Protein modifier that is covalently attached to lysine residues of substrate proteins, thereby targeting them for proteasomal degradation. The tagging system is termed pupylation (By similarity). Protein degradation; proteasomal Pup-dependent pathway. Strongly interacts with the proteasome-associated ATPase ARC through a hydrophobic interface; the interacting region of Pup lies in its C-terminal half. There is one Pup binding site per ARC hexamer ring (By similarity). The N-terminal unstructured half of Pup provides a signal required to initiate unfolding and degradation by the proteasome but is not needed for pupylation, while the C-terminal helical half of Pup interacts with ARC to target proteins to the proteasome. Is modified by deamidation of its C-terminal glutamine to glutamate by the deamidase Dop, a prerequisite to the subsequent pupylation process. Belongs to the prokaryotic ubiquitin-like protein family.