Proteasome activator 11S α subunit, pAb

The catalytic core of this unusually large (2000kDa, 450Å in length) complex (31 subunits) is formed by the 20S proteasome, a barrel shaped structure shown by electron microscopy to comprise of four rings each containing seven subunits. 20S Proteasomes degrade only unfolded proteins in an energy-independent manner, whereas 26S proteasomes degrade native and ubiquitinylated proteins in an ATP-dependent manner. The native protein substrates are recognised by subunits, some with ATP binding sites, of the outer 19S caps of the 26S proteasome. The 19S regulatory subunit recognizes ubiquitinated proteins and plays an essential role in unfolding and translocating targets into the lumen of the 20S subunit. A second activator which can associate with the 20S proteasome in the absence of ATP is known as PA28 or the 11S regulator. The pure PA28 activator is a complex of two alternating subunits, PA28α and PA28β, which share approximately 50% homology but also show considerable similarity (30-40%) to a nuclear protein of unknown function, the Ki autoantigen (recently referred to as PA28γ). These subunits, with an apparent relative molecular weight of approximately 29kDa, form ringlike heteromeric complexes of  ~200kDa possibly with an α₃β₃ stoichiometry. Electron microscopic studies have shown PA28 to be a ring shaped particle which, like the 19S, caps the 20S proteasome, by binding to the α-rings, at both or either end. The complex may, however, be readily dissociated. The finding that PA28 modulates the proteasome-catalysed production of antigenic peptides presented to the immune system on MHC class I molecules indicates a cellular function of this activator in antigen processing. Several genetic diseases are associated with defects in the ubiquitin-proteasome pathway. Some examples of affected proteins include those linked to cystic fibrosis, Angelman’s syndrome, and Liddle syndrome.