A HUMAN EXCHANGE FACTOR FOR ARF CONTAINS SEC7-HOMOLOGY AND PLECKSTRIN-HOMOLOGY DOMAINS

THE small G protein ARF1 is involved in the coating of vesicles that b ud from the Golgi compartments(1,2). Its activation is controlled by a s-yet unidentified guanine-nucleotide exchange factors(3,4). Gea1, the first ARF exchange factor to be discovered in yeast(5), is a large pr otein containing a domain of homology with Sec7, another yeast protein that is also involved in secretion(6) Here we characterized a smaller human protein (relative molecular mass 47K) named ARNO, which contain s a central Sec7 domain that promotes guanine-nucleotide exchange on A RF1. ARNO also contains an amino-terminal coiled-coil motif and a carb oxy-terminal pleckstrin-homology (PH) domain. The PH domain mediates a n enhancement of ARNO exchange activity by negatively charged phosphol ipid vesicles supplemented with phosphatidylinositol bisphosphate. The exchange activity of ARNO is not inhibited by brefeldin A, an agent k nown to block vesicular transport and inhibit the exchange activity on ARF1 in cell extracts(7,8). This suggests that a regulatory component which is sensitive to brefeldin A associates with ARNO in vivo, possi bly through the amino-terminal coiled-coil. We propose that other prot eins with a Sec7 domain regulate different members of the ARF family.