The Ca²⁺/calmodulin-dependent phosphatase, calcineurin, promotes yeast survival during environmental stress. We identified Slm1 and Slm2, two related PH-domain containing proteins that are redundant for an essential function, as novel calcineurin substrates required for the execution of sphingolipid-dependent processes during heat stress (1). Slm1 and Slm2 bind to calcineurin via docking sites that are required for their dephosphorylation by calcineurin, and are related to the PxIxIT motif identified in NF-AT. In vivo, calcineurin mediates prolonged dephosphorylation of Slm1 and Slm2 during heat stress, and this response can be mimicked by exogenous addition of the sphingoid base, phytosphingosine. Slm proteins also promote the growth of yeast cells in the presence of myriocin, an inhibitor of sphingolipid biosynthesis, and regulation of Slm proteins by calcineurin is required for their full activity under these conditions. In addition, Slm proteins function downstream of phytosphingosine, since cells lacking Slm activity are not rescued by exogenous phytosphingosine. During heat stress, sphingolipids signal turnover of the uracil permease, Fur4. In cells lacking Slm protein activity, stress induced endocytosis of Fur4 is blocked, and Fur4 accumulates at the cell surface in an ubiquitinated form. Furthermore, cells expressing a version of Slm2 that cannot be dephosphorylated by calcineurin display an increased rate of Fur4 turnover during heat stress. Thus, calcineurin may modulate sphingolipid-dependent events through regulation of Slm1 and Slm2. These findings, in combination with previous work identifying Slm1 and Slm2 as targets of Mss4/PIP₂ and TORC2 signaling (2), suggest that Slm proteins are at an important regulatory node integrating information from a variety of signaling pathways, including Ca²⁺/calcineurin and sphingolipid signaling, to coordinate the cellular response to heat stress.

(1) Bultynck G. et al., Mol Cell Biol, in press

(2) Audhya A. et al., EMBO J (2004) 23(19): 3747–57