Electron cryomicroscopy and angular reconstitution used to visualize the skeletal muscle calcium release channel

We exploit the random orientations of ice-embedded molecules imaged in an electron cryomicroscope to determine the three-dimensional structure of the Ca(2+)-release channel from the sarcoplasmic reticulum (SR) in its closed state, without tilting the specimen holder. Our new reconstruction approach includes an exhaustive search of all different characteristic projection images in the micrographs and the assignment of Euler angle orientations to these views. The 30 A map implied reveals a structure in which the transmembrane region exhibits no apparent opening on the SR lumen side. The extended cytoplasmic region has a hollow appearance and consists, in each monomer, of a clamp-shaped and a handle-shaped domain.