The 19S regulatory complex (RC) of 26S proteasomes is a 900-1000 kDa particle composed of 18 distinct subunits (S1-S15) ranging in molecular mass from 25 to 110 kDa. This particle confers ATP-dependence and polyubiquitin (polyUb) recognition to the 26S proteasome. The symmetry and homogenous structure of the proteasome contrasts sharply with the remarkable complexity of the RC. Despite the fact that the primary sequences of all the subunits are now known, insight has been gained into the function of only eight subunits. The six ATPases within the RC constitute a subfamily (S4-like ATPases) within the AAA superfamily and we have shown that they form specific pairs in vitro. We have now determined that putative coiled-coils within the variable N-terminal regions of these proteins are likely to function as recognition elements that direct the proper placement of the ATPases within the RC. We have also begun mapping putative interactions between non-ATPase subunits and S4-like ATPases. These studies have allowed us to build a model for the specific arrangement of 9 subunits within the human regulatory complex. This model agrees with recent findings by Glickman et al. who have reported that two subcomplexes, termed the base and the lid, form the RC of budding yeast 26S proteasomes.
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