REGγ: A Shortcut to Destruction

256 Cell 124, January 27, 2006 ©2006 Elsevier Inc. The proteasome is a major nonlysosomal proteolytic apparatus that destroys damaged or misfolded proteins as well as regulators governing fundamental cellular processes such as the cell cycle, transcription, cell signaling, cell death, and the immune response (Goldberg, 2003). The catalytic core of this large multisubunit proteolytic complex is the 20S proteasome, which consists of 14 α and 14 β subunits arranged in a cylindrical particle of four heptameric rings with a central channel of 17Å in diameter and resembles a barrel. Two inner β rings harbor all of the proteolytic sites for substrate cleavage inside the central catalytic chamber, whereas two outer α rings provide the gated entry sites for substrate peptides. The 20S proteasome is a latent protease complex, as the N-terminal tails of α subunits occlude the central proteolytic channel. Activation of the 20S proteasome is achieved by either the 19S (PA700 or RC) regulatory particle (to form the 26S proteasome) or the 11S (REG or PA28) regulator, both of which associate with the top or bottom surface of the 20S “barrel” to trigger opening of the gate to allow entry of unfolded substrates. The REG/11S particle consists of REG subunits that are also arranged in a ring-shaped heptameric complex (Rechsteiner and Hill, 2005). Of the three REG family members, REGα can form either homoheptamers or heteroheptamers with REGβ throughout the cell to participate in MHC class I antigen presentation. In contrast, REGγ (also called Ki antigen or PSME3) resides primarily in the nucleus in a homoheptameric form, and its function has remained largely elusive. In this issue of Cell, Li et al. (2006) provide evidence that the REGγ-20S complex is capable of selectively targeting an intact substrate—in this case, the nuclear receptor SRC-3—for destruction. During the investigation of the homeostasis of steroid-hormone receptors and coactivators, Li et al. (2006) discovered that REGγ interacts selectively with SRC-3, a member of the p160 nuclear receptor coactivator family involved in transcriptional regulation of genes activated through steroid receptors (Liao et al., 2002). Overexpression of REGγ accelerated the turnover rate of SRC-3, whereas silencing REGγ expression with small interfering RNAs (siRNAs) resulted in accumulation of SRC-3 protein, suggesting an unexpected role for REGγ in determining the stability of an intact protein in vivo. The most striking feature of the Li et al. (2006) study is the biochemical evidence for an unconventional mode of SRC-3 degradation by the REGγ-20S proteasome, which was thought only to hydrolyze small peptides (Rechsteiner and Hill, 2005). In a cell-free proteasome proteolysis assay, fulllength SRC-3 was degraded in the presence of the 20S proteasome and heptameric REGγ without exogenous ATP or ubiquitin pathway components. The REGα/β complex did not promote the degradation of SRC-3. Furthermore, the closely related SRC-1, which is incapable of binding to REGγ, was resistant to REGγ-mediated degradation. These data provide compelling evidence for the presence of an alternative mechanism of targeting an intact protein to a distinct REGγ-20S proteasome complex for destruction. This unexpected finding by Li et al. (2006) may provoke reassessment of the biochemical properties and physiological functions of the REG-20S proteasome. Unlike the 19S complex, the REG heptamer does not contain ATPase subunits to catalyze protein unfolding and translocation into the 20S particle. SRC-3 is a 160 kDa protein that contains distinct structural motifs, including a bHLH-PAS domain in the N terminus and a histone acetyltransferase (HAT) domain in the C terminus that is specifically responsible for binding to REGγ. Given that ATP and “reverse chaperones” were absent in the cell-free proteolytic assay, it is unclear how the individual domain structures unfold to yield the linear polypeptide chain for threading into the central proteasome channel for proteolytic cleavage. An intriguing hypothesis is that SRC-3 degradation is not mediated by conventional unfolding and processive threading of its free N or C terminus into the proteasome. Rather, internal unstructured sequences of SRC-3 may serve as endoproteolytic sites for entry into the central opening of the REGγ20S complex. Interestingly, an endoproteolytic activity has been revealed for the 20S proteasome, which cleaves at internal peptide bonds of naturally unfolded or unstructured proteins, including the cyclin-dependent kinase inhibitor p21cip1 and α-synuclein (Liu et REGγ: A Shortcut to Destruction