REGULATION OF THE CFTR CHLORIDE CHANNEL FROM HUMANS AND SHARKS

The cystic fibrosis transmembrane conductance regulator (CFTR) is an A TP-dependent channel which mediates cAMP-stimulated chloride secretion by epithelia, particularly those of the pancreas, airways, and intest ine. CFTR homologues have been found in all higher vertebrates examine d to date and also in some lower vertebrates, although only the human, shark, and Xenopus genes have been heterologously expressed and shown to generate protein kinase A-activated Cl channels. Once phosphorylat ed, CFTR channels require hydrolyzable nucleotides to be active, but t hey can be locked in an open burst state when exposed to mixtures of A TP and its hydrolysis-resistant analogue AMP-PNP. This locking require s low-level phosphorylation at unidentified sites that are not among t he ten ''strong'' (dibasic) PKA consensus sequences on CFTR. Mutagenes is of the dibasic PKA sites, which reduces in vitro phosphorylation by >98%, reduces open probability (P-o) by about 50% whilst having no ef fect on burst duration. Thus, incremental phosphorylation of these sit es under normal conditions does not increase P-o by slowing down ATP h ydrolysis and stabilizing the open burst state, although locking does strictly require low-level phosphorylation at one or more cryptic site s. In addition to serving as a Cl channel, there is compelling evidenc e that CFTR inhibits the amiloride-sensitive, epithelial sodium channe l (ENaC). The mechanism of coupling is not known but most likely invol ves physical interactions between the channels, perhaps mediated by an intermediate protein that impinges on other transport proteins. CFTR does not function as a conductive channel for ATP; however, extracellu lar ATP does regulate epithelial channels through activation of P-2U p urinergic receptors and, after being hydrolyzed extracellularly, throu gh activation of adenosine receptors which elevate cAMP. (C) 1996 Wile y-Liss, Inc.