Automated cryoelectron microscopy of “single particles” applied to the 26S proteasome

The 26S proteasome is a large molecular machine with a central role in intracellular protein degradation in eukaryotes. The 2.5 MDa complex, which is built from two copies each of more than 30 different subunits, is labile and prone to dissociation into subcomplexes. Hence it is difficult if not impossible, to obtain structurally homogeneous preparations and, as a consequence, it is very cumbersome to obtain large numbers of images of the holocomplex. In this communication, we describe an automated procedure for the acquisition of large data sets of cryoelectron micrographs. The application of this procedure to the 26S proteasome from Drosophila has allowed us to determine the three-dimensional structure of the complex to a resolution of 2.9 nm and the prospects for further improvements are good.