INTERACTIONS OF 3 DOMAINS DISTINGUISHING THE RAS-RELATED GTP-BINDING PROTEINS YPT1 AND SEC4

THE genes SEC4 and YPT1 encode Ras-related GTP-binding proteins in the yeast Saccharomyces cerevisiae. Ypt1 is necessary for vesicular transport from the endoplasmic reticulum to the Golgi1-4, whereas Sec4 is required for fusion of post-Golgi secretory vesicles to the plasma membrane5. Recently, three structural domains have been proposed to specify the stage in cellular transport at which members of the Sec4/Ypt1/Rab family act: the effector domain6, the C-terminal hypervariable region7, and a region corresponding to loop 7 in the structure of p21ras (ref. 8). Here we use Sec4/Ypt1 chimaeras to show that these three regions cooperate to specify Ypt1 function and that the C-terminal hypervariable region is needed for Ypt1 localization to the Golgi. Unexpectedly, we found that a single chimaera can function as either Ypt1 or Sec4 without missorting carboxypeptidase Y or invertase.