title: The basis of asymmetry in the SecA:SecB complex.
abstract: During export in Escherichia coli, SecB, a homotetramer structurally organized as a dimer of dimers, forms a complex with two protomers of SecA, which is the ATPase that provides energy to transfer a precursor polypeptide through the membrane via the SecYEG translocon. There are two areas of contact on SecB that stabilize the SecA:SecB complex: the flat sides of the SecB tetramer and the C-terminal 13 residues of SecB. These contacts within the complex are distributed asymmetrically. Breaking contact between SecA and the sides of SecB results in release of only one protomer of SecA yielding a complex of stoichiometry SecA1:SecB4. This complex mediates export; however, the coupling of ATP hydrolysis to movements of the precursor through the translocon is much less efficient than the coupling by the SecA2:SecB4 complex. Here we used heterotetrameric species of SecB to understand the source of the asymmetry in the contacts and its role in the functioning of the complex. The model of interactions presented suggests a way that binding between SecA and SecB might decrease the affinity of precursor polypeptides for SecB and facilitate the transfer to SecA.
authors.affiliation: Department of Biochemistry, University of Missouri, Columbia, MO 65211, USA.
authors.forename: Yuying
authors.initials: Y
authors.lastname: Suo
authors.affiliation: Department of Biology, University of York, York YO10 5DD, UK.
authors.forename: Simon J S
authors.initials: SJS
authors.lastname: Hardy
authors.affiliation: Department of Biochemistry, University of Missouri, Columbia, MO 65211, USA. Electronic address: craneje@missouri.edu.
authors.forename: Linda L
authors.initials: LL
authors.lastname: Randall
mesh_terms: D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000378:metabolism; D001426:Bacterial Proteins / Q000378:metabolism*; D058947:Bacterial Secretion Systems / Q000502:physiology*; D004926:Escherichia coli / Q000378:metabolism*; D029968:Escherichia coli Proteins / Q000378:metabolism; D026901:Membrane Transport Proteins / Q000378:metabolism*; D008958:Models, Molecular; D055503:Protein Multimerization; D021381:Protein Transport; D000069816:SEC Translocation Channels; D000081416:SecA Proteins
chemical_list: D001426:Bacterial Proteins; D058947:Bacterial Secretion Systems; D029968:Escherichia coli Proteins; D026901:Membrane Transport Proteins; D000069816:SEC Translocation Channels; C057842:SecB protein, Bacteria; D000255:Adenosine Triphosphate; D000251:Adenosine Triphosphatases; D000081416:SecA Proteins
keywords: Sec system; asymmetric binding; hybrid SecB species; protein interaction; ΔG stabilization