Acyltransferases that Modify Cell Surface Polymers Across the Membrane

Cell surface oligosaccharides and related polymers are commonly decorated with acyl esters that alter their structural properties and influence their interactions with other molecules. In many cases, these esters are added to polymers that are already positioned on the extracytoplasmic side of a membrane, presenting cells with a chemical challenge because the high-energy acyl donors used for these modifications are made in the cytoplasm. How activated acyl groups are passed from the cytoplasm to extra-cytoplasmic polymers has been a longstanding question. Recent mechanistic work has shown that many bacterial acyl transfer pathways operate by shuttling acyl groups through two covalent intermediates to their final destination on an extracellular polymer. Key to these and other pathways are cross-membrane acyltransferases─enzymes that catalyze transfer of acyl groups from a donor on one side of the membrane to a recipient on the other side. Here we review what has been learned recently about how cross-membrane acyltransferases in polymer acylation pathways function, highlighting the chemical and biosynthetic logic used by two key protein families, membrane-bound O-acyltransferases (MBOATs) and acyltransferase-3 (AT3) proteins. We also point out outstanding questions and avenues for further exploration.