Despite being known and studied for nearly 200 years, taurine has not been found covalently incorporated into proteins in any animal tissue. It is known to be found in some covalent complexes, as in bile salts, and small brain peptides, but in only one macromolecular complex: as a clinically significant posttranscriptional modification to a mitochondrial tRNA . Tubulin is the heterodimeric (αβ) subunit of microtubules (MT), found in all eukaryotic species and in almost all eukaryotic cells . The sequence is highly conserved except for the carboxyl-terminal ~3% (10-20 residues), which is highly variable, disordered, acidic, and the site of many reversible posttranslational modifications (PTM) . An important PTM on α-tubulin is the removal and religation of the final carboxyl residue, tyrosine: detyrosination by a carboxypeptidase and retyrosination by tubulin-tyrosine ligase . We here describe a PTM that requires detyrosination and prevents retyrosination: the addition of taurine to detyrosinated a-tubulin of avian erythrocytes. Further, this is to our knowledge, the first instance of taurine incorporation into a large protein.