Opposite Regulation of F508del-CFTR Biogenesis by Four Poly-Lysine Ubiquitin Chains

Abstract

Background: As a misfolding protein, more than 99% of F508del-CFTR (F508del) is degraded by the ubiquitin-proteasome system before its maturation, which results in almost no membrane expression of CFTR and thereby no chloride secretion across epithelial cells of cystic fibrosis patients. The conjugation of ubiquitin chains to the protein substrates is necessary for ubiquitin-mediated proteasomal degradation. Ubiquitin contains seven lysine (K) residues (K6, K11, K27, K29, K33, K48, and K63), all of them can be conjugated one to another forming poly-ubiquitin chains on substrates by mixing together or by only one type of lysine that provides sorting signals for different pathways. Results: Here we report that four lysine linked poly-Ub chains (LLPUCs) were involved in F508del biogenesis: two LLPUCs linked by K11 or K48 of Ub facilitated F508del degradation; whereas the other two linked by K63 and K33 of Ub protected F508del from degradation. LLPUC K11 is more potent for F508del degradation than K48. Moreover, E3 ligase CHIP and RNF5 catalyzed LLPUCs K48 formation and RNF5 also catalyzed LLPUC K11 formation on F508del. Importantly, Those LLPUCs provide F508del with different affinities to the proteasomal shuttle protein (Rad23a) and the proteasomal receptors (Adrm1 and S5a), offering a mechanistic insight of differential regulation F508del different by different LLPUCs. Conclusions: F508del utilizes four specific lysine-linked poly Ub chains and during its biogenesis for opposite destiny through different identification by proteasomal shuttle protein or receptors. These findings provide new insights to understand the CF pathogenesis and are expected to facilitate the development of therapies for this devastating disease.