stimulation of the mitogen-activated protein kinase ERK2. Focal adhesion kinase (FAK) associates with integrin receptors, and FN-stimulated phosphorylation of FAK at Tyr-397 and Tyr-925 promotes the binding of Src family PTKs and Grb2, respectively. To investigate the mechanisms by which FAK, c-Src, and Grb2 function in FN-stimulated signaling events to ERK2, we expressed wild type and mutant forms of FAK in human 293 epithelial cells by transient transfection. FAK overexpression enhanced FN-stimulated activation of ERK2 approximately 4-fold. This was blocked by co-expression of the dominant negative Asn-17 mutant Ras, indicating that FN stimulation of ERK2 was Ras-dependent. FN-stimulated c-Src PTK activity was enhanced by wild type FAK expression, whereas FN-stimulated activation of ERK2 was blocked by expression of the c-Src binding site Phe-397 mutant of FAK. Expression of the Grb2 binding site Phe-925 mutant of FAK enhanced activation of ERK2, whereas a kinase-inactive Arg-454 mutant FAK did not. Expression of wild type and Phe-925 FAK, but not Phe-397 FAK, enhanced p130(Cas) association with FAK, Shc tyrosine phosphorylation, and Grb2 binding to Shc after FN stimulation. FN-induced Grb2-Shc association is another pathway leading to activation of ERK2 via Ras. The inhibitory effects of Tyr-397 FAK expression show that FAK-mediated association and activation of c-Src is essential for maximal signaling to ERK2. Moreover, multiple signaling pathways are activated upon the formation of an FAK.c-Src complex, and several of these can lead to Ras-dependent ERK2 mitogen-activated protein kinase activation.

Signal transduction: clamping down on Src activity

Recent high-resolution structures of members of the Src family of protein-tyrosine kinases illustrate how a series of cooperative intramolecular interactions represses the catalytic activity of these kinases, but allows for their rapid activation by a variety of regulatory inputs.

Modular peptide recognition domains in eukaryotic signaling

A characteristic feature of cellular signal transduction pathways in eukaryotes is the separation of catalysis from target recognition. Several modular domains that recognize short peptide sequences and target signaling proteins to these sequences have been identified. The structural bases of the specificities of recognition by SH2, SH3, and PTB domains have been elucidated by X-ray crystallography and NMR, and these results are reviewed here. In addition, the mechanism of cooperative interactions between these domains is discussed.

Overexpression of c-src and n-src in the developing Xenopus retina differentially impairs axonogenesis

To compare the roles of the nonreceptor tyrosine kinase c-src and its neuronal splice form n-src in developing neurons, Xenopus retinal precursors were transfected in vivo with c-src, n-src, or constitutively active mutants. Axonogenesis of retinal ganglion cells was markedly impaired by the expression of constitutively active c-src and only mildly affected by the expression of constitutively active n-src. This differential phenotype could not be accounted for by raised levels of intracellular tyrosine phosphorylation alone because the average anti-phosphotyrosine staining intensity of retinal neurons expressing mutant n-src was almost twofold greater than that of neurons expressing mutant c-src. The expression of either constitutively active isoform inhibited photoreceptor differentiation by 72% but did not influence other cell fates. These results suggest that c-src and n-src have both overlapping and distinct activities in differentiating retinal neurons.