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Tyrosine O-sulfation

The O-sulfation of tyrosine residues of plasma membrane and secretory proteins that transit through the secretory pathway of eukaryotic cells is a widespread post-translational modification. This enzymatic reaction is catalyzed by trans-Golgi-associated tyrosylprotein sulfotransferases, which recognize tyrosine residues located in a specific acidic amino acid sequence. Tyrosine sulfation promotes extracellular protein-protein interactions involved in diverse biological processes, ranging from the receptor binding of regulatory peptides to the interaction of viral envelope proteins with the cell surface. This unit outlines procedures to determine whether a protein of interest contains sulfated tyrosine residues, using methods based on labeling proteins with inorganic [35S]-sulfate, alkaline hydrolysis, and one-dimensional thin-layer electrophoresis.

Existence of distinct tyrosylprotein sulfotransferase genes: molecular characterization of tyrosylprotein sulfotransferase-2

Tyrosylprotein sulfotransferase (TPST) is a 54- to 50-kDa integral membrane glycoprotein of the trans-Golgi network found in essentially all tissues investigated, catalyzing the tyrosine O-sulfation of soluble and membrane proteins passing through this compartment. Here we describe (i) an approach to identify the TPST protein, referred to as MSC (modification after substrate crosslinking) labeling, which is based on the crosslinking of a substrate peptide to TPST followed by intramolecular [35S]sulfate transfer from the cosubstrate 3'-phosphoadenosine 5'-phosphosulfate (PAPS); and (ii) the molecular characterization of a human TPST, referred to as TPST-2, whose sequence is distinct from that reported [TPST-1; Ouyang, Y.-B., Lane, W. S. & Moore, K. L. (1998) Proc. Natl. Acad. Sci. USA 95, 2896-2901] while this study was in progress. Human TPST-2 is a type II transmembrane protein of 377 aa residues that is encoded by a ubiquitously expressed 1.9-kb mRNA originating from seven exons of a gene located on chromosome 22 (22q12.1). A 304-residue segment in the luminal domain of TPST-2 shows 75% amino acid identity to the corresponding segment of TPST-1, including conservation of the residues implicated in the binding of PAPS. Expression of the TPST-2 cDNA in CHO cells resulted in an approximately 13-fold increase in both TPST protein, as determined by MSC labeling, and TPST activity. A predicted 359-residue type II transmembrane protein in Caenorhabditis elegans with 45% amino acid identity to TPST-2 in a 257-residue segment of the luminal domain points to the evolutionary conservation of the TPST protein family.

Intracellular transport, sorting, and proteolytic processing of regulated secretory proteins does not require protein sulfation

The biological significance of protein sulfation is poorly understood. To study a possible role of protein sulfation in the regulated secretory pathway, neurointermediate lobes (NIL) of the pituitary of South-African clawed toads (Xenopus laevis) were treated with chlorate, a potent in vivo inhibitor of protein sulfation. We monitored the fates of newly synthesized proopiomelanocortin (POMC), prohormone convertase (PC2), and secretogranin III (SgIII), which are sulfated and regulated secretory proteins. Inhibition of protein sulfation had no effect on the proteolytic processing of these precursor proteins and the kinetics of release of their cleavage products. The release was sensitive to apomorphine, a drug that blocks the release of proteins via the regulated secretory pathway, indicating that no missorting to the constitutive pathway had occurred. Our results suggest that protein sulfation is not required for the intracellular transport, sorting, and proteolytic processing of regulated secretory proteins.

Expression of tyrosine-sulfated secretory proteins in Xenopus laevis oocytes. Differential export of constitutive and regulated proteins

Xenopus laevis oocytes were used to study the tyrosine sulfation and secretion of exogenous proteins. Secretogranin II (SgII), a tyrosine-sulfated protein found in secretory granules of a wide variety of endocrine cells and neurons, became tyrosine-sulfated by the oocytes when expressed by injection of poly(A)-rich RNA isolated from the neuroendocrine cell line PC12. The same result was observed when SgII was expressed from cloned SgII cRNA, showing that its tyrosine sulfation did not require the coexpression of exogenous tyrosylprotein sulfotransferase (TPST) but occurred by means of the endogenous oocyte TPST. Sulfophilin, an artificial protein consisting of 12 repeats of a heptapeptide tyrosine-sulfation site, was highly sulfated upon injection of its RNA, indicating the presence of TPST levels sufficient for stoichiometric sulfation of appropriate reporter proteins. Comparison of the secretion of [35S]sulfate-labelled SgII with that of sulfophilin and an exogenous heparan sulfate proteoglycan (HSPG), two proteins delivered to the cell surface by the constitutive pathway of secretion, revealed striking differences. The majority of sulfophilin and the HSPG was found in the medium, whereas that of SgII was found intracellularly. Prolactin, another secretory granule protein, showed the same secretion behaviour as SgII. These results show that oocytes express TPST and that these cells secrete constitutive and regulated secretory proteins in a differential manner.

Protein tyrosine sulfation, 1993--an update

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.