Dystonia-associated Torsins sustain CLCC1 function to promote membrane fusion of the nuclear envelope for NPC biogenesis

DYT1 early-onset dystonia is a severe, incurable disorder of the central nervous system caused by mutations in the gene encoding Torsin1A (Tor1A, DYT1). Torsins are ER-resident AAA+-ATPases implicated in lipid metabolism, nuclear pore complex (NPC) biogenesis, and lipoprotein secretion, yet their molecular function that underlies the disease pathology has remained incompletely understood. Here, we have utilized Drosophila melanogaster and human somatic cells as experimental models to shed light on their mode-of-action. Fly germ cells lacking dTorsin are arrested in development and display defects in the final steps of NPC biogenesis due to a failure in fusion of the inner and outer nuclear membranes. Using proximity labelling of Torsin1A in human cells, we identify the conserved membrane protein chloride channel CLIC-like protein 1 (CLCC1) as a novel Torsin binding partner. Absence of human CLCC1 or its Drosophila homolog dClcc1 phenocopied the membrane fusion defects at NPC assembly sites observed upon Torsin deletion. Furthermore, CLCC1 is enriched at arrested fusion sites, suggesting it to be a candidate for the elusive NE membrane fusogen. Importantly, CLCC1/dClcc1 overexpression is sufficient to rescue NPC biogenesis and developmental defects associated with Torsin-loss-of-function. Taken together, our data suggest that Torsin-regulated CLCC1 activity drives membrane fusion during NPC biogenesis and reveal that modulating CLCC1 expression is a promising therapeutic prospect for DYT1 dystonia.