Sulfation is the most abundant post-translational modification of tyrosine residues and occurs in many soluble and membrane proteins passing through the secretory pathway of metazoan cells. The sulfation reaction is catalysed by tyrosylprotein sulfotransferase, a membrane-bound enzyme of the trans-Golgi-network. Tyrosylprotein sulfotransferase has been purified and its substrate specificity characterized. Tyrosine sulfation has been shown to be important for protein-protein interactions occurring during the intracellular transport of proteins and upon their secretion.