SH2 domains mediate the sequential phosphorylation of HS1 protein by p72syk and Src-related protein tyrosine kinases.

The protein tyrosine kinase p72syk readily phosphorylates hematopoietic linkage cell-specific protein p50/HS1 with high stoichiometry (up to 4 mol of Pi/mol of protein) and favorable kinetic constants (Km 77 nM, kcat 0.37 s-1), at sites that display the motif that is specifically recognized by the HS2 domains of Src tyrosine kinases. Such a phosphorylation converts p50/HS1 into a good substrate for c-Fgr, which in contrast is nearly inactive on nonphosphorylated p50/HS1. A phosphopeptide reproducing one of the main p50/HS1 site affected by p72syk, but neither its dephosphorylated derivative nor other phosphopeptides with different structure, blocks the secondary phosphorylation of phospho(p50/HS1) by c-Fgr but not its primary phosphorylation by p72syk. It also prevents the coimmunoprecipitation of phospho(HS1) with c-Fgr by anti-(c-Fgr) antibodies. In contrast the HS1[393-402] phosphopeptide is ineffective on the kinase activity of c-Fgr when tested with peptide substrates, showing that inhibition of p50/HS1 phosphorylation is not exerted at the catalytic site of c-Fgr. The sequential phosphorylation of p50/HS1 as well as its specific blockage by the HS1 phosphopeptide is also observable if c-Fgr is replaced by two other Src-related kinases, namely, Lyn and Fyn, as secondary phosphorylating agents. None of these Src-related kinases, however, can carry out the phosphorylation of p50/HS1 at the sites affected by p72syk, even after prolonged incubation. Our data suggest that sequential phosphorylation might represent a general mechanism by which p72syk and other Syk-related kinases generate substrates for Src-related protein tyrosine kinases. They also show that sequential phosphorylation (requiring the concerted action of a primary and a secondary kinases) cannot be surrogated by "processive" phosphorylation where a single kinase catalyzes both the primary and secondary phosphorylation, although both these modes of multiple phosphorylation are based on interactions between SH2 domains of the kinases and phosphotyrosyl sites of the substrate.