>The subunit topography of the Thermoplasma acidophilum proteasome was determined by iminunoelectron microscopy using monospecific antibodies directed against the two constituent subunits (α,β). Anti-α-subunit IgG was found to bind to the outer disks of the cylinder- or barrel-shaped molecule, while the binding sites of the anti-β-subunit IgG were mapped on the two inner rings. Probably the homologues of the two subunits in the compositionally more complex but isomorphous eukaryotic proteasomes occupy equivalent positions.
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