MOLECULAR-CLONING OF SKAP55, A NOVEL PROTEIN THAT ASSOCIATES WITH THEPROTEIN-TYROSINE KINASE P59(FYN) IN HUMAN T-LYMPHOCYTES

In human T-lymphocytes the Src family protein tyrosine kinase p59(fyn) associates with three phosphoproteins of 43, 55, and 85 kDa (pp43, pp 55, and pp85), Employing a GST-Fyn-Src homology 2 (SH2) domain fusion protein pp55 was purified from lysates of Jurkat T-cells, Molecular cl oning of the pp55 cDNA reveals that the pp55 gene codes for a so far n ondescribed polypeptide of 359 amino acids that comprises a pleckstrin homology domain, a C-terminal SH3 domain, as well as several potentia l tyrosine phosphorylation sites, among which one fulfills the criteri a to bind Src-like SH2 domains with high affinity, Consistent with thi s observation, pp55 selectively binds to isolated SH2 domains of Lck, Lyn, Src, and Fyn but not to the SH2 domains of ZAP70, Syk, Shc, SLP-7 6, Grb2, phosphatidylinositol 3-kinase, and c-abl in vitro, Based on t hese properties the protein was termed SKAP55 (src kinase-associated p hosphoprotein of 55 kDa). Northern blot analysis shows that SKAP55 mRN A is preferentially expressed in lymphatic tissues, SKAP55 is detected in resting human T-lymphocytes as a constitutively tyrosine phosphory lated protein that selectively interacts with p59(fyn). These data sug gest that SKAP55 represents a novel adaptor protein likely involved in Fyn-mediated signaling in human T-lymphocytes.