Rpn1 is the largest proteasomal receptor for polyubiquitin and shuttle proteins contains one known binding site for polyubiquitin and shuttle proteins. We used a combination of NMR spectroscopy, photo-crosslinking, mass spectrometry, and mutagenesis,to confirm the presence of additional ubiquitin recognition site on RPN1. This novel site is situated in the N-terminal section of Rpn1, a region previously surmised to be devoid of functionality.