VIMENTIN SERVES AS A PHOSPHATE SINK DURING THE APPARENT ACTIVATION OFPROTEIN-KINASES BY OKADAIC ACID IN MAMMALIAN-CELLS

The vimentin contents of four mammalian cell lines originating from ra t and human tissues were determined by immunoblotting and scanning den sitometry. On per cell volume basis, vimentin content in 9L, KD, and H eLa cells was found to be 206.6, 151.6, and 19.1 ng/mul, respectively. A431 cells were devoid of vimentin. Protein phosphorylation was augme nted by treatment of 600 nM okadaic acid for 1 h in these cells. Durin g the apparent activation of protein kinases, vimentin became hyperpho sphorylated and the phosphorylation level of other nonvimentin phospho proteins was relatively little affected in 9L and KD cells. In contras t, cytokeratins and other nonvimentin proteins were heavily phosphoryl ated in OA-treated HeLa and A431 cells. Regression analysis indicated that the relative increase in phosphorylation level of nonvimentin pho sphoproteins was inversely correlated to the contents of vimentin in t he four cell lines [r2 = -0.985]. These observations strongly suggest that vimentin acts as a phosphate sink by which the effects of ''exces s kinase activity'' inflicted by phosphatases inhibition was attenuate d. (C) 1993 Wiley-Liss, Inc.