Beta-turn formation in the processing region is important for efficient maturation of Escherichia coli maltose-binding protein by signal peptidase I in vivo

Signal peptidase I (also called leader peptidase) is the endopeptidase that removes the signal peptides of most secreted proteins during or after translocation in Escherichia coli. Precursor recognition is contingent in part on the presence of small, uncharged residues in the -3 and -1 positions relative to the cleavage site, and may also depend on the structure of the processing region. Most precursor processing regions include residues likely to form a beta-turn. Mutations were introduced into the processing region of maltose-binding protein (MBP) that altered the prediction of beta-turn formation in this region. MBP species with a decreased probability of beta-turn formation were processed slowly or not at all, whereas MBP species with an increased probability of beta-turn formation were processed efficiently. Mutations altering the prediction of beta-turn formation in the MBP processing region were also made in cis to a proline in the +1 position. Cleavage at the normal processing site is blocked by proline in the +1 position; this MBP species, MBP27-P, inhibits processing of other proteins by signal peptidase I. Decreasing the probability of beta-turn formation in the processing region of MBP27-P eliminated the inhibition of signal peptidase I, and these MBP27-P derivatives remained unprocessed, suggesting that the formation of a beta-turn in the MBP processing region was necessary for recognition by signal peptidase I. Increasing the probability of beta-turn formation in cis to proline at +1 in MBP did not alter recognition of the protein by the processing enzyme. The results presented here are consistent with the hypothesis that the efficiency of recognition and processing by signal peptidase I is increased by the formation of a beta-turn in the processing region of the MBP signal peptide.