ESCHERICHIA-COLI SECA SHAPE AND DIMENSIONS

SecA shape and conformational flexibility in solution were studied by small angle X-ray scattering, Dimeric SecA is a very elongated molecul e, 15 nm long and 8 nm wide. SecA is therefore four times as long as t he membrane is wide. The two globular protomers are distinctly separat ed and share limited surface of intermolecular contacts. ATP, ADP or a denylyl-imidodiphosphate (AMP-PNP) binding does not alter the SecA rad ius of gyration. A SecA mutant that catalyzes multiple rounds of ATP h ydrolysis does not undergo conformational changes detectable by small angle X-ray scattering (SAXS). We conclude that SecA conformational al terations observed biochemically during nucleotide interaction are onl y small-scale and localized. The ramifications of these findings on Se cA/SecYEG interaction are discussed. (C) 1998 Federation of European B iochemical Societies.