Cooh-Terminal Truncations Promote Proteasome-Dependent Degradation of Mature Cystic Fibrosis Transmembrane Conductance Regulator from Post-Golgi Compartments

Benharouga, Mohamed; Haardt, Martin; Kartner, Norbert; Lukacs, Gergely L.

Wt and T70 CFTR were pulse labeled for 15 min and those molecules that arrived at the cell surface were biotinylated during a 1-h chase. Biotinylated CFTR was affinity isolated and visualized by fluorography ( Fig. 1 E, biot) . Radioactivity of biotinylated forms was quantified with PhosphorImage analysis and expressed as a percentage of the respective pulse-labeled core-glycosylated CFTR ( Fig. 1 E, lysate). The cell surface targeting efficiency of T70 CFTR was 74 Ϯ 4% (mean Ϯ SEM, n ϭ 4) of wt CFTR. Since this measurement is compounded by the rapid degradation of the complex-glycosylated T70 CFTR ( t 1/2 ‫ف‬ 1.5-2 h, see also Fig. 6 B; Haardt et al., 1999) , the calculated targeting efficiency of the T70 CFTR represents an underestimate and suggests that the plasma membrane delivery of the complex-glycosylated T70 CFTR is largely preserved.