Cryo-EM structure of the calcium release-activated calcium channel Orai in an open conformation

Area of Science:

• Structural Biology
• Molecular Physiology
• Immunology

Background:

• Orai channels regulate calcium (Ca2+) influx into non-excitable cells, a process vital for immune responses.
• Pathologic mutations can cause Orai channels to remain constitutively open, disrupting normal cellular function.
• Previous low-resolution X-ray data provided limited insight into the open conformation of Drosophila melanogaster Orai.

Purpose of the Study:

• To determine the high-resolution structure of the constitutively open H206A mutant of Drosophila melanogaster Orai.
• To elucidate the conformational changes and molecular mechanisms underlying Orai channel gating in its open state.

Main Methods:

• Fiducial-assisted cryo-electron microscopy (cryo-EM) was employed to obtain the structure.
• The study achieved a resolution of 3.3 Å, significantly improving upon previous structural data.

Main Results:

• The high-resolution structure reveals detailed conformations of amino acids within the open Orai pore.
• Channel dilation occurs through outward movements of the Orai subunits.
• A phenylalanine ring repositions, exposing glycine residues and forming the channel gate in conjunction with other hydrophobic residues.
• Previously unobserved structured M1-M2 turrets were identified at the channel's extracellular entrance.

Conclusions:

• The study provides a near-atomic resolution model of the open Orai channel.
• This detailed structure elucidates the gating mechanism and the role of specific amino acid residues in channel function.
• The findings offer critical insights into the regulation of calcium signaling and its implications for cellular processes, including immunity.