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 s ecretion of exogenous proteins. Secretogranin II (SgII), a tyrosine-su lfated protein found in secretory granules of a wide variety of endocr ine cells and neurons, became tyrosine-sulfated by the oocytes when ex pressed by injection of poly(A)-rich RNA isolated from the neuroendocr ine cell line PC12. The same result was observed when SgII was express ed 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. Sulfophili n, an artificial protein consisting of 12 repeats of a heptapeptide ty rosine-sulfation site, was highly sulfated upon injection of its RNA, indicating the presence of TPST levels sufficient for stoichiometric s ulfation of appropriate reporter proteins. Comparison of the secretion of [S-35]sulfate-labelled SgII with that of sulfophilin and an exogen ous heparan sulfate proteoglycan (HSPG), two proteins delivered to the cell surface by the constitutive pathway of secretion, revealed strik ing 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 behavio ur as SgII. These results show that oocytes express TPST and that thes e cells secrete constitutive and regulated secretory proteins in a dif ferential manner.