Evidence for posttranslational translocation of beta-lactamase across the bacterial inner membrane

Secretion of beta-lactamase was studied in Salmonella typhimurium infected with P22 phage carrying wild-type and mutant alleles of the structural gene. Cellular location of precursor and mature products of wild-type and temperature-sensitive and chain-terminating mutants was analyzed by cell fractionation and by trypsin accessibility in intact and lysed spheroplasts. The precursors of wild-type and all these mutants (none of which alter the signal peptide) are found sequestered within the cell, while all the mature forms have at least partially been translocated across the inner membrane. Thus most beta-lactamase molecules traverse the membrane after completion of their translation. It seems that the carboxyl terminus of beta-lactamase is not required for translocation across the inner membrane but is required for the protein to appear in the periplasm as a soluble species.