VAMP-associated Proteins (VAP) as Receptors That Couple Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Proteostasis with Lipid Homeostasis

• Published in: The Journal of biological chemistry 2016-03, Vol.291 (10), p.5206-5220
• Main Authors: Ernst, Wayne L., Shome, Kuntala, Wu, Christine C., Gong, Xiaoyan, Frizzell, Raymond A., Aridor, Meir
• Format: Article
• Language: English
• Subjects: Adaptor Proteins, Signal Transducing; Adenosine Triphosphatases; amyotrophic lateral sclerosis (ALS) (Lou Gehrig disease); Apoptosis Regulatory Proteins; Binding Sites; Cell Cycle Proteins; Cholesterol - metabolism; cholesterol regulation; cystic fibrosis transmembrane conductance regulator (CFTR); Cystic Fibrosis Transmembrane Conductance Regulator - metabolism; DNA-Binding Proteins - metabolism; ER-associated degradation; HEK293 Cells; HeLa Cells; Homeostasis; Humans; Membrane Proteins - metabolism; Protein Binding; Protein Stability; Protein Synthesis and Degradation; Proteolysis; proteostasis; Ubiquitin-Protein Ligases - metabolism; Valosin Containing Protein; Vesicular Transport Proteins - metabolism
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M115.692749

Unesterified cholesterol accumulates in late endosomes in cells expressing the misfolded cystic fibrosis transmembrane conductance regulator (CFTR). CFTR misfolding in the endoplasmic reticulum (ER) or general activation of ER stress led to dynein-mediated clustering of cholesterol-loaded late endosomes at the Golgi region, a process regulated by ER-localized VAMP-associated proteins (VAPs). We hypothesized that VAPs serve as intracellular receptors that couple lipid homeostasis through interactions with two phenylalanines in an acidic track (FFAT) binding signals (found in lipid sorting and sensing proteins, LSS) with proteostasis regulation. VAPB inhibited the degradation of ΔF508-CFTR. The activity was mapped to the ligand-binding major sperm protein (MSP) domain, which was sufficient in regulating CFTR biogenesis. We identified mutations in an unstructured loop within the MSP that uncoupled VAPB-regulated CFTR biogenesis from basic interactions with FFAT. Using this information, we defined functional and physical interactions between VAPB and proteostasis regulators (ligands), including the unfolded protein response sensor ATF6 and the ER degradation cluster that included FAF1, VCP, BAP31, and Derlin-1. VAPB inhibited the degradation of ΔF508-CFTR in the ER through interactions with the RMA1-Derlin-BAP31-VCP pathway. Analysis of pseudoligands containing tandem FFAT signals supports a competitive model for VAP interactions that direct CFTR biogenesis. The results suggest a model in which VAP-ligand binding couples proteostasis and lipid homeostasis leading to observed phenotypes of lipid abnormalities in protein folding diseases.