The N-terminal 22 amino acid residues in the lactose permease of Escherichia coli are not obligatory for membrane insertion or transport activity.

When the lactose (lac) permease of Escherichia coli is expressed from the lac promoter at relatively low rates, deletion of amino acid residues 2-8 (delta 7) or 2-9 (delta 8) from the hydrophilic N terminus has a relatively minor effect on the ability of the permease to catalyze active lactose transport. Activity is essentially abolished, however, and the permease is hardly detected in the membrane when two additional amino acid residues are deleted (delta 10), and mutants deleted of residues 2-23 (delta 22) or 2-39 (delta 38) also exhibit no activity and are not inserted into the membrane. Dramatically, when the defective deletion mutants are overexpressed at high rates via the T7 promoter, delta 10 and delta 22 are inserted into the membrane in a stable form and catalyze active lactose transport in a highly significant manner, whereas delta 38 is hardly detected in the membrane and exhibits no activity. Interestingly, a fusion protein consisting of delta 38 and the ompA leader peptide is inserted into the membrane but exhibits no transport activity. The results indicate that the N-terminal hydrophilic domain of lac permease and the N-terminal half of the first putative transmembrane alpha-helix are not mandatory for either membrane insertion or transport activity.