Calcineurin regulates the yeast synaptojanin Inp53/Sjl3 during membrane stress.
Bottom Line: By activating Inp53, calcineurin repolarizes the actin cytoskeleton and maintains normal plasma membrane morphology in synaptojanin-limited cells.This response has physiological and molecular similarities to calcineurin-regulated activity-dependent bulk endocytosis in neurons, which retrieves a bolus of plasma membrane deposited by synaptic vesicle fusion.We propose that activation of Ca(2+)/calcineurin and PI(4,5)P2 signaling to regulate endocytosis is a fundamental and conserved response to excess membrane in eukaryotic cells.
Affiliation: Department of Biology, Stanford University, Stanford, CA 94305.Show MeSH
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Mentions: A diverse array of >40 proteins involved in polarized growth, cell cycle regulation, mating, autophagy, and many other cellular processes have been identified as CN targets (Goldman et al., 2014). Where and when these proteins are regulated by CN have yet to be elucidated. We show here that CN undergoes major localization changes during hyperosmotic shock, which may restrict the substrates it dephosphorylates. In particular, we find that CN colocalizes with a subset of actin patches, perhaps indicating transient localization to sites of endocytosis, and binds preferentially to Inp53 during hyperosmotic shock. In contrast, activation of the CN-dependent Crz1 transcription factor is blocked (Figure 10A). Intense osmotic shock delays multiple signaling events, including Crz1 nuclear translocation, possibly because of impaired diffusion caused by water loss (Miermont et al., 2013). However, changing CN's binding partners also affects downstream signaling; artificially strengthening the CN-Crz1 interaction, for example, causes growth defects by blocking the interaction of CN with other substrates (Roy et al., 2007). Therefore CN signaling outcomes during hyperosmotic stress are likely specified by its subcellular distribution and protein–protein interactions, in agreement with important roles for CN localization in other fungi and in mammals (Oliveria et al., 2007; Juvvadi et al., 2011; Kozubowski et al., 2011). The mechanism(s) responsible for the CN localization changes caused by hyperosmotic stress are unknown, as significant colocalization of CN and Inp53 was not observed (Supplemental Figure S2, B and C). However, similar approaches to those described here will likely identify additional scaffold candidates. We also suggest that interactions between Inp53 and CN during hyperosmotic stress bring CN into proximity with substrates like Bsp1 and Sla1 that lack canonical docking motifs but show CN-dependent dephosphorylation (Goldman et al., 2014). Thus hyperosmotic shock illustrates how a specific environmental condition can tune CN signaling by modifying localization and substrate access.
Affiliation: Department of Biology, Stanford University, Stanford, CA 94305.