Site-specific functionalization of biomolecules is becoming increasingly important and researchers have focused much effort on the selective functionalization of a certain natural amino acid residue in peptides and proteins.¹ Recently, an efficient sulfate Click reaction between an aryl silyl ether and an aryl fluorosulfate in the presence of a base catalyst was developed by Sharpless’ group and was named sulfur fluoride exchange (SuFEx) reaction.² We envisioned that this SuFEx reaction could be exploited for selective tyrosine bioconjugation if the reaction conditions met the requirements for bioconjugation. In this approach, we have discovered that tyrosine (Tyr) embedded in a peptide or a proein reacts selectively with aryl fluorosulfate in the presence of various amino acid residues in bio-tolerable conditions.³ In the conjugations of amino acids, a model peptide, and a protein, extremely high chemoselectivity of this unique SuFEx reaction was confirmed. The functions of peptides and proteins were well preserved in two cases we examined. Tyr-specific modification of a cell-penetrating TAT 47-57 peptide with a fluorescent dye showed that cell penetration capability of the peptide was not altered upon selective Tyr-conjugation. Moreover, selective tyr-conjugation on the surface of recombinant human erythropoietin (rhEPO) was possible on Tyr-49 of erythropoietin with a short PEG unit containing fluorosulfonylated phenol unit. Structures of the Tyr-conjugated TAT peptide and rhEPO were identified through the use of extensive MALDI-TOF analyses. We also confirmed that the hematogenous function of the PEGylated rhEPO (PEG-rhEPO) was retained after the PEGylation in an in vivo expreiment using Balb/c mice. We believe that this method is readily applicable to residue-specific modification of native proteins, and thus would expand the versatility of bio-conjugation in peptide and protein chemistry.