RESEARCH COMMUNICATION Expression of Proteasome Activator REGγ in Human Laryngeal Carcinoma and Associations with Tumor Suppressor Proteins

The 20S proteasomes are large complexes that play crucial roles in protein quality control and in regulating many cellular functions in eukaryotic cells such as the cell cycle, transcription, cell signaling, cell death (Ciechanover et al., 2000). At least two families of proteasome activator complexes have been found, the 19S proteasome (also known as PA700) and the 11S proteasome (also known as REG or PA28) (Mao et al., 2008; Luo et al., 2010). The 19S activator binds to 20S proteasome to form the 26S proteasome in an ATP-dependent manner, which mainly performs degradation of proteins in a ubiquitindependent manner. In contrast, the REG activates the 20S proteasome in an ATP-independent manner and is primarily responsible for ubiquitin-independent protein degradation (Realini et al., 1997). There are three REG homologs, called α, β and γ. The α and β subunits form a heteroheptamer and is mainly cytoplasmic, whereas γ has a nuclear-restricted expression pattern and can be found independently or associated with 20S proteasomes as a homoheptameric lid (Ahn et al., 1996; Soza et al., 1997; Wojcik et al., 1998). REGα/β is induced by interferon IFN-γ and infection and its main function has been implicated in MHC class I antigen presentation. REGγ is unaffected by IFN-γ and can be markedly reduced during infection (Mao et al., 2008; Luo et al., 2010).