A ubiquityl-calmodulin synthetase that effectively recognizes the Ca(2+)-free form of calmodulin

Ubiquityl-calmodulin synthetase (uCaM-synthetase) activity as detected in reticulocyte lysate and the crude extracts of rabbit tissues [FEBS Lett. 294 (1991) 229-233] has been well characterized as being essentially Ca(2+)-dependent (-Ca2+/+Ca2+ activity ratio: 0.15-0.2). However, during the purification of this enzyme on ubiquitin-Sepharose the Ca(2+)-dependent activity is lost and an essentially Ca(2+)-independent enzyme (-Ca2+/+Ca2+ activity ratio: 1.0-1.5) is obtained which was purified 90-fold (uCaM-Syn F1) to a final specific activity of 0.32 pkat/mg. During the purification procedure a second protein factor (uCaM-Syn F2) was isolated that has no catalytic activity by itself but restores Ca2+ dependence to the uCaM-Syn F1 fraction (-Ca2+/+Ca2+ activity ratio: 0.1) and enhances the catalytic activity in uCaM-Syn F1 in the presence of Ca2+ over 40-fold. It is concluded that several (possibly interdependent) forms of uCaM-synthetase exist which display different substrate specificities for calmodulin.