DIFFERENTIAL TRANSLOCATION OF PROTEIN PRECURSORS ACROSS SECY-DEFICIENT MEMBRANES OF ESCHERICHIA-COLI - SECY IS NOT OBLIGATORILY REQUIRED FOR TRANSLOCATION OF CERTAIN SECRETORY PROTEINS IN-VITRO

SecY, a component of the protein translocation system in Escherichia c oli, was depleted at a nonpermissive temperature in a strain which had a temperature-sensitive polar effect on the the expression of its sec Y. Membrane vesicles prepared from these cells, when grown at the nonp ermissive temperature, contained about 5% SecY and similarly low level s of SecG. As expected, translocation of alkaline phosphatase precurso rs across these SecY-deficient membranes was severely impaired and app eared to be directly related to the decrease of SecY amounts. However, despite such a dramatic reduction in SecY and SecG levels, these memb ranes exhibited 50 to 70% of the wild-type translocation activity incl uding the processing of the signal peptide, of OmpA precursor (proOmpA ). This translocation activity in SecY-deficient membranes was still S ecA and ATP dependent and was not unique to proOmpA, as lipoprotein an d lambda receptor protein precursors were also transported efficiently . Membranes that were reconstituted from these SecY-depleted membranes contained undetectable amounts of SecY yet were also shown to possess substantial translocation activity fur proOmpA. These results indicat e that the requirement of SecY for translocation is not obligatory for all secretory proteins and may depend on the nature of precursors. Co nsequently, it is unlikely that SecY is the essential core channel thr ough which all precursors traverse across membranes; rather, SecY prob ably contributes to efficiency and specificity.