Intestinal brush border membranes contain regulatory subunits of adenylyl cyclase

Abstract

Cholera toxin alters intestinal function by stimulation of adenylyl cyclase [ATP pyrophosphate-lyase (cyclizing) or adenylate cyclase, EC 4.6.1.1]. The mechanism of this activation is unknown and particularly puzzling because adenylyl cyclase is confined to the basal lateral membrane of enterocytes, whereas it is the brush border membrane that binds the toxin and contains proteins that undergo cholera toxin-catalyzed ADP ribosylation. It is shown that cholate extracts from cholera toxin-treated brush border membranes can efficiently reconstitute adenylyl cyclase activity in the guanine nucleotide-binding regulatory component (Gs)-deficient cyc- variant of the S49 mouse lymphoma cell line (cyc- cells lack the alpha subunit of Gs needed to activate the catalytic subunit of adenylyl cyclase). Moreover, NaF (in the presence of Al3+) and guanyl-5'-yl imidodiphosphate mediate strong activation of cyc- adenylyl cyclase provided the cholate extracts of brush border membranes are also present. Therefore, it appears that brush border membranes contain high levels of regulatory subunits of adenylyl cyclase in the absence of catalytic subunits. This represents a previously unrecognized feature of this transduction system that presumably plays an important role in the derangement of intestinal cell function by cholera toxin.