 This breakage is catalyzed by a protein disulfide-isomerase that is in the endoplasmic reticulum.Following separation, the A1 domain unfolds and is redirected back to the cytosol where it refolds and catalyzes ADP-ribosylation of certain G protein alpha subunits.In doing so, the downstream effects of the G protein signal transduction pathway is disrupted by activating adenylate cyclase.