Description
S5a (Rpn10) is a major ubiquitin binding protein that binds preferentially to polyubiquitin chains. It is found as a subunit of the 26S proteasome, but unlike other proteasome subunits, S5a also exists predominantly as a free protein in the cytosol. S5a contains two stretches of approximately 15 amino acids called the ubiquitin interacting motifs (UIMs), which are responsible for its high affinity for ubiquitin chains (Kim and Goldberg, 2012)