EXPORTATION OF MOUSE VAS-DEFERENS PROTEIN, A PROTEIN WITHOUT A SIGNALPEPTIDE, FROM MOUSE VAS-DEFERENS EPITHELIUM - A MODEL OF APOCRINE SECRETION

Mouse vas deferens protein (MVDP) is a major androgen-dependent protei n of deferential fluid. It is specifically expressed in the epithelium of the mouse vas deferens. Its amino acid sequence as deduced from th e nucleotidic sequence of its cDNA does not possess a signal sequence characteristic of secretory proteins. In vitro, transcription of MVDP cDNA followed by translation of mRNA in the rabbit reticulocyte system , in the absence or the presence of microsomes, demonstrated that ther e was no internalization of MVDP into microsomes that could protect it from degradation by proteinase K; this confirmed the absence of signa l sequence. Moreover, MVDP has its NH2-terminus blocked. To understand how MVDP can be exported, its ultrastructural distribution and secret ion process were analyzed by means of electron microscopy. Immunolocal ization of MVDP revealed that it was distributed in the whole cytoplas m; it was never detected in the lumen of endoplasmic reticulum, Golgi apparatus, or vesicles but was abundant in apical protrusions and in t he fluid, where it was associated with cellular material undergoing de gradation. These data clearly demonstrated that exportation of MVDP in to the luminal fluid does not occur in the classical manner for secret ory proteins but rather involves an apocrine secretion process.