The inhibitory effect of actin on protein phosphorylation by three distinct protein kinases (CK-II, A-kinase and MAP-kinase) was examined in vitro. It was found that: (i) actin inhibits the activities of α-monomeric CK-II (CK-IIα) as well as oligomeric CK-II (α2β2) in a dose-dependent manner, but has no effect on the activities of the two other kinases; and (ii) actin-induced inhibition of CK-II activity is due to the binding of actin to the α-subunit of CK-II and is non-competitive with its phosphate acceptors. In addition, it is demonstrated that actin binds directly to CK-II: both actin and CK-II are coprecipitated by anti-serum against Drosophila CK-IIβ or by specific IgG against Ascaris suum muscle actin. The results presented here suggest that actin can suppress CK-II-mediated signal transduction.
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