S5a (Rpn10), a component of the 19S regulatory complex, functions as a receptor by binding to and recognizing poly-ubiquitinated proteins destined for 26S proteasome degradation. The protein recognizes ubiquitin chains and ubiquitin protein conjugates via two UIM (ubiquitin-interacting motif) domains. Detection with avidin-linked reagents allows for a higher efficiency and detection sensitivity than with other antibodies.