G. Matsumoto et al., SECY AND SECA INTERACT TO ALLOW SECA INSERTION AND PROTEIN TRANSLOCATION ACROSS THE ESCHERICHIA-COLI PLASMA-MEMBRANE, EMBO journal, 16(21), 1997, pp. 6384-6393
SecA, the preprotein-driving ATPase in Escherichia coli, was shown pre
viously to insert deeply into the plasma membrane in the presence of A
TP and a preprotein; this movement of SecA was proposed to be mechanis
tically coupled with preprotein translocation. We now address the role
played by SecY, the central subunit of the membrane-embedded heterotr
imeric complex, in the SecA insertion reaction. We identified a secY m
utation (secY205), affecting the most carboxy-terminal cytoplasmic dom
ain, that did not allow ATP and preprotein-dependent productive SecA i
nsertion. while allowing idling insertion without the preprotein. Thus
, the secY205 mutation might affect the SecYEG 'channel' structure in
accepting the preprotein-SecA complex or its opening by the complex. W
e isolated secA mutations that allele-specifically suppressed the secY
205 translocation defect in vivo. One mutant protein, SecA36, with an
amino acid alteration near the high-affinity ATP-binding site, was pur
ified and suppressed the in vitro translocation defect of the inverted
membrane vesicles carrying the SecY205 protein. The SecA36 protein co
uld also insert into the mutant membrane vesicles in vitro. These resu
lts provide genetic evidence that SecA and SecY specifically interact,
and show that SecY plays an essential role in insertion of SecA in re
sponse to a preprotein and ATP and suggest that SecA drives protein tr
anslocation by inserting into the membrane in vivo.