Protein modifier that is covalently attached to lysine residues of substrate proteins, thereby targeting them for proteasomal degradation. The tagging system is termed pupylation. Among the identified substrates are the SodA and Ino1 proteins. 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 probably by the deamidase Dop, a prerequisite to the subsequent pupylation process. Belongs to the prokaryotic ubiquitin-like protein family.