IDENTIFICATION AND CHARACTERIZATION OF SPRK, A NOVEL SRC-HOMOLOGY-3 DOMAIN-CONTAINING PROLINE-RICH KINASE WITH SERINE THREONINE KINASE-ACTIVITY/

Protein kinases play important roles in the growth and differentiation of cells. We have isolated cDNA clones from the human megakaryocytic cell line CMK11-5 that encode a novel protein kinase, which we call SP RK (src-homology 3 (SH3) domain containing proline-rich kinase). The g ene sequence predicts an 847-amino acid protein kinase with a unique d omain arrangement. An amino-terminal glycine-rich region is followed b y an SH3 domain and a kinase domain that is similar to both tyrosine a nd serine/threonine kinases. Adjacent to the kinase domain are two clo sely spaced leucine/isoleucine zipper motifs and a stretch of basic am ino acids that resembles karyophilic nuclear localization signals. The COOH-terminal half of SPRK is basic, and proline accounts for 24% of the COOH-terminal 216 amino acids. The sprk gene is widely expressed a s a 4-kilobase transcript in adult and fetal human tissues. Transfecti on of 293 cells with a vector encoding an epitope-tagged SPRK results in the expression of a 95-kDa protein. The epitope-tagged SPRK becomes phosphorylated on serine and threonine residues in an in vitro kinase assay, whereas SPRK variants with point mutations in the predicted AT P-binding site fail to become phosphorylated. These data indicate that SPRK has serine/threonine kinase activity. The SH3 domain of SPRK is interrupted by a unique 5-amino acid insert whose location in the SH3 consensus sequence is the same as that of the inserts found in the SH3 domains of neuronal SRC and of the p85 subunit of phosphatidylinosito l 3-kinase.