MOLECULAR-CLONING OF A 2ND FORM OF RAC PROTEIN-KINASE

A novel serine/threonine protein kinase (termed rac-PK) has recently been identified and cloned from cDNA libraries derived from the human cell lines MCF-7 and Wl38. A second form of this protein kinase, termed rac protein kinase-beta, has been identified from cDNAs derived from the same cell lines. These two closely related forms show 90% homology, although the beta-form with a predicted M(r) 60 200 has a carboxyl terminal extension of 40 amino acids in comparison to the alpha-form. This extension has a high serine content with 11 serine residues in the last 30 amino acids. The beta-form of the protein has been shown by both in vitro translation and bacterial expression to be approximately 5000 Da larger than the alpha-form. rac protein kinase-beta is encoded by a 3.4-kb transcript and the alpha-form is encoded by a 3.2-kb mRNA. Using gene-specific probes both transcripts were detected in all cell types analyzed, although levels of expression were different for the two forms. The catalytic domain of rac protein kinase-beta shows a high degree of homology to both the protein kinase C and cyclic AMP-dependent protein kinase families, and hence rac protein kinases appear to represent a new subfamily of the second messenger serine/threonine protein kinases.