Lipid perturbations sensitize osmotic down-shock activated Ca2+ influx, a yeast "deletome" analysis

Osmotic down shock causes an immediate influx of Ca2+ in yeast, likely through a membrane stretch-sensitive channel. To see how this channel is constituted and regulated, we screened the collection of 4,906 yeast gene deletants for major changes in this response by luminomtery. We discovered deletants that responded very strongly to much milder down shocks than wild-type required, but show little changes in up-shock response. Of all the possibilities (general metabolism, ion distribution, cytoskeleton, cell wall, membrane receptors, etc.), most of the over-responders turned out to be deleted of proteins functioning in the biogenesis of phospholipids, sphingolipids, or ergosterol. Other over-responders are annotated to have vesicular transport defects, traceable to lipid defects in some cases. The deletant lacking the de novo synthesis of phosphatidylcholine, opi3delta, is by far the strongest over-responder. opi3 deletion does not cause non-specific leakage but greatly sensitizes the force-sensing Ca2+-influx mechanism. Choline supplementation normalizes the opi3delta response. Thus, the osmotic-pressure induced stretch force apparently controls channel activities through lipids. This unbiased examination of the yeast genome supports the view that forces intrinsic to the bilayer are determined by the geometry of the lipids and these forces, in turn, govern the activities of proteins embedded therein.