REGULATION OF MICROTUBULE DYNAMICS BY CA2+ CALMODULIN-DEPENDENT KINASE IV GR-DEPENDENT PHOSPHORYLATION OF ONCOPROTEIN-18/

Oncoprotein 18 (Op18; also termed p19, 19K, p18, prosolin, and stathmi n) is a regulator of microtubule (MT) dynamics and is phosphorylated b y multiple kinase systems on four Ser residues, In addition to cell cy cle-regulated phosphorylation, external signals induce phosphorylation of Op18 on Ser-25 by the mitogen-activated protein kinase and on Ser- 16 by the Ca2+/calmodulin-dependent kinase IV/Gr (CaMK IV/Gr). Here we show that induced expression of a constitutively active mutant of CaM K IV/Gr results in phosphorylation of Op18 on Ser-16, In parallel, we also observed partial degradation of Op18 and a rapid increase of tota l cellular MTs, These results suggest a link between CaMK IV/Gr, Op18, and MT dynamics, To explore such a putative link, we optimized a gene tic system that allowed conditional coexpression of a series of CaMK I V/Gr and Op18 derivatives, The result shows that CaMK IV/Gr can suppre ss the MT-regulating activity of Op18 by phosphorylation on Ser-16, In line with these results, by employing a chemical cross-linking protoc ol, it was shown that phosphorylation of Ser-16 is involved in weakeni ng of the interactions between Op18 and tubulin, Taken together, these data suggest that the mechanism of CaMK IV/Gr-mediated suppression of Op18 activity involves both partial degradation of Op18 and direct mo dulation of the MT-destabilizing activity of this protein, These resul ts show that Op18 phosphorylation by CaMK IV/Gr may couple alterations of MT dynamics in response to external signals that involve Ca2+.