Three-dimensional structure of ryanodine receptor isoform three in two conformational states as visualized by cryo-electron microscopy

Using cryo-electron microscopy and single particle image processing techniq ues, we present the first three-dimensional reconstructions of isoform 3 of the ryanodine receptor/calcium release channel (RyR3), Reconstructions wer e carried out on images obtained from a purified, detergent-solubilized rec eptor for two different buffer conditions, which were expected to favor ope n and closed functional states of the channel, As for the heart (RyR2) and skeletal muscle (RyR1) receptor isoforms, RyR3 is a homotetrameric complex comprising two main components, a multidomain cytoplasmic assembly and a sm aller (similar to 20% of the total mass) transmembrane region. Although the isoforms show structural similarities, consistent with the similar to 70% overall sequence identity of the isoforms, detailed comparisons of RyR3 wit h RyR1 showed one region of highly significant difference between them. Thi s difference indicated additional mass present in RyR1, and it likely corre sponds to a region of the RyR1 sequence (residues 1303-1406, known as diver sity region 2) that is absent from RyR3, The reconstructions of RyR3 determ ined under "open" and "closed" conditions were similar to each other in ove rall architecture. A difference map computed between the two reconstruction s reveals subtle changes in conformation at several widely dispersed locati ons in the receptor, the most prominent of which is a similar to 4 degrees rotation of the transmembrane region with respect to the cytoplasmic assemb ly.