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 molecule, 15 nm long and 8 nm wide. SecA is therefore four times as long as the membrane is wide. The two globular protomers are distinctly separated and share limited surface of intermolecular contacts. ATP, ADP or adenylyl-imidodiphosphate (AMP-PNP) binding does not alter the SecA radius of gyration. A SecA mutant that catalyzes multiple rounds of ATP hydrolysis does not undergo conformational changes detectable by small angle X-ray scattering (SAXS). We conclude that SecA conformational alterations observed biochemically during nucleotide interaction are only small-scale and localized. The ramifications of these findings on SecA/SecYEG interaction are discussed.