Syntaxin 6 is Involved in the Maintenance of Secretory Granules in Parotid Acinar Cells

Abstract

Secretory granules (SGs) of salivary glands are considered to be generated as an immature granule and to mature by condensing their contents. Syntaxin 6 and vesicle associated membrane protein 4 (VAMP4) are detected in the membrane fractions of immature but not mature SGs, suggesting that syntaxin 6 and VAMP4 are transported from SGs to other organelles during the process of granule maturation. To study the role of syntaxin 6 in the biogenesis of granules, we transfected syntaxin 6 fused with enhanced green fluorescent protein(EGFP)into primary cultured parotid acinar cells. The EGFPsyntaxin 6 signal overlapped with that of an early endosome marker, early endosome antigen 1(EEA1), which was detected using immunofluorescence. The localization of EGFP-fused VAMP4 was also similar to that of EEA1, suggesting that syntaxin 6 and VAMP4 are transported from immature SGs to early endosomes. Transfection of a syntaxin 6 mutant that lacks a SNARE-binding domain caused a decrease in the number of SGs although the localization of the mutant was unchanged from that of the wild type syntaxin 6. These results suggest that syntaxin 6 has an important role in the maintenance of SGs in parotid acinar cells.