PRLA4 PREVENTS THE REJECTION OF SIGNAL SEQUENCE DEFECTIVE PREPROTEINSBY STABILIZING THE SECA-SECY INTERACTION DURING THE INITIATION OF TRANSLOCATION

In Escherichia coli, precursor proteins are translocated across the cy toplasmic membrane by translocase. This multisubunit enzyme consists o f a preprotein-binding and ATPase domain, SecA, and the SecYEG complex as the integral membrane domain. PrlA4 is a mutant of SecY that enabl es the translocation of preproteins with a defective, or missing, sign al sequence. Inner membranes of the prlA4 strain efficiently transloca te Delta 8proOmpA, a proOmpA derivative with a non-functional signal s equence. Owing to the signal sequence mutation, Delta 8proOmpA binds t o the translocase with a lowered affinity and the recognition is not r estored by the prlA4 SecY, At the ATP-dependent initiation of transloc ation, the binding affinity of SecA for SecYEG is lowered causing the premature loss of bound preproteins from the translocase. The prlA4 me mbranes, however, bind SecA with a much higher affinity than the wild- type, and during initiation, the SecA and preprotein remain bound at t he translocation site allowing an improved efficiency of translocation , It is concluded that the prlA4 strain prevents the rejection of defe ctive preproteins from the export pathway by stabilizing SecA at the S ecYEG complex.