Structural and Dynamic Basis of Molecular Recognition between Acyltransferase and Carrier Protein in E. coli Fatty Acid Synthesis

Fatty acid synthases (FASs) and polyketide synthases (PKSs) iteratively elongate and often reduce two-carbon ketide units in de novo fatty acid and polyketide biosynthesis. Cycles of chain extensions in FAS and PKS are initiated by an acyltransferase (AT), which loads monomer units onto acyl carrier proteins (ACPs), small, flexible proteins that shuttle covalently linked intermediates between catalytic partners. Formation of productive ACP-AT interactions is required for catalysis and specificity within primary and secondary FAS and PKS pathways. Here, we use the Escherichia coli FAS AT, FabD, and its cognate ACP, AcpP, to interrogate type II FAS ACP-AT interactions. We utilize a covalent crosslinking probe to trap transient interactions between AcpP and FabD to elucidate the first x-ray crystal structure of a type II ACP-AT complex. Our structural data are supported using a combination of mutational, crosslinking, and kinetic analyses, and long timescale molecular dynamics (MD) simulations. Together, these complementary approaches reveal key catalytic features of FAS ACP-AT interactions. These mechanistic inferences suggest that AcpP adopts multiple, productive conformations at the AT binding interface, allowing the complex to sustain high transacylation rates. Furthermore, MD simulations support rigid body subdomain motions within the FabD structure that may play a key role in AT activity and substrate selectivity. Significance StatementThe essential role of acyltransferases (ATs) in fatty acid synthase (FAS) and polyketide synthase (PKS) pathways, namely the selection and loading of starter and extender units onto acyl carrier proteins (ACPs), relies on catalytically productive ACP-AT interactions. Here, we describe and interrogate the first structure of a type II FAS malonyl-CoA:ACP-transacylase (MAT) in covalent complex with its cognate ACP. We combine structural, mutational, crosslinking and kinetic data with molecular dynamics simulations to describe a highly flexible and robust protein-protein interface, substrate-induced active site reorganization, and key subdomain motions that likely govern FAS function. These findings strengthen a mechanistic understanding of molecular recognitions between ACPs and partner enzymes and provide new insights for engineering AT-dependent biosynthetic pathways.