Regulation of NDR2 protein kinase by multi-site phosphorylation and the S100B calcium-binding protein.

Nuclear Dbf2-related (NDR) protein kinases are a family of AGC group kinases that are involved in the regulation of cell division and cell morphology. We describe the cloning and characterization of the human and mouse NDR2, a second mammalian isoform of NDR protein kinase. NDR1 and NDR2 share 86% amino acid identity and are highly conserved between human and mouse. However, they differ in expression pattern; mouse Ndr1 is expressed mainly in spleen, lung and thymus, whereas mouse Ndr2 shows highest expression in the gastrointestinal tract. NDR2 is potently activated in cells following treatment with the protein phosphatase 2A inhibitor okadaic acid, which also results in phosphorylation on the activation segment residue Ser-282 and the hydrophobic motif residue Thr-442. We show that Ser-282 becomes autophosphorylated in vivo, whereas Thr-442 is targeted by an upstream kinase. This phosphorylation can be mimicked by replacing the hydrophobic motif of NDR2 with a PRK2-derived sequence, resulting in a constitutively active kinase. Similar to NDR1, the autophosphorylation of NDR2 protein kinase was stimulated in vitro by S100B, an EF-hand Ca(2+)-binding protein of the S100 family, suggesting that the two isoforms are regulated by the same mechanisms. Further we show a predominant cytoplasmic localization of ectopically expressed NDR2.