Structure of voltage-modulated sodium-selective NALCN-FAM155A channel complex

Abstract

Resting membrane potential determines the excitability of the cell and is essential for the cellular electrical activities. The NALCN channel mediates sodium leak currents, which positively adjust resting membrane potential towards depolarization. The NALCN channel is involved in several neurological processes and has been implicated in a spectrum of neurodevelopmental diseases. Here, we report the cryo-EM structure of rat NALCN and mouse FAM155A complex to 2.7 Å resolution. The structure reveals detailed interactions between NALCN and the extracellular cysteine-rich domain of FAM155A. We find that the non-canonical architecture of NALCN selectivity filter dictates its sodium selectivity and calcium block, and that the asymmetric arrangement of two functional voltage sensors confers the modulation by membrane potential. Moreover, mutations associated with human diseases map to the domain-domain interfaces or the pore domain of NALCN, intuitively suggesting their pathological mechanisms.

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

“…The structure of the NALCN-FAM155A subcomplex is overall similar to our previous structure at 2.65 Å (Fig. 1b-f) 15 . The UNC79-UNC80 heterodimer forms a large helix-rich structure in the cytosol.…”

“…Guided by our previous high resolution structure of NALCN-FAM155A subcomplex 15 , we found that the combination of I753A, L754A, R761A, R764A and R765A mutations (CIH-5A) is sufficient to disrupt such interaction, shown by GST pull-down assay (Fig. 5f).…”

“…Previous 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.…”

“…Recently advances on 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, the high-resolution structure allowed the identification of a CTD Interacting Helix (CIH) on the linker of NALCN domain II-III (D II-III ) 15 . The interaction between CTD and CIH has never been observed in related Na V or Ca V channels before.…”

Analysis of the Mechanosensor Channel Functionality of TACAN

“…specifying four output classes. The best class with 245,031 particles was selected, then subjected to a resolution-based classification workflow similar to a previous study (Kang et al, 2020). In brief, 40 iterations of global search 3D classification (K=1) in Relion 3.1 with an angular sampling step of 7.5° was performed to determine the initial alignment parameters using the initial model generated from cryoSPARC.…”