The mycobacterial proteasomal ATPase Mpa forms a gapped ring to engage the 20S proteasome

Area of Science:

• Biochemistry
• Structural Biology
• Microbiology

Background:

• Actinobacteria, including Mycobacterium tuberculosis, utilize proteasome systems for protein degradation, unlike many other bacteria.
• The mycobacterial proteasome ATPase (Mpa) shares structural similarities with archaeal and eukaryotic ATPases but possesses a unique β-grasp domain.
• The mechanism of Mpa interaction with the 20S proteasome core particle (CP) remains unclear due to steric hindrance suggested by crystal structures.

Purpose of the Study:

• To elucidate the structural mechanism by which the mycobacterial proteasome ATPase Mpa interacts with the 20S proteasome core particle (CP).
• To investigate the role of ATP and degradation substrates in Mpa's interaction with the proteasome.
• To understand the functional significance of Mpa's unique structural features, including its β-grasp domain and linker regions.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was used to determine the structure of the Mpa hexamer in the presence of ATP and a degradation substrate.
• Analysis of conformational changes in Mpa's domains and linker regions.

Main Results:

• The Mpa hexamer forms a gapped ring structure with two flexible ATPase domains when bound to ATP and substrate.
• Conformational changes in the linkers between the oligonucleotide-binding and ATPase domains are crucial for Mpa function.
• The gapped ring configuration is proposed as an intermediate state facilitating Mpa's engagement with the proteasome CP, involving rearrangement of β-grasp domains.

Conclusions:

• Mpa utilizes a unique gapped ring conformation, driven by ATP hydrolysis, to unfold substrates.
• Linker regions play a critical, previously unappreciated role in ATP-driven protein unfolding by Mpa.
• This study provides novel insights into the interaction between bacterial ATPases and proteasomes, essential for protein turnover in actinobacteria.