The
cDNA clone encoding a novel
isoform of
protein kinase PKN, termed
PKNbeta, was isolated from a HeLa cDNA library.
PKNbeta had high sequence homology with PKNalpha, originally isolated PKN, especially in the repeats of charged
amino acid-rich region with leucine-zipper like sequences (CZ region/HR1), in the carboxyl-terminal catalytic domain, and in approximately 130
amino acid stretch (D region/HR2), located between CZ region/HR1 and the catalytic domain. However, the amino acid sequence of
PKNbeta differed from that of PKNalpha in the region immediately amino-terminal to the catalytic domain, which contained two distinct
proline-rich sequences consistent with the class II consensus sequence, PXXPXR, for binding to SH3 domain. Distribution of
PKNbeta differed from that of PKNalpha in the following two respects: (1) Northern blotting indicated that
PKNbeta mRNA could not be detected in human adult tissues, but was expressed abundantly in human
cancer cell lines; (2) immunochemical analysis indicated that
PKNbeta localized in nucleus and perinuclear Golgi apparatus, and was almost absent in cytoplasmic region in NIH3T3 cells. Recombinant
PKNbeta expressed in COS7 cells displayed autophosphorylation and
peptide kinase activity, but was found to be significantly less responsive to
arachidonic acid than PKNalpha. The identification of this novel
isoform underscores the diversity of PKN signaling pathway.