Figure 7.

A model showing the dynamic relationships among PR500, 19S regulatory particle, and other proteasome structures. Under this model, 19S regulatory particle and PR500 are both present in the cell, and cellular stresses and other signals can modulate their equilibrium. Note that the presence of ATP (+) will help assembly of the 26S proteasome, whereas absence of ATP (−) will cause the disassociation of the 26S proteasome into 20S and 19S components. Free subunits and subcomplexes apart from the 19S particle may also participate in other unknown cellular activities. It is not clear whether the incorporation of PR500 into 19S or 26S proteasome would constitute a normal or distinct lid subcomplex, which would have different functional implications. See DISCUSSION for more detail.