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J. Biol. Chem., Vol. 278, Issue 6, 3628-3638, February 7, 2003
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§,§,§,§,,,,
From the 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.
Department of Biological Sciences, Columbia
University, New York, New York 10027 and the ¶ Departments of
Biochemistry & Molecular Biology and Chemistry, University of
Massachusetts, Amherst, Massachusetts 01003
To whom correspondence should be addressed. Tel.:
212-854-5443; Fax: 212-865-8246; E-mail:
hunt@sid.bio.columbia.edu.
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