Three-dimensional structure of the anaphase-promoting complex

The anaphase-promoting complex (APC) is a cell cycle-regulated ubiquitin-pr otein ligase, composed of at least 11 subunits, that controls progression t hrough mitosis and G1. Using cryo-electron microscopy and angular reconstit ution, we have obtained a three-dimensional model of the human APC at a res olution of 24 Angstrom. The APC has a complex asymmetric structure 140 Angs trom x 140 Angstrom x 135 Angstrom in size, in which an outer protein wall surrounds a large inner cavity. We discuss the possibility that this cavity represents a reaction chamber in which ubiquitination reactions take place , analogous to the inner cavities formed by other protein machines such as the 26S proteasome and chaperone complexes. This cage hypothesis could help to explain the great subunit complexity of the APC.