STRUCTURE OF GUANINE-NUCLEOTIDE-EXCTRANGE FACTOR HUMAN MSS4 AND IDENTIFICATION OF ITS RAB-INTERACTING SURFACE

GUANINE-NUCLEOTIDE-EXCHANGE factors (GEFs) promote the exchange of GDP for GTP in Ras GTPases, and thereby positively regulate their functio ns(1,2). Members of the Sec4/Ypt1/Rab branch of the Ras superfamily ar e essential for vesicular transport(3-6). A GEF for a subset of Rab pr oteins, termed mammalian suppressor of Sec4 (Mss4), has been identifie d(7). Here we use multidimensional NMR to determine the structure of h uman Mss4 (hMss4), which is the first tertiary structure established f or a protein with GEF activity. Mss4 contains a central beta-sheet san dwiched between two small sheets. It also binds a Zn ion through Cys 2 3, Cys 26, Cys 94 and Cys 97. The Rab-binding surface of hMss4 has sub sequently been delineated using chemical-shift perturbation experiment s and site-directed mutagenesis. The active site of hMss4 involves the Zn2+-binding region and a neighbouring loop.