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.

Evidence suggests that nonnative states of soluble polypeptides induced by mutations or thermal or chemical denaturation can lead to accelerated degradation (Parsell and Lindquist, 1993; Hershko and Ciechanover, 1998; Brodsky and McCracken, 1999; Plemper and Wolf, 1999) , a paradigm that may apply to the degradation of nonnative plasma membrane proteins, including the truncated CFTR, as well. Based on the enhanced protease susceptibility and the decreased in vivo thermostability of T70 CFTR (Figs. 8 and 9), we speculate that structural destabilization of the native form is induced by disrupting interaction(s) of the COOH terminus. Using various yeast screening assays and site-directed mutagenesis of the Deg-1 proteolytic motif in the MAT␣2 transcription factor, it has been established that ubiquitin-conjugating enzymes (Ubc6 and Ubc7) can recognize hydrophobic stretches and exposed hydrophobic surfaces on amphipathic helices (Sadis et al., 1995; Gilon et al., 1998; Johnson et al., 1998) . These or similar motifs, normally buried inside the globular domains of CFTR or inserted into the membrane plane, may serve as recognition signal(s). In support of this notion, the decreased plasma membrane residence time and impaired expression level of the ␤ 2 -and the ␣ 2A -adrenergic receptors were also attributed to structural destabilization (Gether et al., 1997; Wilson and Limbird, 2000) . However, the mechanism responsible for the downregulation of these G protein-coupled receptors, which could be partially rescued with ligand stabilization, has remained obscure.