The N-terminal region of Escherichia coli lactose permease mediates membrane contact of the nascent polypeptide chain

Plasmids encoding N-terminal segments of the Escherichia coli lactose permease (also referred to as lactose carrier) have been used to analyze the biosynthesis and membrane insertion of this complex integral protein of the cytoplasmic membrane. Such truncated polypeptides were found to be stably associated with the membrane and to resemble the full-length protein with respect to their solubilization characteristics. Membrane-bound and free cytoplasmic polysomes were prepared from plasmid-bearing cells and incubated in the presence of 35S]methionine to permit completion of polypeptides initiated in vivo. Under these conditions, lactose permease was found to be radiolabeled in the fraction of membrane-bound polysomes; beta-galactosidase, used as a control, was translated almost exclusively by free polysomes. From similar experiments with N-terminal segments of lactose permease, we estimate that at most a polypeptide of 120 amino acid residues emerging from the ribosome is needed to target the nascent chain to the lipid bilayer and to mediate attachment of the ribosome to the membrane during elongation. Additional data support the idea that even shorter N-terminal sequences of 50 and 71 amino acid residues contain sufficient 'information' to provide contact with the membrane.