Internal Ca2+ release in yeast is triggered by hypertonic shock and mediated by a TRP channel homologue

Abstract

Calcium ions, present inside all eukaryotic cells, are important second messengers in the transduction of biological signals. In mammalian cells, the release of Ca(2+) from intracellular compartments is required for signaling and involves the regulated opening of ryanodine and inositol-1,4,5-trisphosphate (IP3) receptors. However, in budding yeast, no signaling pathway has been shown to involve Ca(2+) release from internal stores, and no homologues of ryanodine or IP3 receptors exist in the genome. Here we show that hyperosmotic shock provokes a transient increase in cytosolic Ca(2+) in vivo. Vacuolar Ca(2+), which is the major intracellular Ca(2+) store in yeast, is required for this response, whereas extracellular Ca(2+) is not. We aimed to identify the channel responsible for this regulated vacuolar Ca(2+) release. Here we report that Yvc1p, a vacuolar membrane protein with homology to transient receptor potential (TRP) channels, mediates the hyperosmolarity induced Ca(2+) release. After this release, low cytosolic Ca(2+) is restored and vacuolar Ca(2+) is replenished through the activity of Vcx1p, a Ca(2+)/H(+) exchanger. These studies reveal a novel mechanism of internal Ca(2+) release and establish a new function for TRP channels.