Expression of Tyrosine‐Sulfated Secretory Proteins in Xenopus laevis Oocytes

Abstract

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.