Involvement of FtsH in protein assembly into and through the membrane. I. Mutations that reduce retention efficiency of a cytoplasmic reporter.

To identify cellular factors that assist in membrane protein biogenesis, we looked for mutants affected in the "stop transfer" anchoring process. Using a SecY-PhoA fusion protein in which alkaline phosphatase (PhoA) mature sequence is attached to the last cytoplasmic domain following the 10th transmembrane segment of SecY, we isolated a mutation (std101) that allowed significant export of the PhoA moiety across the membrane. The mutation did not cause nonspecific leakage of cytoplasmic proteins. The mutation was identified as a single base change in the ftsH gene, causing an amino acid substitution in the proposed periplasmic region of FtsH, a putative membrane-bound ATPase. In addition, the ftsH1 temperature-sensitive mutation caused a similar phenotype. Disruption of the chromosomal ftsH in combination with a lac promoter-controlled copy of ftsH on a plasmid rendered the cell viability dependent on lac induction. Repression of this system resulted in a strong Std phenotype as well as significant export defects of beta-lactamase and OmpA. Thus, the loss of ftsH function enhances translocation of normally anchored protein segments and retards that of normally translocated proteins. These results suggest that FtsH participates in assembly of proteins into and through the membrane. It is needed for the cell to assure efficient stop-transfer of some transmembrane proteins.