National Center for
2L22: Mupirocin didomain ACP
Nat. Chem. Biol. (2013) 9 p.685-692
Type I polyketide synthases often use programmed beta-branching, via enzymes of a 'hydroxymethylglutaryl-CoA synthase (HCS) cassette', to incorporate various side chains at the second carbon from the terminal carboxylic acid of growing polyketide backbones. We identified a strong sequence motif in acyl carrier proteins (ACPs) where beta-branching is known to occur. Substituting ACPs confirmed a correlation of ACP type with beta-branching specificity. Although these ACPs often occur in tandem, NMR analysis of tandem beta-branching ACPs indicated no ACP-ACP synergistic effects and revealed that the conserved sequence motif forms an internal core rather than an exposed patch. Modeling and mutagenesis identified ACP helix III as a probable anchor point of the ACP-HCS complex whose position is determined by the core. Mutating the core affects ACP functionality, whereas ACP-HCS interface substitutions modulate system specificity. Our method for predicting beta-carbon branching expands the potential for engineering new polyketides and lays a basis for determining specificity rules.