SELECTIVE UBIQUITINATION OF CALMODULIN BY UBC4 AND A PUTATIVE UBIQUITIN PROTEIN LIGASE (E3) FROM SACCHAROMYCES-CEREVISIAE

A putative ubiquitin protein ligase (E3-CaM) which cooperates with UBC 4 in selectively ubiquitinating calmodulin has been partially purified from Saccharomyces cerevisiae. Ca2+ was required for this activity an d monoubiquitinated calmodulin was the main product of the reaction. T he apparent K(m) of E3-CaM for calmodulin was approximately 1 muM whic h is of the same order of magnitude as the concentration of calmodulin in yeast cells. Proteins which are good substrates for other E3s (E3a lpha or E3-R) were not ubiquitinated by E3-CaM. Lower but significant activities of E3-CaM were observed when UBC1 replaced UBC4.