PHOSPHORYLATION OF RAT-LIVER HETEROGENEOUS NUCLEAR RIBONUCLEOPROTEIN-A2 AND RIBONUCLEOPROTEIN-C CAN BE MODULATED BY CALMODULIN

It was previously reported that the phosphorylation of three proteins of 36, 40 to 42, and 50 kDa by casein kinase 2 is inhibited by calmodu lin in nuclear extracts from rat liver cells (R, Bosser, R. Aligue, D, Guerini, N, Agell, E, Carafoli, and O. Bachs, J. Biol. Chem. 268:1547 7-15483, 1993), By immunoblotting, peptide mapping, and endogenous pho sphorylation experiments, the 36- and 40 to 42-kDa proteins have been identified as the A2 and C proteins, respectively, of the heterogeneou s nuclear ribonucleoprotein particles, To better understand the mechan ism by which calmodulin inhibits the phosphorylation of these proteins , they were purified by using single-stranded DNA chromatography, and the effect of calmodulin on their phosphorylation by casein kinase 2 w as analyzed, Results revealed that whereas calmodulin inhibited the ph osphorylation of purified A2 and C proteins in a Ca2+-dependent manner , it did not affect the casein kinase 2 phosphorylation of a different protein substrate, i.e., beta-casein. These results indicate that the effect of calmodulin was not on casein kinase 2 activity but on speci fic protein substrates. The finding that the A2 and C proteins can bin d to a calmodulin-Sepharose column in a Ca2+-dependent manner suggests that this association could prevent the phosphorylation of the protei ns by casein kinase 2. Immunoelectron microscopy studies have revealed that such interactions could also occur in vivo, since calmodulin and A2 and C proteins colocalize on the ribonucleoprotein particles in ra t liver cell nuclei.