Involvement of FtsH in protein assembly into and through the membrane. II. Dominant mutations affecting FtsH functions.

FtsH is an Escherichia coli protein with its amino-terminal region anchored to the cytoplasmic membrane and with its cytoplasmic domain significantly homologous to the members of an ATPase family found in eukaryotic cells. We previously showed that the loss of ftsH function results in reduced cytoplasmic retention of the alkaline phosphatase moiety that was attached to cytoplasmic regions of membrane proteins (the Std phenotype) and also in translocation retardation of some exported proteins. We now report that expression of some FtsH variants from plasmids causes dominant Std and translocation phenotypes. Such variants include carboxyl-terminally truncated forms of FtsH and some missense mutations in the cytoplasmic domain, notably Lys to Asn changes at the two ATP binding consensus sequences. In contrast, amino-terminally truncated variants lacked the dominant phenotypes. It was suggested that the amino-terminal membrane region of FtsH interacts with other component(s), and that the two putative ATP binding sites are of vital importance for the FtsH functions.