The consequences of stepwise deletions from the signal-processing site of beta-lactamase

Amino acids have been deleted from the processing site of pre-beta-lactamase, either into the signal sequence or into the mature protein. Whereas the loss of more than 2 amino acid residues from the C-terminal end of the signal sequence prevents the translocation of the protein into the periplasm, the removal of two or more amino acids from the beginning of the mature protein has no effect on the translocation of the truncated protein. The insertion of an additional one to three amino acids at the processing site has no detectable phenotypic consequence either. It appears that many sequences for the first few residues of the mature protein allow successful translocation and processing. In sharp contrast, the removal of one (but not both) of the amino acids that flank the processing site results in a severe growth defect in the host cell and very low expression of the protein. Yet removal of two amino acids from either side of the processing site, or removal of both the flanking residues of the processing site, results in normal secretion and signal cleavage. These results illustrate the limits on the amino acid sequence around the processing junction and suggest that interference with the signal cleavage step can lead not only to aborted secretion but also to pleiotropic consequences for the growth of the host organism.