title: Phospholipid-induced monomerization and signal-peptide-induced oligomerization of SecA.
abstract: The SecA ATPase drives the processive translocation of the N terminus of secreted proteins through the cytoplasmic membrane in eubacteria via cycles of binding and release from the SecYEG translocon coupled to ATP turnover. SecA forms a physiological dimer with a dissociation constant that has previously been shown to vary with temperature and ionic strength. We now present data showing that the oligomeric state of SecA in solution is altered by ligands that it interacts with during protein translocation. Analytical ultracentrifugation, chemical cross-linking, and fluorescence anisotropy measurements show that the physiological dimer of SecA is monomerized by long-chain phospholipid analogues. Addition of wild-type but not mutant signal sequence peptide to these SecA monomers redimerizes the protein. Physiological dimers of SecA do not change their oligomeric state when they bind signal sequence peptide in the compact, low temperature conformational state but polymerize when they bind the peptide in the domain-dissociated, high-temperature conformational state that interacts with SecYEG. This last result shows that, at least under some conditions, signal peptide interactions drive formation of new intermolecular contacts distinct from those stabilizing the physiological dimer. The observations that signal peptides promote conformationally specific oligomerization of SecA while phospholipids promote subunit dissociation suggest that the oligomeric state of SecA could change dynamically during the protein translocation reaction. Cycles of SecA subunit recruitment and dissociation could potentially be employed to achieve processivity in polypeptide transport.
authors.affiliation: Department of Biological Sciences, Columbia University, New York, New York 10027, USA.
authors.forename: Jordi
authors.initials: J
authors.lastname: Benach
authors.forename: Yi-Te
authors.initials: YT
authors.lastname: Chou
authors.forename: John J
authors.initials: JJ
authors.lastname: Fak
authors.forename: Anna
authors.initials: A
authors.lastname: Itkin
authors.forename: Daita D
authors.initials: DD
authors.lastname: Nicolae
authors.forename: Paul C
authors.initials: PC
authors.lastname: Smith
authors.forename: Guenther
authors.initials: G
authors.lastname: Wittrock
authors.forename: Daniel L
authors.initials: DL
authors.lastname: Floyd
authors.forename: Cyrus M
authors.initials: CM
authors.lastname: Golsaz
authors.forename: Lila M
authors.initials: LM
authors.lastname: Gierasch
authors.forename: John F
authors.initials: JF
authors.lastname: Hunt
mesh_terms: D000251:Adenosine Triphosphatases / Q000737:chemistry / Q000378:metabolism*; D000595:Amino Acid Sequence; D001426:Bacterial Proteins; D019281:Dimerization; D029968:Escherichia coli Proteins / Q000737:chemistry / Q000378:metabolism*; D026901:Membrane Transport Proteins / Q000737:chemistry / Q000378:metabolism*; D008969:Molecular Sequence Data; D010743:Phospholipids / Q000378:metabolism*; D021382:Protein Sorting Signals; D021381:Protein Transport; D000069816:SEC Translocation Channels; D000081416:SecA Proteins; D013050:Spectrometry, Fluorescence
chemical_list: D001426:Bacterial Proteins; D029968:Escherichia coli Proteins; D026901:Membrane Transport Proteins; D010743:Phospholipids; D021382:Protein Sorting Signals; D000069816:SEC Translocation Channels; D000251:Adenosine Triphosphatases; D000081416:SecA Proteins