Structure of human sodium leak channel NALCN in complex with FAM155A

Abstract

NALCN, a sodium leak channel mainly expressed in the central nervous systems, is responsible for the resting Na+ permeability that controls neuronal excitability. Dysfunctions of the NALCN channelosome, NALCN with several auxiliary subunits, are associated with a variety of human diseases. Here, we reported the cryo-EM structure of human NALCN in complex with FAM155A, at an overall resolution of 3.1 angstrom. FAM155A forms extensive interactions with the extracellular loops of NALCN that help stabilize NALCN in the membrane. A Na+ ion-binding site, reminiscent of a Ca2+ binding site in Cav channels, is identified in the unique EEKE selectivity filter. Despite its ‘leaky’ nature, the intracellular gate is sealed by S6I, II-III linker and III-IV linker. Our study establishes the molecular basis of Na+ permeation and voltage sensitivity, and provides important clues to the mechanistic understanding of NALCN regulation and NALCN channelosome-related diseases.

Structure and mechanism of NALCN-FAM155A-UNC79-UNC80 channel complex

“…Architecture of the NALCN-FAM155A-UNC79-UNC80 quaternary complex. Previous structural studies on the NALCN-FAM155A heterodimer showed that this subcomplex is wellfolded in the absence of UNC79 and UNC80 [15][16][17] . However, both UNC79 and UNC80 are crucial for the NALCN currents in HEK293T cells 1 , indicating that UNC79 and UNC80 might not be necessary for channel folding, but rather affect the plasma membrane localization of the channel or play other regulatory roles.…”

“…1b), suggesting they could form a stable heterodimer. However, the quaternary complex tends to dissociate during purification 16,17 , probably due to the low affinity between the NALCN-FAM155A subcomplex and UNC79-UNC80 heterodimer. To overcome this obstacle, we exploited the tight binding between GFP and its nanobody 19 .…”

“…In agreement with this, genetic mutations of the UNC80 gene also lead to IHPRF diseases (IHPRF2; OMIM 616801) in human 13 , and knock-out of UNC79 leads to disrupted breathing rhythms 14 , phenocopying the loss-of-function of the NALCN channel in mice 6 . Recently advances in the structure determination of mammalian NALCN-FAM155A subcomplex have provided insights into how FAM155A interacts with the NALCN channel, how the functional and degenerate voltage sensors are spatially arranged, and how the non-canonical selectivity filter allows the permeation of sodium [15][16][17] . Moreover, our previous high-resolution structure identified a CTD-Interacting Helix (CIH) on the linker of NALCN domain II-III (D II-III ) 15 .…”

Architecture of the human NALCN channelosome

“…We have previously reported the structure of human NALCN-FAM155A subcomplex, revealing that FAM155A stabilizes NALCN by attaching to the extracellular loops of NALCN 17 . UNC79 and UNC80, however, were invisible in the previous structure, although they were co-expressed with NALCN and FAM155A.…”

“…However, the auxiliary subunits of the NALCN channelosome are unique and share no sequence homology to the auxiliary subunits of other 4 × 6 TM channels. Among the auxiliary subunits, FAM155A may facilitate the folding and membrane translocation of NALCN through forming a stable subcomplex with NALCN [17][18][19] . UNC79 and UNC80 are large proteins that have been reported indispensable for neuronal localization in mice 9 and robust circadian locomotor rhythms in Drosophila 6 .…”