ATPase which is responsible for recognizing, binding, unfolding and translocation of pupylated proteins into the bacterial 20S proteasome core particle. May be essential for opening the gate of the 20S proteasome via an interaction with its C-terminus, thereby allowing substrate entry and access to the site of proteolysis. Thus, the C-termini of the proteasomal ATPase may function like a 'key in a lock' to induce gate opening and therefore regulate proteolysis. Is required but not sufficient to confer resistance against the lethal effects of reactive nitrogen intermediates (RNI), antimicrobial molecules produced by activated macrophages and other cell types. ATPase activity is inhibited by EDTA, N-ethylmaleimide (NEM) and sodium azide. Optimum pH is 7.4-7.5. Protein degradation; proteasomal Pup-dependent pathway. Homohexamer. Assembles into a hexameric ring structure that caps the 20S proteasome core. Strongly interacts with the prokaryotic ubiquitin-like protein Pup through a hydrophobic interface; the interacting region of Mpa lies in its N-terminal coiled-coil domain. There is one Pup binding site per Mpa hexamer ring; the K(D) measured is about 3.8 uM. Upon ATP-binding, the C-terminus of Mpa interacts with the alpha-rings of the proteasome core, possibly by binding to the intersubunit pockets. Consists of three main regions, an N-terminal coiled-coil domain (residues 1-96) that binds to protein Pup and functions as a docking station, an interdomain (residues 97-245) involved in Mpa hexamerization, and a C-terminal ATPase domain of the AAA type (residues 246-609). Pupylated at Lys-591 by the prokaryotic ubiquitin-like protein Pup, which leads to its degradation by the proteasome. Mpa thus promotes its own turnover. Mpa is a target of RNI, thereby is S-nitrosylated in the phagosome of immunologically activated host macrophages, which causes enzyme inhibition. Cells lacking this gene accumulate pupylated proteins. These cells also become hypersensitive to reactive nitrogen intermediates (RNI) and are severely attenuated in both wild-type and nitric oxide synthase 2 deficient mice. Moreover, they display increased resistance to hydrogen peroxide. Was identified as a natural substrate of the M.tuberculosis proteasome. Belongs to the AAA ATPase family.