Role of the microcin B17 propeptide in substrate recognition: solution structure and mutational analysis of McbA1−26 Original Research Article

Abstract Background: The peptide antibiotic microcin B17 (MccB17) contains oxazole and thiazole heterocycles formed by the post-translational modification of four cysteine and four serine residues. An amino-terminal propeptide targets the 69 amino acid precursor of MccB17 (preproMccB17) to the heterocyclization enzyme MccB17 synthetase. The mode of synthetase recognition has been unclear, because there has been limited structural information available on the MccB17 propeptide to date. Results: The solution structure of the MccB17 propeptide (McbA1−26), determined using nuclear magnetic resonance, reveals that McbA1−26 is an amphipathic a helix. Mutational analysis of 13 propeptide residues showed that PheB and Leu12 are essential residues for MccB17 synthetase recognition. A domain of the propeptide was putatively identified as the region that interacts with the synthetase. Conclusions: MccB17 synthetase recognizes key hydrophobic residues within a helical propeptide, allowing the selective heterocyclization of downstream cysteine and serine residues in preproMccB17. The determination of the solution structure of the propeptide should facilitate the investigation of other functions of the propeptide, including a potential role in antibiotic secretion.