Crystal structure of a membrane-bound O-acyltransferase.

Abstract

Membrane-bound O-acyltransferases (MBOATs) represent a superfamily of integral transmembrane enzymes found in all kingdoms of life1. In bacteria, MBOATs modify protective cell surface polymers. In vertebrates, some MBOAT enzymes such as acyl-CoA:cholesterol acyltransferase (ACAT) and diacylglycerol acyltransferase 1 (DGAT1) are responsible for lipid biosynthesis and phospholipid remodeling2,3. Some other MBOATs, including porcupine (PORCN), hedgehog acyltransferase (HHAT) and ghrelin acyltransferase (GOAT), catalyze essential lipid modifications of secreted proteins such as Wnt, hedgehog and ghrelin, respectively4–10. Although many MBOAT proteins are important drug targets, little is known about their molecular architecture and functional mechanisms. Here we present crystal structures of DltB, a MBOAT responsible for D-alanylation of cell wall teichoic acid (TA) of Gram-positive bacteria11–16, by itself and in complex with the D-alanyl donor protein, DltC. DltB contains a ring of 11 peripheral transmembrane helices, which shield a highly conserved extracellular structural “funnel” extending into the middle of lipid bilayer. The conserved catalytic histidine residue is located at the bottom of this funnel and connected to the intracellular DltC through a narrow tunnel. Mutation of either the catalytic histidine or the DltC binding site of DltB abolishes LTA D-alanylation, and sensitizes the Gram-positive bacterium Bacillus subtilis to cell wall stress, suggesting cross-membrane catalysis involving the tunnel. Structure-guided sequence comparison among DltB and vertebrate MBOATs reveals a conserved structural core and suggests similar catalytic mechanisms. Our structures provide a template for understanding MBOAT structure-function relationships and for developing therapeutic MBOAT inhibitors.