Small deletion in v-src SH3 domain of a transformation defective mutant of Rous sarcoma virus restores wild type transforming properties

The SH3 domain of Src can downregulate its kinase activity in the absence of the SH2 domain-pY527 interaction

The contact between the SH2 domain and the C-terminal tail of c-Src inhibits its kinase activity via a complex network of interactions, including the SH3 domain. We examined the role of the SH3 domain in v-Src, where the C-terminal tail is mutated and unbound. We used the v-Src variants Prague C (PRC) and Schmidt-Ruppin A (SRA), which are of low and high kinase activities, respectively, to measure phosphorylation in vitro by immunoprecipitated kinases produced in Saccharomyces cerevisiae. Swapping the regulatory domains between SRA and PRC revealed that N117D, I96T, and V124L mutations in the n-src- and RT-loops of the SH3 domain of PRC are responsible for the low kinase activity of PRC. Moreover, introducing D117N, R95W, T96I, and L124V into activated c-Src(Y527F) caused a 2.5-fold increase in its activity. The mutations in the CD linker KP249,250DG and L255A, which were shown to activate c-Src, had no effect on the activity of the "SH2-activated" Src kinases. Together our data suggest that in the "SH2-activated" forms of Src, the SH3 domain continues to influence the kinase activity via the direct contacts of the n-src- and RT-loops with the kinase N-terminal lobe.

Human immunodeficiency virus type 1 Nef binds directly to Lck and mitogen-activated protein kinase, inhibiting kinase activity

It is now well established that human immunodeficiency virus type I (HIV-1) Nef contributes substantially to disease pathogenesis by augmenting virus replication and markedly perturbing T-cell function. The effect of Nef on host cell activation could be explained in part by its interaction with specific cellular proteins involved in signal transduction, including at least a member of the src family kinase, Lck, and the serine/threonine kinase, mitogen-activated protein kinase (MAPK). Recombinant Nef directly interacted with purified Lck and MAPK in coprecipitation experiments and binding assays. A proline-rich repeat sequence [(Pxx)4] in Nef occurring between amino acid residues 69 to 78 is highly conserved and bears strong resemblance to a defined consensus sequence identified as an SH3 binding domain present in several proteins which can interact with the SH3 domain of various signalling and cytoskeletal proteins. Binding and coprecipitation assays with short synthetic peptides corresponding to the proline-rich repeat sequence [(Pxx)4] of Nef and the SH2, SH3, or SH2 and SH3 domains of Lck revealed that the interaction between these two proteins is at least in part mediated by the proline repeat sequence of Nef and the SH3 domain of Lck. In addition to direct binding to full-length Nef, MAPK was also shown to bind the same proline repeat motif. Nef protein significantly decreased the in vitro kinase activity of Lck and MAPK. Inhibition of key members of signalling cascades, including those emanating from the T-cell receptor, by the HIV-1 Nef protein undoubtedly alters the ability of the infected T cell to respond to antigens or cytokines, facilitating HIV-1 replication and contributing to HIV-1-induced disease pathogenesis.

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.

Biochemistry of the Src protein-tyrosine kinase: regulation by SH2 and SH3 domains

pp60c-Srs (c-Src) is the prototype for a family of cytoplasmic protein-tyrosine kinases involved in the control of signal transduction. In addition to the enzymatic kinase domain, c-Src has several noncatalytic domains which regulate Src tyrosine kinase activity in both a positive and a negative fashion. Phosphorylation of c-Src at Tyr527 in the noncatalytic C-terminal tail is a key mechanism for repression of c-Src tyrosine kinase activity. This inhibitory phosphorylation is apparently catalyzed by another cytoplasmic tyrosine kinase (Csk). Recent evidence suggests that the c-Src SH2 domain participates in this phosphorylation-dependent repression of kinase activity through an intramolecular association with the phosphotyrosine-containing C-terminus. The SH3 domain of c-Src also negatively regulates c-Src tyrosin kinase activity, although the mechanism is as yet unknown. However, in the background of constitutively active transforming Src variants, such as a c-Src mutant with an amino acid substitution eliminating Tyr527 (527F c-Src) or the retroviral oncogene v-src product pp60v-src (v-Src), both the SH2 and SH3 domains contribute positively to the enzymatic and biological activities of the Src tyrosine kinase through interactions with Src substrates and/or cellular regulators.