Binding of amyloid β protein to the 20 S proteasome

Abstract

Neurodegenerative disorders of aging are characterized by the intraneuronal accumulation of ubiquitin conjugates into tangles and inclusions. Ubiquitin conjugates are degraded by cellular particles known as proteasomes. We have previously shown that amyloid β protein (Aβ) inhibits proteasomal activity and thereby blocks ubiquitin conjugate degradation. In the present studies, we found that Aβ binds the 20 S proteasome and forms a proteasome-Aβ complex. The complex was detected by Western blot with anti- Aβ antibodies. Using a 1.4 nm Nanogold-labeled Aβ, we visualized proteasome-Aβ complexes by scanning transmission electron microscopy (STEM). Analysis of the side-on oriented proteasome-Aβ complexes revealed a single gold particle, corresponding to one gold-labeled Aβ, in the middle portion of the proteasome. On end-on views of proteasome-Aβ complexes, gold was detected within the area delimited by the proteasome circular projection. Both STEM views are consistent with Aβ localization inside the proteasome along the peptide channel. Direct interaction of Aβ with the inner catalytic compartment of the proteasome may explain the generation of ubiquitin- containing lesions in Alzheimer's disease and other neurodegenerative disorders. In addition, detection of Nanogold-labeled peptide inside the 20 S eukaryotic proteasome suggests that conformational constraints for protein degradation in eukaryotic proteasomes are different from those in archaebacteria proteasomes.