Sequential truncation of the lactose permease over a three-amino acid sequence near the carboxyl terminus leads to progressive loss of activity and stability.

Previous experiments are consistent with the notion that residues 396-401 (... SVFTLS ...) at the carboxyl terminus of the last putative transmembrane helix of the lactose (lac) permease of Escherichia coli are important for protection against proteolytic degradation and suggest that this region of the permease may be necessary for proper folding. Stop codons (TAA) have now been substituted sequentially for amino acid codons 396-401 in the lacY gene, and the termination mutants were expressed from the plasmid pT7-5. With respect to transport, permease truncated at residue 396 or 397 is completely defective, while molecules truncated at residues 398, 399, 400, and 401, respectively, exhibit 15-25%, 30-40%, 40-45%, and 70-100% of wild-type activity. As judged by pulse-chase experiments with [35S]methionine, wild-type permease or permease truncated at residue 401 is stable, while permease molecules truncated at position 400, 399, 398, 397, or 396 are degraded at increasingly rapid rates. The findings indicate that either the last turn of putative helix XII or the region immediately distal to helix XII is important for proper folding and protection against proteolytic degradation.