Structure-Based Analysis of Transient Interactions between Ketosynthase-like Decarboxylase and Acyl Carrier Protein...

Ketosynthase-like decarboxylase (KS_Q) domains are widely distributed in the loading modules of modular type I polyketide synthases (PKSs) and catalyze the decarboxylation of the (alkyl-)malonyl unit bound to the acyl carrier protein (ACP) in the loading module for the construction of the PKS starter unit. Previously, we performed a structural and functional analysis of the GfsA KS_Q domain involved in the biosynthesis of macrolide antibiotic FD-891. We furthermore revealed the recognition mechanism for the malonic acid thioester moiety of the malonyl-GfsA loading module ACP (ACP_L) as a substrate. However, the exact recognition mechanism for the GfsA ACP_L moiety remains unclear. Here, we present a structural basis for the interactions between the GfsA KS_Q domain and GfsA ACP_L. We determined the crystal structure of the GfsA KS_Q-acyltransferase (AT) didomain in complex with ACP_L (ACP_L=KS_QAT complex) by using a pantetheine crosslinking probe. We identified the key amino acid residues involved in the KS_Q domain-ACP_L interactions and confirmed the importance of these residues by mutational analysis. The binding mode of ACP_L to the GfsA KS_Q domain is similar to that of ACP to the ketosynthase domain in modular type I PKSs. Furthermore, comparing the ACP_L=KS_QAT complex structure with other full-length PKS module structures provides important insights into the overall architectures and conformational dynamics of the type I PKS modules.