Campylobacter jejuni free oligosaccharides: function and fate

The Campylobacter jejuni N-linked protein glycosylation pathway produces a heptasaccharide that is added to >65 periplasmic and membrane proteins and is also released into the periplasm as the free oligosaccharide (fOS). The fOS is a novel soluble component of the C. jejuni periplasmic space that exists in 10-fold greater quantities than its asparagine-linked counterpart. Structurally, fOS is the same heptasaccharide that is found attached to asparagine residues on C. jejuni glycoproteins and both glycans are cleaved from the undecaprenylpyrophosphate anchor by the previously identified oligosaccharyltransferase PglB, which we have now shown to be a bifunctional enzyme also displaying hydrolase activity. The fOS levels in C. jejuni, similar to bacterial periplasmic glucans, can be manipulated by altering the salt and osmolyte concentrations in the growth environment. Here, we outline potential functions of fOS and raise new questions about the underlying mechanism involved in PglB-mediated fOS release from its lipid anchor and fOS retention within the C. jejuni periplasm.