The Regulators of Calcineurin are chaperones for a conserved phosphatase

The Regulators of Calcineurin (RCANs) are a conserved family of proteins capable of both stimulating and inhibiting intracellular signaling via calcineurin, the only known Ca2+/CaM-dependent protein phosphatase. Calcineurin is responsible for controlling a wide array of biological processes in almost all eukaryotes. RCANs are implicated in a number of disorders, such as Down syndrome, which feature disregulated calcineurin signaling, and understanding RCAN function is essential to dissecting the etiology of these conditions and the development of novel treatments. Herein, I present studies of RCANs from both yeast and humans, which provide evidence that RCANs function like dedicated molecular chaperones to stimulate calcineurin activity in vivo. Detailed structure-function studies, based on regions conserved in all RCANs, show that RCANs undergo a highly organized and multi-step chaperone-like activation cycle during calcineurin stimulation. This requires the coordinated function of a pair of stereotypical calcineurin docking motifs along with a pair of novel motifs that specifically activate calcineurin. Phosphorylation and proteasomal degradation of RCANs are required to drive this cycle and release activated calcineurin. RCANs stimulate calcineurin signaling on their own in vivo, as their function does not require co-chaperones such as HSP90, and real-time imaging of calcineurin signaling dynamics in live cells demonstrates that RCANs must be present before the onset of a Ca 2+ signal in order for calcineurin to achieve full activity. I also identify and characterize a novel subfamily of highly divergent RCANs, found only in yeasts, that likely act as feedback inhibitors of calcineurin. My studies provide a new appreciation for these critical calcineurin regulators.