ftsE(Ts) Affects Translocation of K+-Pump Proteins into the Cytoplasmic Membrane of Escherichia coli

The ftsE(Ts) mutation of Escherichia coli causes defects in cell division and cell growth. We expressed alkaline phosphatase (PhoA) fusion proteins of KdpA, Kup, and TrkH, all of which proved functional in vivo as K⁺ ion pumps, in the mutant cells. During growth at 41°C, these proteins were progressively lost from the membrane fraction. The reduction in the abundance of these proteins inversely correlated with cell growth, but the preformed proteins in the membrane were stable at 41°C, indicating that the molecules synthesized at the permissive temperature were diluted in a growth-dependent manner at a high temperature. Pulse-chase experiments showed that KdpA-PhoA was synthesized, but the synthesized protein did not translocate into the membrane of the ftsE(Ts) cells at 41°C and degraded very rapidly. The loss of KdpA-PhoA from the membrane fractions of ftsE(Ts) cells was suppressed by a multicopy plasmid carrying the ftsE⁺ gene. While cell growth stopped when the abundance of these proteins decreased 15-fold, the addition of a high concentration of K⁺ ions specifically alleviated the growth defect of ftsE(Ts) cells but not cell division, and the cells elongated more than 100-fold. We conclude that one of the causes of growth cessation in the ftsE(Ts) mutants is a defect in the translocation of K⁺-pump proteins into the cytoplasmic membrane.