Complement yourself: Transcomplementation rescues partially folded mutant proteins

Abstract

Cystic Fibrosis (CF) is an autosomal disease associated with malfunction in fluid and electrolyte transport across several mucosal membranes. The most common mutation in CF is an in-frame three-base pair deletion that removes a phenylalanine at position 508 in the first nucleotide-binding domain of the cystic fibrosis conductance regulator (CFTR) chloride channel. This mutation has been studied extensively and leads to biosynthetic arrest of the protein in the endoplasmic reticulum and severely reduced channel activity. This review discusses a novel method of rescuing ΔF508 with transcomplementation, which occurs when smaller fragments of CFTR containing the wild-type nucleotide binding domain are co-expressed with the ΔF508 deletion mutant. Transcomplementation rescues the processing and channel activity of ΔF508 and reduces its rate of degradation in airway epithelial cells. To apply transcomplementation as a therapy would require that the cDNA encoding the truncated CFTR be delivered to cells. We also discuss a gene therapeutic approach based on delivery of a truncated form of CFTR to airway cells using adeno-associated viral vectors.

Fig. 1

CFTR is a chloride channel and conductance regular (Schwiebert et al. 1999). It has two sets of transmembrane domains, two sets of nucleotide binding domains, and a unique R domain (Fuller and Benos 1992)

Fig. 2

ΔF508 is recognized as a mutant protein and degraded in the proteasome. Small amounts may reach the plasma membrane but the defective channel has reduced channel gating, not enough to support normal physiological function (Fuller and Benos 1992)

Fig. 3

Co-expression of ∆27-264 CFTR restores ΔF508 trafficking and channel function through transcomplementation

Fig. 4

Chaperone displacement theory: because ∆264 CFTR binds avidly to molecules involved in the quality control mechanism, it sequesters these molecules from ΔF508, allowing ΔF508 to proceed through the Golgi to the plasma membrane

Fig. 5

∆27-264 may act as a molecular chaperone by binding to ΔF508 and allowing it to fold properly restoring its native function. Binding is likely to occur via the NBD1 domains because the NBD1 domain is a common requirement for transcomplementation to occur