Functional analysis of a Novel Human Protein Kinase, Nori-2p (MRPK) in the Proliferation of Cancer Cells

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A novel human serine/threonine kinase was identified in a Lymphokine-activated T-killer (T-LAK) cell subtraction library and full length cDNA of both human and mouse were cloned. This protein kinase, namely membrane lymphotoxin-related protein kinase (MRPK/Nori-2p), possesses a serine/threonine kinase motif and appears to be a homologue of a yeast serine/threonine kinase, YGR262c. Similar with YGR 262c, MRPK fails to have some typical sequence patterns of protein kinase but it exerts casein phosphorylation in vitro. The expression of MRPK mRNA was noted markedly in the testicular tissue. The activated but not deactivated form of T-LAK cells express MRPK mRNA.Natural MRPK transcript was identified in some cancer cell lines along with the mRNA expression. MRPK indicated to have a bipartite nuclear localization signal (NLS_BP) that does not exist in the yeast YGR262c. Immunohistochemical study showed that MRPK localizes in the nucleus. A complementation assay using YGR262c-disrupted yeast showed that MRPK may not be a functional homologue of YGR262c. A synchronized cell cycle study showed that MRPK mRNA appeared only in the S-phase. These results suggest that a novel protein kinase, MRPK, may play some important roles in the proliferation of IL-2 driven T-LAK cells and some cancer cells.