Evaluating the role of conserved amino acids in bacterial O-oligosaccharyltransferases by in vivo, in vitro and limited proteolysis assays.

Bacterial O-Oligosaccharyltransferases (O-OTases) constitute a growing family of enzymes that catalyze the transfer of a glycan from a lipid carrier to protein acceptors. O-OTases are inner membrane proteins that display limited sequence similarity, except for the Wzy_C signature domain also present in a predicted periplasmic loop of the WaaL ligase, the enzyme responsible for transferring the O antigen to the lipid A core. The mechanism of O-OTase-dependent glycosylation is poorly understood. In this work, conserved amino acid residues in the O-OTases were replaced with alanine in PglL, the O-OTase of Neisseria meningitidis. The activity of wild-type PglL and its mutant derivatives were analyzed in vivo in engineered Escherichia coli cells, and in in vitro assays. We identified two additional sites of pilin glycosylated exclusively by PglL in E. coli. Both sites are modified with phosphoglycerol (PG) by different enzymes in Neisseria gonorrhoeae and Neisseria meningitidis. Limited proteolysis experiments revealed a conformational change that is triggered upon interaction of the C-terminal region of PglL with the lipid-linked oligosaccharide (LLO) substrate. These experiments showed that Q178 and Y405 are required for optimal function, whereas H349 is essential for activity and plays a critical role in the interaction with LLO. The equivalent His residue is also essential for WaaL activity, which suggests a common mechanism for both enzymes, and supports the hypothesis that O-glycosylation and lipopolysaccharide (LPS) synthesis are evolutionarily related. These results contribute to the elucidation of the mechanism of O-OTases, which are promising targets for novel antibiotics and present an enormous potential for glycoengineering novel vaccines and therapeutics.