Ha. Cook et Ca. Kumamoto, Overproduction of SecA suppresses the export defect caused by a mutation in the gene encoding the Escherichia coli export chaperone SecB, J BACT, 181(10), 1999, pp. 3010-3017
SecB is a cytosolic protein required for rapid and efficient export of part
icular periplasmic and outer membrane proteins in Escherichia coli, SecB pr
omotes export by stabilizing newly synthesized precursor proteins in a nonn
ative conformation and by targeting the precursors to the inner membrane. B
iochemical studies suggest that SecB facilitates precursor targeting by bin
ding to the SecA protein, a component of the membrane-embedded translocatio
n apparatus. To gain more insight into the functional interaction of SecB a
nd SecA, in vivo, mutations in the secA locus that compensate for the expor
t defect caused by the secB missense mutation secBL75Q were isolated. Two s
uppressors were isolated, both of which led to the overproduction of wild-t
ype SecA protein. In vivo studies demonstrated that the SecBL75Q mutant pro
tein releases precursor proteins at a lower rate than does wild-type SecB,
Increasing the level of SecA protein in the cell was found to reverse this
slow-release defect, indicating that overproduction of SecA stimulates the
turnover of SecBL75Q-precursor complexes. These findings lend additional su
pport to the proposed pathway for precursor targeting in which SecB promote
s targeting to the translocation apparatus by binding to the SecA protein.