CRYOELECTRON MICROSCOPY AND 3-DIMENSIONAL RECONSTRUCTION OF THE CALCIUM-RELEASE CHANNEL RYANODINE RECEPTOR FROM SKELETAL-MUSCLE

The calcium release channel (CRC) from skeletal muscle is an unusually large tetrameric ion channel of the sarcoplasmic reticulum, and it is a major component of the triad junction, the site of excitation contr action coupling. The three-dimensional architecture of the CRC was det ermined from a random conical tilt series of images extracted from ele ctron micrographs of isolated detergent-solubilized channels prepared in a frozen-hydrated state. Three major classes of fourfold symmetric images were identified, and three-dimensional reconstructions were det ermined for two of these. The two independent reconstructions were alm ost identical, being related to each other by a 180 degrees rotation a bout an axis in the plane of the specimen grid. The CRC consists of a large cytoplasmic assembly (29 x 29 x 12 nm) and a smaller transmembra ne assembly that protrudes 7 nm from one of its faces. A cylindrical l ow-density region, 2-3 nm in apparent diameter, extends down the cente r of the transmembrane assembly, and possibly corresponds to the trans membrane Ca2+-conducting pathway. At its cytoplasmic end this channel- like feature appears to be plugged by a globular mass of density. The cytoplasmic assembly is apparently constructed from 10 or more domains that are loosely packed together such that greater than 50% of the vo lume enveloped by the assembly is suggestive of a scaffolding and seem s well adapted to maintain the structural integrity of the triad junct ion while allowing ions to freely diffuse to and away from the transme mbrane assembly.