The SCF Cdc4 ubiquitin ligase regulates calcineurin signaling through degradation of phosphorylated Rcn1, an inhibitor of calcineurin

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Abstract
The highly conserved RCN family of proteins regulates the serine/threonine protein phosphatase calcineurin, which is required for the expression of genes involved in Ca(2+)-dependent processes, such as the control of memory, apoptosis, T cell activation, cell cycle, Ca(2+)-homeostasis, and skeletal and cardiac muscle growth and differentiation. However, RCNs regulate calcineurin through two paradoxical actions: they act as feedback inhibitors of calcineurin, whereas their phosphorylation stimulates calcineurin. Here we show that phosphorylation of yeast RCN, Rcn1, triggers degradation through the SCF(Cdc4) ubiquitin ligase complex. Degradation of phosphorylated Rcn1 is required to mitigate inhibition of calcineurin by Rcn1 and results in activation of calcineurin activity in response to Ca(2+) as well as in reactivation of calcineurin in response to changes in Ca(2+) concentration. The SCF(Cdc4)-dependent degradation required phosphorylation of Rcn1 by Mck1, a member of the GSK3 family of protein kinases, and was promoted by Ca(2+). However, such degradation was counteracted by dephosphorylation of Rcn1, which was promoted by Ca(2+)-stimulated calcineurin. Thus, calcineurin activity is fine-tuned to Ca(2+) signals by mechanisms that have opposite functions. Our results identify the molecular mechanism of Rcn1 phosphorylation-induced stimulation of the phosphatase activity of calcineurin. The results provide insight into the mechanism involved in maintaining proper responses to Ca(2+) signals.