Fig. 9.

Speculative model for PI4P regulation during polarized growth versus stress conditions. a Under normal growth conditions, PM PI4P is regulated in mother cells at ER-PM contact sites through its production by Stt4 PIK patches and Osh protein-mediated turnover by Sac1. Osh3-mediated PI4P metabolism may also be coupled to exchange for an as yet unknown lipid at the PM and Osh3 function may also occur outside ER-PM contacts. PI4P levels at the PM are higher in the growing daughter cell than in the mother cell, possibly due to the lack of an established cortical ER network in the newly formed daughter cell. As a consequence, PI4P- and PI(4,5)P₂-regulated vesicle trafficking is directed to the growing daughter cell. Plasma membrane maintenance in mother cells may be controlled by Osh protein-mediated non-vesicular lipid exchange [52]. b Under heat stress conditions, Osh3 localization and function at ER-PM contacts is impaired as Osh3 forms internal cytoplasmic aggregates. Diminished Osh3-mediated PI4P turnover contributes to increases in PI4P and possibly PI(4,5)P₂ signals at the PM in mother cells. Loss of PI4P and PI(4,5)P₂ polarization at the PM triggers isotropic (non-polarized) secretion, as well as additional stress responses necessary to maintain PM integrity