The 26S proteasome, a eukaryotic ATP-dependent protease complex, is responsible for selective degradation of malformed proteins and certain short-lived proteins. The 26S proteasome complex consists of the 20S proteasome (multifunctional protease), which is the catalytic core, and an ATPase-containing P700 regulatory complex. This complex contains five highly related putative ATPases (TBP1, TBP7, S4, MSS1, p45) that belong to the ATPase family designated AAA (ATPases Associated with a variety of cellular Activities). They share a highly conserved 200 amino acid ATPase domain (AAA module) and participate in a range of cellular functions. p45 (Trip1, hSUG1) is the homolog of the S. cerevisiae and mouse SUG1 proteins. In a ligand-enhanced manner, mouse SUG1 interacts with a range of nuclear receptors via their AF-2 domains and yeast SUG1 functions as a transcription factor that mediates the transcriptional response of the GAL4 protein. p45 is thought to be their functional homolog. Thus, in addition to its participation in the P700 regulatory complex, p45 may function as a mammalian transcriptional modulator.