| The 20S proteasome has a barrel - shaped structure arranged as four heptomeric rings of alphabetabetaalpha. In eukaryotes, each of alpha and beta ring is composed of seven different proteins. The beta1, beta2 and beta5 subunits have 'caspase-like, 'trypsin-like and 'chymotypsin-like activities, respectively. In 26S proteasome-mediated protein degradation, to entry the beta chamber of the 20S proteasome that houses the proteolytic sites, a substrate protein has to pass through a substrate translocation channel consisting of the double-ring formed by six ATPases of PA700 and the alpha chamber formed by alpha subunits of the 20S proteasome. |
