Complex behavior in solution of homodimeric SecA

SecA, a homodimeric protein involved in protein export in Escherichia coli, exists in the cell both associated with the membrane translocation apparatus and free in the cytosol. SecA is a multifunctional protein involved in protein localization and regulation of its own expression. To carry out these functions, SecA interacts with a variety of proteins, phospholipids, nucleotides, and nucleic acid and shows two enzymic activities. It is an ATPase and a helicase. Its role during protein localization involves interaction with the precursor polypeptides to be exported, the cytosolic chaperone SecB, and the SecY subunit of the membrane-associated translocase, as well as with acidic phospholipids. At the membrane, SecA undergoes a cycle of binding and hydrolysis of ATP coupled to conformational changes that result in translocation of precursors through the cytoplasmic membrane. The helicase activity of SecA and its affinity for its mRNA are involved in regulation of its own expression. SecA has been reported to exist in at least two conformational states during its functional cycle. Here we have used analytical centrifugation, as well as column chromatography coupled with multiangle light scatter, to show that in solution SecA undergoes at least two monomer-dimer equilibrium reactions that are sensitive to temperature and to concentration of salt

Complex behavior in solution of homodimeric SecA

SecA, a homodimeric protein involved in protein export in Escherichia coli, exists in the cell both associated with the membrane translocation apparatus and free in the cytosol. SecA is a multifunctional protein involved in protein localization and regulation of its own expression. To carry out these functions, SecA interacts with a variety of proteins, phospholipids, nucleotides, and nucleic acid and shows two enzymic activities. It is an ATPase and a helicase. Its role during protein localization involves interaction with the precursor polypeptides to be exported, the cytosolic chaperone SecB, and the SecY subunit of the membrane-associated translocase, as well as with acidic phospholipids. At the membrane, SecA undergoes a cycle of binding and hydrolysis of ATP coupled to conformational changes that result in translocation of precursors through the cytoplasmic membrane. The helicase activity of SecA and its affinity for its mRNA are involved in regulation of its own expression. SecA has been reported to exist in at least two conformational states during its functional cycle. Here we have used analytical centrifugation, as well as column chromatography coupled with multiangle light scatter, to show that in solution SecA undergoes at least two monomer-dimer equilibrium reactions that are sensitive to temperature and to concentration of salt

Sites of interaction between SecA and the chaperone SecB, two proteins involved in export

SecB, a small tetrameric cytosolic chaperone in Escherichia coli, facilitates the export of precursor polypeptides by maintaining them in a normative conformation and passing them to SecA, which is a peripheral member of the membrane-bound translocation apparatus. It has been proposed by several laboratories that as SecA interacts with various components along the export pathway, it undergoes conformational changes that are crucial to its function. Here we report details of molecular interactions between SecA and SecB, which may serve as conformational switches. One site of interaction involves the final C-terminal 21 amino acids of SecA, which are positively charged and contain zinc. The C terminus of each subunit of the SecA dimer makes contact with the flat P-sheet that is formed by each dimer of the SecB tetramer. Here we demonstrate that a second interaction exists between the extreme C-terminal alpha-helix of SecB and a site on SecA, as yet undefined but different from the C terminus of SecA. We investigated the energetics of the interactions by titration calorimetry and characterized the hydrodynamic properties of complexes stabilized by both interactions or each interaction singly using sedimentation velocity centrifugation