Carrier proteins modified to incorporate one or more pilin glycotags and applications thereof for O-linked glycosylation are provided. In particular, a modified carrier protein comprising a carrier protein that comprises at least one GlycoTag, wherein the at least one GlycoTag is a Neisseria gonorrhoeae PglL GlycoTag (NgGlycoTag), Neisseria lactamica PglL GlycoTag (NlGlycoTag), or Neisseria shayeganii GlycoTag (NsGlycoTag), or combinations thereof is provided, together with nucleic acids and vectors encoding the modified carrier protein, host cells comprising these modofoed carrier proteins or nucleic acids encoding them, bioconjugates, methods of making bioconjugates and uses of the bioconjugates.