HALOGENATED BENZIMIDAZOLE INHIBITORS OF PHOSPHORYLATION, IN-VITRO ANDIN-VIVO, OF THE SURFACE ACIDIC PROTEINS OF THE YEAST RIBOSOMAL 60S SUBUNIT BY ENDOGENOUS PROTEIN-KINASES CK-II AND PK60S

Several halogeno benzimidazoles and 2-azabenzimidazoles, previously sh own to be relatively selective inhibitors of protein kinases CK-I and/ or CK-II from various sources, including CK-II from yeast [Szyszka et al. (1995) Biochem. Biophys. Res. Commun. 208, 418-424] inhibit also t he yeast ribosomal protein kinase PK60S. The most effective inhibitor of CK-II and PK60S was tetrabromo-2-azabenzimidazole (TetraBr-2-azaBz) , which was competitive with respect to ATP (and GTP in the case of CM -II) with K-i values of 0.7 mu M for CK-II, and 0.1 mu M for PK60S. PK 60S phosphorylates only three (YP1 beta, YF1 beta',YF2 alpha) out of f ive polypeptides of pp13 kDa acidic proteins of 60S subunit phosphoryl ated by CK-II [Szyszka et al. (1995) Acta Biochim, Polon. 42, 357-362] . Accordingly, TetraBr-2-azaBz inhibits phosphorylation only of these polypeptides, catalysed by PK60S. Addition of TetraBr-2-azaBz to cultu res of yeast cells, at concentrations which were without effect on cel l growth, led to inhibition of intracellular phosphorylation of riboso mal acidic proteins, paralleling that observed in vitro. TetraBr-2-aza Bz is shown to be a useful tool for studies on the intracellular regul ation of phosphorylation of the ribosomal 60S acidic proteins, which a re involved in formation of active ribosomes.