PHOSPHOLIPID TRANSFER ACTIVITY IS RELEVANT TO BUT NOT SUFFICIENT FOR THE ESSENTIAL FUNCTION OF THE YEAST SEC14 GENE-PRODUCT

To investigate several key aspects of phosphatidylinositol transfer protein (PI-TP) function in eukaryotic cells, rat PI-TP was expressed in yeast strains carrying lesions in SEC14, the structural gene for yeast PI-TP (SEC14p), whose activity is essential for Golgi secretory function in vivo. Rat PI-TP expression effected a specific complementation of sec14ts growth and secretory defects. Complementation of sec14 mutations was not absolute as rat PI-TP expression failed to rescue sec14 null mutations. This partial complementation of sec14 lesions by rat PI-TP correlated with inability of the mammalian protein to stably associate with yeast Golgi membranes and was not a result of rat PI-TP stabilizing the endogenous sec14ts gene product. These collective data demonstrate that while the in vitro PI-TP activity of SEC14p clearly reflects some functional in vivo property of SEC14p, the PI-TP activity is not the sole essential activity of SEC14p. Those data further identify an efficient Golgi targeting capability as a likely essential feature of SEC14p function in vivo. Finally, the data suggest that stable association of SEC14p with yeast Golgi membranes is not a simple function of its lipid-binding properties, indicate that the amino-terminal 129 SEC14p residues are sufficient to direct a catalytically inactive form of rat PI-TP to the Golgi and provide the first evidence to indicate that a mammalian PI-TP can stimulate Golgi secretory function in vivo.