Methods for the Study of Intermolecular and Intramolecular Interactions Regulating CFTR Function

Abstract

CFTR (Cystic Fibrosis Transmembrane Conductance Regulator) is a cAMP-activated chloride channel present on the apical surfaces of epithelial cells. This protein has been shown to be responsible for salt and water transport across epithelia (1). CFTR has been implicated in two major diseases, namely, cystic fibrosis (CF) and secretory diarrhea. In CF, the synthesis and or functional activity of the CFTR Cl^- channel is reduced. This autosomal recessive disorder affects approx 1 in 2500 Caucasians in the United States (1). Excessive CFTR activity is implicated in cases of toxin-induced secretory diarrhea (e.g., by cholera toxin and heat stable Escherichia coli enterotoxin) that stimulate cAMP or cGMP production in the gut (2). The protein encoded by the CF gene (CFTR) is a 1480-amino acid membrane-bound protein containing five major cytosolic domains, the N- and C-terminal tails, two nucleotide-binding domains (NBD 1 and 2), and a regulatory domain (R-domain). CFTR also has two sets of six transmembrane spanning domains. Physical interactions have been reported for various domains of CFTR with different proteins. The two opposing tails (N and C) of this Cl^- channel connect to different regulatory networks by interacting with distinct proteins. The amino-terminal tail interacts directly with syntaxin 1A and in part is responsible for inhibiting CFTR Cl” current activity. Syntaxin 1A is highly expressed in the brain (3) and to a lesser extent in the lung and colon (4). Members of the syntaxin family of proteins have been implicated in membrane fusion (3,5). In the synapse, N-type calcium channels bind to syntaxin 1A (8,8), which negatively modulates the gating of these channels (9,11). The carboxy terminal tail interacts with PDZ (PSD-95, disc large, ZO-1) domain-containing proteins, and this interaction has been implicated in CFTR targeting to the apical (luminal) surface of polarized epithelial cells (12). A recent review summarizes some of these interactions (13). In this chapter, detailed methods will be presented and discussed that aid in the study of these interactions, with special emphasis on pulldown assays and pairwise binding assays.