THE SPLEEN PROTEIN-TYROSINE KINASE TPK-IIB IS HIGHLY SIMILAR TO THE CATALYTIC DOMAIN OF P72(SYK)

TPK-IIB is a protein kinase that is predominant in the cytosol of sple en and is characterized by a high specific activity toward acidic pept ide substrates and a low auto-phosphorylation activity. A prominent 52 -kDa component purifies with the kinase [Marin, O., Donella-Deana, A., Brunati, A. M., Fischer, S. & Pinna, L. A. (1991) J. Biol. Chem. 266, 17798-17803]. Here we demonstrate that the 52-kDa protein displays se quence identity with the Miller-Dieker lissencephaly protein (LIS 1). The protein is not related to any known protein kinase and lacks an AT P-binding motif. The ATP binding and phosphotransferase activities of TPK-IIB can be fully accounted for by a minor 38-kDa protein band (p38 /TPK-IIB) which can be separated from the 52-kDa protein by Mono-Q/FPL C in the presence of EDTA. Sequence analysis of p38/TPK-IIB reveals a high level of similarity, if not identity: with the catalytic domain o f p72(syk), a protein-tyrosine kinase implicated in the activation of hematopoietic cells. Antibodies raised against the catalytic domain of p72(syk), but not antibodies raised against its N-terminal segment, c ross-react with p38/TPK-IIB. The peptide substrate specificity of p72( syk) is almost identical to that of p38/TPK-IIB, which also supports t he classification of TPK-IIB as a close relative (possibly a proteolyt ic or alternative spliced form) of p72(syk). p38/TPK-IIB, however, exh ibits a specific activity which is sixfold higher than that of p72(syk ) and appears to be 50-fold more sensitive to inhibition by heparin. T hus, the observation that Ca2+-dependent degradation of p72(syk) by pa rticulate fraction of rat spleen is accompanied by increased catalytic activity and increased sensitivity to heparin would be consistent wit h the possibility that hyperactive p38/TPK-IIB might be proteolyticall y generated from p72(syk) in response to an Increase of intracellular Ca2+.