Scholars have discovered the cryo-EM structure of the sodium leakage channel complex

Recently, Chen Lei, a researcher at the Institute of Molecular Medicine of the School of Future Technology of Peking University, discovered the cryo-EM structure of the sodium leakage channel NALCN-FAM155A-UNC79-UNC80 complex and the mechanism of UNC79-UNC80 regulating NALCN-FAM155A. The study was published in Nature Communications on May 12.

The resting membrane potential (RMP) of nerve cells affects the excitability of nerve cells and is essential for maintaining the normal physiological function of nerve cells. Sodium leak channel NALCN (Sodium Leak Channel, Nonselective) mediates the sodium leakage current of nerve cells, which can depolarize the resting membrane potential, thereby improving the excitability of nerve cells.

NALCN is highly conserved in mammals and has high homology with voltage-gated calcium ion channels (CaV) and voltage-gated sodium channels (NaV). It is also involved in many important biological processes related to the nervous system, including the regulation of respiratory rhythms, pain perception, the regulation of the biological clock and rapid eye movement sleep.

“In the population, single-point mutations in NALCN cause a variety of serious neurodevelopmental genetic disorders, including psychomotor retardation and pediatric hypotonia with characteristic facial features, as well as congenital contractures of the extremities and face, hypotonia and developmental delay. Although the NALCN channel has such an important function, its working mechanism is still unclear. Chen Lei told China Science Daily.

In 2020, Chen Lei’s research group analyzed the high-resolution structure of the NALCN-FAM155A subcomplex, elucidated the structural basis of THE SODIUM ion selectivity, extracellular calcium ion blocking and voltage regulation characteristics of the NALCN, and found that the CIH helix located on the II-III linker unique in the NALCN channel can be bound to its intracellular domain. But the structure of UNC79 and UNC80 and how they activate THEN is unclear.

Previous studies have shown that UNC79 and UNC80 are prone to dissociation from the NALCN-FAM155A subcomplex. In this study, the authors fused GFP at the C-terminus of the NALCN, and the N-terminus of UNC80 fused nanoantibodies bound to the high affinity of GFP to stabilize the interaction between UNC79/80 and THE NALCN.

After homologous protein screening and other steps, the researchers determined that the complex composed of rat NALCN and mouse FAM155A, UNC79 and UNC80 subunits was used as the research object, and after overcoming difficulties such as sample preparation and data processing, the electron density of the quaternary complex with an overall resolution of 3.2 angstroms was obtained by single-particle cryo-EM technology, and an atomic model was constructed.

Structures show that both UNC79 and UNC80 are made up of spiral-rich structures that are further assembled into superhelix structures such as HEAT repeats or ARM repeats. The N-terminal of UNC79 interacts closely with the C-terminal of UNC80, the middle hinge region between UNC79 and UNC80, and the C-end of UNC79 and the N-terminal of UNC80, forming a plier-like complex with an overall shape similar to the infinity number “∞”.

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Structure of the NALCN-FAM155A-UNC79-UNC80 complex (Courtesy of the interviewee, the same below)

Further studies have found that NALCN interacts with UNC79-UNC80 mainly through the intracellular loop region: a β-card structure (UNIM-A) in the I-II linker on the cytoplasmic side of NALCN interacts with UNC79, a loop-helical structure (UNIM-B) in the II-III linker and an L-shaped spiral structure (UNIM-C) interact with UNC80 (Figure 1). The authors named the basis sequence in which NALCN interacted with UNC79/80 UNC Interacting Motif (UNIM).

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Schematic diagram of the mechanism by which UNC79-UNC80 activates THE NASALCN

Chen Lei introduced that the study also found that the three subsidiary subunits of UNC79, UNC80 and FAM155A are essential for the correct transport of NALCN to the cell membrane. “This may be because these interactions allow UNC79/80 to block the endoplasmic reticulum retention signal on the cytoplasmic side of the NALCN, thereby promoting the NALCN upper membrane.” In addition, these interactions also release the self-suppression of the NALCN by ciH, causing it to activate. This provides a foundation for a deep understanding of how the NALCN complex works. He said. (Source: China Science Daily Liu Runan)

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