Cloning and characterization of RLPK, a novel RSK-related protein kinase

A novel protein kinase whose activity can be stimulated by mitogen in vivo was cloned and characterized. The cDNA of this gene encodes an 802-amino ac id protein (termed RLPK) with the highest homology (37% identity) to the tw o protein kinase families, p90(RSK) and p70(RSK). Like p90(RSR), but not p7 0(RSK), RLPK also contains two complete nonidentical protein kinase domains . RLPK mRNA is widely expressed in all human tissues examined and is enrich ed in the brain, heart, and placenta. In HeLa cells, transiently expressed epitope-tagged RLPK can be strongly induced by epidermal growth factor, ser um, and phorbol 12-myristate 13-acetate, but only moderately up-regulated b y tumor necrosis factor-ct and other stress-related stimuli. The activity o f RLPK stimulated by epidermal growth factor was not inhibited by several k nown protein kinase C inhibitors nor by rapamycin, a known specific inhibit or for p70(RSK), but could be inhibited by herbimycin A, a tyrosine kinase inhibitor, and partially inhibited by PD98059 or SB203580, inhibitors for t he mitogen-activated protein kinase pathways. Recombinant RLPK possesses hi gh phosphorylation activity toward histone 2B and the S6 peptide, RRRLSSLRA . Although purified recombinant RLPK can be phosphorylated by ERK2 and p38 alpha in vitro, its activity is not affected by this phosphorylation. Moreo ver, the treatment of RLPK with acid phosphatase did not reduce its in vitr o kinase activity. These data suggest that RLPK is structurally similar to previously isolated RSKs, but its regulatory mechanism may be distinct from either p70(RSK) or p90(RSK)S.