Maintenance of proper phosphatidylinositol-4-phosphate level by Stt4 and Sac1 contributes to vesicular transport to and from the plasma membrane

Growing evidence suggests that counter-transport of phosphatidylinositol-4-phosphate (PtdIns(4)P) and phosphatidylserine (PS) at endoplasmic reticulum (ER)-plasma membrane (PM) contact sites is required for intracellular vesicle transport. PtdIns(4)P is metabolized by Stt4 PI 4-kinase residing at the PM and by Sac1 PtdIns(4)P phosphatase at the ER, and ER-PM contact sites are believed to be important for its efficient turnover. Recently, Stt4 has been shown to extensively localize to ER-PM contact sites. However, the precise location of Stt4 and the mechanism of localization to these sites have not been clarified. Additionally, although several studies have suggested a requirement for PS/PtdIns(4)P and sterol/PtdIns(4)P exchange at ER-PM contact sites in endocytosis, it is still unclear whether contact between the ER and the PM, turnover of PtdIns(4)P or PS, or maintenance of PtdIns(4)P or PS levels is more important. Here we found that Stt4 localizes to the cER regions where Scs2 and Ist2 are localized abundantly, and that localization of Stt4 is maintained in the Δtether mutant, which has a reduced number of ER-PM contact sites. We also demonstrated that the Δtether and sac1Δ mutants showed defects at different stages of endocytosis, and that the inactivation mutation of Stt4 restored the endocytosis defect only in the Δtether mutant. Furthermore, these mutants exhibited defective transport in the secretory and recycling pathways, and inactivation of Stt4 restored the secretory pathway in the Δtether mutant, but not the recycling pathway in either mutant. These results suggest that endocytosis, secretion, and recycling pathways are regulated directly or indirectly by different PtdIns(4)P-mediated mechanisms.