A SecE mutation that modulates SecY-SecE translocase assembly, identified as a specific suppressor of SecY defects

The SecY39(Cs) (cold-sensitive) alteration of Arg357 results in a defect of translocation initiation. As a means to dissect the Sec translocation machinery, we isolated mutations that act as suppressors of the secY39 defect. A specific secE mutation, designated secE105, was thus isolated. This mutation proved to be identical with the prlG2 mutation and to suppress a number of cold-sensitive secY mutations. However, other prlG mutations did not effectively suppress the secY defects. Evidence indicates that the Ser105-to-Pro alteration in the C-terminal transmembrane segment of SecE weakens SecY-SecE association. In vitro analyses showed that the SecE(S105P) alteration preferentially stimulates the initial phase of translocation. It is suggested that the S105P alteration affects the SecYEG channel such that it is more prone to open and to accept the translocation initiation domain of a preprotein molecule.