Few tools are available to determine the structure of large integral membrane proteins such as intracellular Ca(2+) release channels, RyRs and IP3Rs. Single particle cryo-EM can readily determine the structure of such channels to intermediate resolution, and can be used to quantitatively assess conformational variability. However, due to the, often low, image contrast of these cryospecimens, methods for validation are critical to insure the accuracy of such structures, and to put limits on their interpretability. The low-resolution structure of RyR has been well established for some time, but high-resolution has been slow to emerge. The structure of IP3R channel by cryo-EM had a number of false-starts, but improved validation methods have recently lead to a demonstrably accurate reconstruction.
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