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.

To test this hypothesis, the disposal rate of the metabolically labeled, complex-glycosylated T70 and T82 CFTR was measured in the presence of lysosomal protease inhibitors after allowing the conversion of the core-into complex-glycosylated form (Fig. 2 A) . The degradation rate was unaltered upon exposing the cells to NH 4 Cl, chloroquin, or bafilomycin B, agents that inhibit endolysosomal proteolysis by dissipating the acidic luminal pH and interfering with cargo delivery (Fig. 2 B) . Similarly, leupeptin and pepstatin A, inhibitors of cathepsins B, H, L, N, S, and T (Seglen, 1983) , have a negligible effect on the degradation rate (Fig. 2 , A and B) and the steady state expression of truncated CFTRs (Fig. 2 C) . In contrast, these drugs stabilized the interleukin 2 receptor ␣ chain (Tac), which is targeted for lysosomal degradation (Fig. 2 D; Hemar et al., 1995) , validating their efficacy and suggesting that the initial proteolysis of the truncated CFTR is largely independent of endolysosomal proteases.