Occurrence of tyrosine sulfate in proteins--a balance sheet. 2. Membrane proteins

1. The abundance of tyrosine sulfate in membrane proteins was quantified in four different cell lines and compared to that in soluble cellular and secreted proteins. 2. Upon metabolic labelling of HepG2, Ltk-, AtT20 and PC12 cells with [35S]sulfate or [3H]tyrosine, a fraction enriched in integral membrane proteins was found to contain small, but significant, amounts of protein-bound tyrosine sulfate (up to 2.5% of the total cellular plus secreted protein-bound tyrosine sulfate). On the other hand, the frequency of sulfation of tyrosine residues of membrane proteins was within the same order of magnitude as that of secreted proteins, indicating that the low abundance of tyrosine sulfate in membrane proteins was largely a reflection of the low abundance of these proteins themselves. Consistent with this conclusion were the results of an analysis showing that 14 out of 32 selected membrane-spanning proteins contain potential tyrosine sulfation sites. 3. In HepG2 cells, three tyrosine-sulfated integral membrane glycoproteins of molecular mass 100, 125 and 150 kDa were identified. Characterization of the 150-kDa tyrosine-sulfated membrane protein revealed that it was protected from proteolysis in intact cells, suggesting a localization in an intracellular organelle. 4. Together with the results reported in the preceding paper in this journal, our data suggest that tyrosine sulfation occurs in various classes of trans-Golgi-derived proteins, soluble as well as membrane, and extracellularly exposed as well as intracellularly retained, proteins. This suggests that tyrosine sulfation may have a variety of physiological functions, depending on the individual tyrosine-sulfated protein or protein class.