STRUCTURE OF THE GUANINE-NUCLEOTIDE EXCHANGE FACTOR SEC7 DOMAIN OF HUMAN ARNO AND ANALYSIS OF THE INTERACTION WITH ARF GTPASE

Sec7-related guanine nucleotide exchange factors (GEFs) initiate vesic le budding from the Golgi membrane surface by converting the GTPase AR F to a GTP-bound, membrane-associated form. Here we report the crystal structure of the catalytic Sec7 homology domain of Arno, a human GEF for ARF1, determined at 2.2 Angstrom resolution. The Sec7 domain is an elongated, all-helical protein with a distinctive hydrophobic groove that is phylogenetically conserved. Structure-based mutagenesis identi fies the groove and an adjacent conserved loop as the ARF-interacting surface. The sites of Sec7 domain interaction on ARF1 have subsequentl y been mapped, by protein footprinting experiments, to the switch 1 an d switch 2 GTPase regions, leading to a model for the interaction betw een ARF GTPases and Sec7 domain exchange factors.