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 rule out that cross talks between lysosomal and proteasomal degradation pathway account for the stabilization of the complex-glycosylated T70 CFTR, the specificity of the drugs was examined on BHK cells expressing Tac-TCR␣ or Tac-Lamp1 chimeras. Although the former polypeptide is a substrate of the ERAD (Bonifacino et al., 1990) , the latter is targeted for lysosomal proteolysis (Marks et al., 1996) . As we anticipated, proteasome inhibitors mitigated the disappearance of Tac-TCR␣, but not Tac-Lamp1, whereas bafilomycin B, but not the proteasome inhibitors, stabilized Tac-Lamp1 (Fig. 5, B and C) . Consistent with the notion that proteasome activity accounts, at least in part, for the proteolysis of the complexglycosylated truncated CFTR, proteasome inhibitors increased the steady state level of the complex-glycosylated T70, T82, and T98 CFTR by two-to fourfold according to densitometric analysis of immunoblots (Fig. 5 D) . This was also reflected by the elevated cAMP-activated anion conductance of the plasma membrane (Fig. 5 E) . Lactacystin treatment significantly increased the protein kinase A-stimulated iodide release from BHK cells constitutively expressing T70 CFTR as compared with nontreated cells (Fig. 5 E) .