Figure 1.

Organization and assembly of mammalian proteasomes. A) The 20S proteasome or core protein (CP) is formed by the illustrated stacking assembly of a and β subunits making a 28-subunit catalytic barrel. B) Binding of 19S proteasome regulators to both ends of the CP generates the 26S proteasome. 26S proteasome is specialized in ATPdependent degradation of ubiquitinated proteins. C) Binding of heptameric 11S proteasome regulators (PA28, 3a/4β) or PA28-g (not shown) to both ends of the CP results in proteasome formations specialized in non-ATP dependent degradation of non-ubiquitinated short peptides. D) Simultaneous binding of 11S and 19S proteasome regulators to CP results in hybrid proteasome formation. Hybrid proteasomes are poorly studied but are suggested to be involved in ATP-dependent degradation of non-ubiquitinated proteins.68