Human brain tissue contains predominately 19S-uncapped 20S proteasome.A and B, frozen mouse, macaque, and human brain samples were collected and applied to native (A) or denaturing (B) gel conditions. For native gels, samples were prepared for AMC assays as previously described, followed by immunoblot analysis using indicated antibodies. For denaturing gels, samples were prepared in denaturing buffer as previously described, followed by SDS-PAGE and immunoblot analysis using indicated antibodies. Detectable capped proteasome signal appears at locations indicated for the 19S-uncapped (20S), singly capped (26S), or doubly capped (30S) proteasome. Note, no AMC signal for the 20S proteasome, no 19S-capped proteasome in PMI 20 h human sample, and significantly reduced capped proteasome for macaque and PMI 0 human brain samples compared to mouse tissue. Also, note equal levels of 20S proteasomes and subunits available in all samples. See also Fig. S4.