A Conserved Docking Site Modulates Substrate Affinity for Calcineurin, Signaling Output, and In Vivo Function

SummaryCalcineurin, the conserved Ca2+/calmodulin-regulated protein phosphatase, mediates diverse aspects of Ca2+-dependent signaling. We show that substrates bind calcineurin with varying strengths and examine the impact of this affinity on signaling. We altered the calcineurin-docking site, or PxIxIT motif, in Crz1, the calcineurin-regulated transcription factor in S. cerevisiae, to decrease (Crz1PVIAVN) or increase (Crz1PVIVIT) its affinity for calcineurin. As a result, the Ca2+-dependent dephosphorylation and activation of Crz1PVIAVN are decreased, whereas Crz1PVIVIT is constitutively dephosphorylated and hyperactive. Surprisingly, the physiological consequences of altering calcineurin-Crz1 affinity depend on the growth conditions. Crz1PVIVIT improves yeast growth under several environmental stress conditions but causes a growth defect during alkaline stress, most likely by titrating calcineurin away from other substrates or regulators. Thus, calcineurin-substrate affinity determines the Ca2+ concentration dependence and output of signaling in vivo as well as the balance between different branches of calcineurin signaling in an overall biological response.