S5a, also known as PSMD4, is a major ubiquitin binding protein that binds preferentially to polyubiquitin chains. It is found as a non-ATPase subunit of the 19S regulatory complex of the 26S proteasome where it functions as a receptor for proteins destined for proteolytic degradation. In yeast, the homologue of S5a, Rpn10, is required for degradation of a subset of cellular proteins by the proteasome. Unlike other proteasome subunits, S5a exists predominantly as a free protein in the cytosol (i.e. not incorporated into the proteasome), though its precise role in the cytosol has yet to be fully elucidated.
S5a contains two approximately 15 amino acid ubiquitin interacting motifs (UIM) which are responsible for its binding to polyubiquitin chains. In vitro S5a binds predominantly to chains of at least four ubiquitins in length and can act as a substrate for a wide range of ubiquitin E3 ligases, which, it has been suggested, occurs by a novel mechanism involving the association of S5a with growing poly-Ub chains.
Recombinant human S5a protein expressed with a N-terminal GST-tag. Protein has utility in ubiquitin binding assays.