STRUCTURE OF THE PROTEASOME ACTIVATOR REG-ALPHA (PA28-ALPHA)

The specificity of the 20S proteasome, which degrades many intracellul ar proteins, is regulated by protein complexes that bind to one or bot h ends of the cylindrical proteasome structure(1-5). One of these regu latory complexes, the 11S regulator (known as REG or PA28), stimulates proteasome peptidase activity(6,7) and enhances the production of ant igenic peptides for presentation by class I molecules of the major his tocompatibility complex (MHC)(8,9). The three REG subunits that have b een identified REG alpha, REG beta and REG gamma (also known as the Ki antigen).