(Glu62, Ala30, Tyr8)n serves as high-affinity substrate for tyrosylprotein sulfotransferase: a Golgi enzyme.

Tyrosylprotein sulfotransferase, the enzyme catalyzing the sulfation of proteins on tyrosine residues, was characterized by using the acidic polymer containing tyrosine (Glu62, Ala30, Tyr8)n (referred to as Glu,Ala,Tyr) as exogenous "protein" substrate. After subcellular fractionation of a bovine adrenal medulla homogenate, tyrosylprotein sulfotransferase activity was found to be highest in fractions enriched in Golgi membrane vesicles. Tyrosylprotein sulfotransferase required the presence of a nonionic detergent for sulfation of exogenous Glu,Ala,Tyr, indicating an orientation of the catalytic site of the enzyme toward the Golgi lumen. Tyrosylprotein sulfotransferase was solubilized by Triton X-100, suggesting that the enzyme was tightly associated with the Golgi membrane, possibly as an integral membrane protein. The apparent Golgi localization of tyrosylprotein sulfotransferase was supported by the observation that tyrosine sulfation of proteins in intact cells was blocked by monensin and was in line with previous observations that all tyrosine-sulfated proteins known so far are secretory. Glu,Ala,Tyr was found to have a very high affinity for tyrosylprotein sulfotransferase (apparent Km, 300 nM), similar to that reported for certain tyrosylprotein kinases. While this may suggest some similarity between these enzymes, the Golgi localization of tyrosylprotein sulfotransferase segregates tyrosine sulfation from the sites of tyrosine phosphorylation of proteins in the intact cell. If, however, tyrosylprotein sulfotransferase was allowed to react with cytoplasmic proteins by using a nonionic detergent, tyrosine sulfation of tubulin was observed.