The heterotrimeric GTP-binding protein, G(s), modulates the Cl- conductance of rat parotid acinar secretory granules

G(s alpha) has been reported to be present in rat parotid acinar secretory granule membrane (SGM) fractions. In the present study, we evaluated epitope orientation of G(s alpha) on the secretory granule (SG) and the ability of G(s) to modulate the Cl- conductance of isolated granules by measuring granule lysis, G(s alpha) was found to be associated with the cytoplasmic face of the SGM. Aluminum fluroide (AlF4-, 20 mu M Al3+ and 10 mM F-) significantly increased granule lysis and this effect was blocked by GDP beta S. Cholera toxin (5 mu g/ml) mimicked the effects of AlF4- on granule lysis, whereas pertussis toxin (0.5 mu g/ml) was without effect, GTP gamma S, however, reduced granule lysis in a concentration-dependent manner, The orientation of G(s alpha) on the SGM as well as the effects of AlF4- and cholera toxin on granule lysis lends support for a role of G(s) in the exocytotic process. (C) 1997 Academic Press.