EMC chaperone-Ca_V structure reveals an ion channel assembly intermediate

Area of Science:

• Structural biology
• Molecular neuroscience
• Membrane protein biogenesis

Background:

• Voltage-gated ion channels (VGICs) are crucial for cellular function but their assembly mechanism is poorly understood.
• High-voltage-activated calcium channels (Ca_Vs) are complex multisubunit proteins requiring proper assembly for function and trafficking.
• The role of chaperone proteins in VGIC assembly, particularly the endoplasmic reticulum membrane protein complex (EMC), remains unclear.

Purpose of the Study:

• To elucidate the structural basis of VGIC assembly by examining Ca_V1.2 channels.
• To investigate the role of the endoplasmic reticulum membrane protein complex (EMC) as a potential chaperone in Ca_V channel biogenesis.
• To determine the structural interactions between Ca_V channel subunits and auxiliary proteins, including the Ca_Vα₂δ subunit.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was employed to determine high-resolution structures.
• Structures were obtained for Ca_V1.2 bound to Ca_Vβ₃ and the EMC, and for the fully assembled Ca_V1.2-Ca_Vβ₃-Ca_Vα₂δ-1 channel.
• Analysis of structural interfaces to identify key interaction sites and conformational changes.

Main Results:

• Cryo-EM structures revealed the Ca_V1.2 channel complexed with Ca_Vβ₃ and the EMC, providing a view of an EMC-client complex.
• Distinct EMC binding sites (transmembrane and cytoplasmic docks) were identified, which interact with the Ca_V channel to facilitate subunit assembly.
• The structures pinpoint the Ca_Vα₂δ-binding site for gabapentinoid drugs and show that EMC and Ca_Vα₂δ interactions with the channel are mutually exclusive, suggesting a hand-off mechanism.
• Disruption of the EMC-Ca_V complex impaired Ca_V channel function, supporting the EMC's role as a holdase chaperone.

Conclusions:

• The EMC functions as a holdase chaperone, facilitating the assembly of multisubunit Ca_V channels.
• Structural insights into the EMC-Ca_V complex reveal key interactions and a hand-off mechanism involving Ca_Vα₂δ subunits.
• These findings have broad implications for understanding the biogenesis of VGICs and other complex membrane proteins.