Methods and compositions are provided for covalently linking a chemical species to a recombinant or synthetic polypeptide. The methods involve the reaction of a thioester-comprising polypeptide with a reagent comprising a reactive amino-thiol group connected to the chemical species which is to be covalently linked to the polypeptide, via a linker. Such chemical species can be a functional group, a label or tag molecule, a biological molecule, a ligand, or a solid support. Efficient and catalyst-free methods for C-terminal protein labeling are also provided. The methods expand current capabilities in the area of protein functionalization, providing useful and complementary tools for the isolation, detection, characterization, and analysis of proteins in a variety of in vitro and in vivo applications.