The S18 ribosomal protein is a putative substrate for Ca2+/calmodulin-activated protein kinase II.

The delta-isoform of Ca(2+)/calmodulin-activated protein kinase II (CaMK II) is abundantly expressed in vascular smooth muscle, but relatively little is known about its regulation or its potential cellular substrates. There are few, if any, known substrates of CaMK II that are physiologically relevant in vascular smooth muscle cells. Studies presented earlier (Mishra-Gorur, K., Singer, H. A., and Castellot, J. J., Jr. (2002) Am. J. Pathol., in press) by our laboratory show an inhibitory effect of heparin on CaMK II phosphorylation and activity. During these studies we observed the specific co-immunoprecipitation of a 20-kDa protein with CaMK II. Purification and sequence analysis indicate that this protein is the S18 protein of the 40 S ribosome. S18 was found to be abundantly phosphorylated in response to serum treatment, and this effect was strongly inhibited by heparin. In addition, KN-93, a specific CaMK II inhibitor, blocks S18 phosphorylation in vascular smooth muscle cells; a concomitant 24% reduction in protein synthesis was observed. Taken together these data support the idea that S18 could be a novel substrate for CaMK II, thus providing a potential link between Ca(2+)-mobilizing agents and protein translation.