Plasma membrane processes are differentially regulated by type I phosphatidylinositol phosphate 5-kinases and RASSF4.

Phosphoinositide lipids regulate many cellular processes and are synthesized by lipid kinases. Type I phosphatidylinositol phosphate 5-kinases (PIP5KIs) generate phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)]. Several phosphoinositide-sensitive readouts revealed the nonequivalence of overexpressing PIP5KIβ, PIP5KIγ or Ras association domain family 4 (RASSF4), believed to activate PIP5KIs. Mass spectrometry showed that each of these three proteins increased total cellular phosphatidylinositol bisphosphates (PtdIns) and trisphosphates (PtdIns) at the expense of phosphatidylinositol phosphate (PtdIns) without changing lipid acyl chains. Analysis of KCNQ2/3 channels and PH domains confirmed an increase in plasma membrane PtdIns(4,5) in response to PIP5KIβ or PIP5KIγ overexpression, but RASSF4 required coexpression with PIP5KIγ to increase plasma membrane PtdIns(4,5) Effects on the several steps of store-operated calcium entry (SOCE) were not explained by plasma membrane phosphoinositide increases alone. PIP5KIβ and RASSF4 increased STIM1 proximity to the plasma membrane, accelerated STIM1 mobilization and speeded onset of SOCE; however, PIP5KIγ reduced STIM1 recruitment but did not change induced Ca entry. These differences imply actions through different segregated pools of phosphoinositides and specific protein-protein interactions and targeting.This article has an associated First Person interview with the first author of the paper.