The present invention provides sequon polypeptides with an amino acid sequence including one or more exogenous O-linked glycosylation sequence of the invention. In addition, the present invention provides methods of making polypeptide conjugates as well as methods of using such conjugates and their pharmaceutical compositions. The invention further provides libraries of sequon polypeptides, wherein each member of such library includes at least one exogenous O-linked glycosylation sequence of the invention. Also provided are methods of making and using such libraries.