A novel CFTR disease-associated mutation causes addition of an extra N-linked oligosaccharide

We have examined the influence of a novel missense mutation in the fourth e xtracytoplasmic loop (EL4) of CFTR detected in a patient with cystic fibros is. This substitution (T908N) creates a consensus sequence (N X S/T) for ad dition of an N-linked oligosaccharide chain near the C-terminal end of EL4. Oligosaccharyl transferase generally does not have access to this consensu s sequence if it is closer than about twelve amino acids from the membrane. However, the T908N site is used, even though it is within four residues of the predicted membrane interface and the oligosaccharide chain added binds calnexin, a resident chaperone of the ER membrane. The chloride channel ac tivity of this variant CFTR is abnormal as evidenced by a reduced rate of C l-36(-) efflux and a noisy single channel open state. This may reflect some displacement of the membrane spanning sequence C-terminal of EL4 since it contains residues influencing the ion pore.