Selective ubiquitination of calmodulin by UBC4 and a putative ubiquitin protein ligase (E3) from Saccharomyces cerevisiae

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Abstract
A putative ubiquitin protein ligase (E3‐CaM) which cooperates with UBC4 in selectively ubiquitinating calmodulin has been partially purified from Saccharomyces cerevisiae. Ca2+ was required for this activity and monoubiquitinated calmodulin was the main product of the reaction. The apparent K m of E3‐CaM for calmodulin was approximately 1 μM which is of the same order of magnitude as the concentration of calmodulin in yeast cells. Proteins which are good substrates for other E3s (E3α or E3‐R) were not ubiquitinated by E3‐CaM. Lower but significant activities of E3‐CaM were observed when UBC1 replaced UBC4.