Yeast Sec14p deficient in phosphatidylinositol transfer activity is functional in vivo

Yeast phosphatidylinositol transfer protein (Sec14p) is essential for Golgi secretory function. It is widely accepted, though unproven, that phosphati dylinositol transfer between membranes represents the physiological activit y of phosphatidylinositol transfer proteins (PITPs). We report that Sec14p( K66,239A) is inactivated for phosphatidylinositol, but not phosphatidylchol ine (PC), transfer activity. As expected, Sec14p(K66,239A) fails to meet es tablished criteria for a PITP in vitro and fails to stimulate phosphoinosit ide production in vivo. However, its expression efficiently rescues the let hality and Golgi secretory defects associated with sec14-1(ts) and sec14 nu ll mutations. This complementation requires neither phospholipase D activat ion nor the involvement of a novel class of minor yeast PITPs. These findin gs indicate that PI binding/transfer is remarkably dispensable for Sec14p f unction in vivo.