P619, A GIANT PROTEIN RELATED TO THE CHROMOSOME CONDENSATION REGULATOR RCC1, STIMULATES GUANINE-NUCLEOTIDE EXCHANGE ON ARF1 AND RAB PROTEINS

We report the identification of a novel human gene, designated p619, t hat encodes a polypeptide of 4861 amino acid residues, one of the larg est human proteins known to dale, The p619 protein contains two region s of seven internal repeats highly related to the cell cycle regulator RCC1, a guanine nucleotide exchange factor for the small GTP binding protein, Ran, Ln addition, p619 possesses seven P-repeat domains chara cteristic of the beta-subunit of heterotrimeric G proteins, three puta tive SN3 binding sites, seven polar amino acid-rich regions, a putativ e leucine zipper and a carboxy-terminal HECT domain characteristic of E3 ubiquitin-protein ligases, p619 is expressed ubiquitously in mouse and human tissues and overexpressed in several human tumor cell Lines, Subcellular localization studies indicate that p619 is located in the cytosol and in the Golgi apparatus, Localization of p619 in the Golgi is altered by Brefeldin A, The carboxy-terminal RCC1-like domain of p 619 interacts specifically with myristoylated ARF1, a small GTP bindin g protein also located in the Golgi, Moreover, the second RCC1-like mo tif located at the amino-terminus of p619 stimulates guanine nucleotid e exchange on ARF1 and on members of the related Rab proteins, but not on other small GTP binding proteins such as Ran or R-Ras2/TC21. These observations suggest that p619 is a Brefeldin A-sensitive Golgi prote in that functions as a guanine nucleotide exchange factor for ARF1 and , possibly, for members of the Rab family of proteins.