The SecA and SecY subunits of translocase are the nearest neighbors of a translocating preprotein, shielding it from phospholipids.

To study the environment of a preprotein as it crosses the plasma membrane of Escherichia coli, unique cysteinyl residues were introduced into proOmpA and the genes for these mutant preproteins were fused to the gene of dihydrofolate reductase (Dhfr). A photoactivable, radiolabeled and reducible cross‐linker was then attached to the unique cysteinyl residue of each purified protein. Partially translocated polypeptides were generated and arrested in their membrane transit by the folded structure of the dihydrofolate reductase domain. After photolysis to label their nearest neighbors and reduction of the disulfide bond between proOmpA‐Dhfr and the cross‐linker, radiolabeled cross‐linker was selectively recovered with the SecA and SecY subunits of preprotein translocase. Strikingly, neither the SecE nor Band 1 subunits were cross‐linked to any of the constructs and the membrane phospholipids were almost entirely shielded from cross‐linking. The fact that SecY and SecA are the only membrane proteins cross‐linked to the translocating chains suggests that they may form an entirely proteinaceous pathway through which secreted proteins pass during membrane transit.