The Spleen Protein‐Tyrosine Kinase TPK‐IIB is Highly Similar to the Catalytic Domain of p7Psyk

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Abstract
TPK‐IIB is a protein kinase that is predominant in the cytosol of spleen and is characterized by a high specific activity toward acidic peptide 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 sequence identity with the Miller‐Dieker lissencephaly protein (LIS 1). The protein is not related to any known protein kinase and lacks an ATP‐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/FPLC in the presence of EDTA. Sequence analysis of p38/TPK‐IIB reveals a high level of similarity, if not identity, with the catalytic domain of p72syk, a protein‐tyrosine kinase implicated in the activation of hematopoietic cells. Antibodies raised against the catalytic domain of p72syk, but not antibodies raised against its N‐terminal segment, cross‐react with p38/TPK‐IIB. The peptide substrate specificity of p72syk is almost identical to that of p38/TPK‐IIB, which also supports the classification of TPK‐IIB as a close relative (possibly a proteolytic or alternative spliced form) of p72syk. p38/TPK‐IIB, however, exhibits a specific activity which is sixfold higher than that of p72syk and appears to be 50‐fold more sensitive to inhibition by heparin. Thus, the observation that Ca2+‐dependent degradation of p72syk by particulate fraction of rat spleen is accompanied by increased catalytic activity and increased sensitivity to heparin would be consistent with the possibility that hyperactive p38/TPK‐IIB might be proteolytically generated from p72syk in response to an increase of intracellular Ca2+.