MDN1

MDN1 (Rea1) is a giant AAA+ ATPase mechanoenzyme that drives the late nucleoplasmic maturation and nuclear export of the pre-60S large ribosomal subunit (PMID:15528184). It engages late pre-60S particles in association with the Rix1 complex and, through ATP-driven remodeling, mechanically extracts UBL-domain-containing assembly factors such as Ytm1 (with its partner Erb1) and Rsa4 from the maturing particle, a remodeling step coordinated with helicase-mediated restructuring of 25S rRNA helices H62/H63 (PMID:15528184, PMID:36929751). Substrate extraction is executed by a C-terminal MIDAS domain that physically clamps the UBL domains of Rsa4 and Ytm1 in an integrin-like interaction and forms catch bonds with them, so that piconewton-scale forces consistent with AAA+ ATPase output stabilize binding before higher forces trigger release (PMID:31296859, PMID:35147499). The MIDAS domain is connected to the hexameric AAA+ ring by an ~20 nm structured linker; conformational changes within the ring are propagated through this linker to dock and undock MIDAS in a nucleotide-state-dependent manner, with ATP hydrolysis required for the linker to engage the ring and then the docked MIDAS domain to transmit force (PMID:30318141, PMID:30460895, PMID:39604383). Intramolecular tethering of MIDAS to the AAA ring normally suppresses docking until preribosome engagement or chemical inducers trigger the mechanochemical cycle, establishing a long-range autoinhibitory allostery, and an α-helical bundle within AAA2 regulates ATPase activity by controlling formation of the MIDAS docking site (PMID:30460895, PMID:32694211).

1. 2004 High

   Established that Rea1/MDN1 is a dedicated late-acting factor for pre-60S maturation and export rather than a general housekeeping ATPase, defining the cellular process it serves.

   Evidence GAL-repressible and ts rea1 alleles, Rpl25-GFP export reporter, and co-purification with the Rix1 complex in yeast

   PMID:15528184

   Open questions at the time

   • Did not reveal the molecular mechanism of action or direct substrates
   • Recruitment to the particle beyond Rix1 association undefined
2. 2018 High

   Resolved the architecture coupling the AAA+ motor to the substrate-engaging MIDAS domain, answering how a single enzyme converts nucleotide cycling into long-range mechanical output.

   Evidence Single-particle cryo-EM of S. pombe Mdn1 and S. cerevisiae Rea1 in distinct nucleotide states, including the ~20 nm linker and an AAA2 α-helical regulatory bundle

   PMID:30318141 PMID:30460895

   Open questions at the time

   • Atomic detail of MIDAS-substrate engagement not resolved
   • Order and directionality of force transmission inferred but not directly demonstrated
3. 2019 High

   Defined the structural basis for substrate recognition, showing MIDAS directly clamps the UBL domains of the assembly factors it extracts.

   Evidence X-ray crystallography of the Rea1 MIDAS domain alone and bound to Rsa4-UBL and Ytm1-UBL, revealing integrin-like contacts, an NLS loop, and an anchoring β-hairpin

   PMID:31296859

   Open questions at the time

   • Static structures do not show the dynamics of extraction
   • Affinity modulation during the catalytic cycle not addressed
4. 2020 High

   Showed that MIDOS-ring docking is autoinhibitory, reframing the linker tether as a suppressor of premature engagement rather than a simple connector.

   Evidence Chemical probes, single-particle EM, native mass spectrometry, and domain-truncation ATPase assays comparing intramolecular vs intermolecular MIDAS docking

   PMID:32694211

   Open questions at the time

   • Single-lab; in vivo relevance of intermolecular docking not established
   • Precise signal from preribosome that relieves autoinhibition unidentified
5. 2022 High

   Demonstrated that MIDAS-UBL bonds are force-activated catch bonds, providing the mechanistic logic for switching between gripping and releasing substrate during the enzymatic cycle.

   Evidence Optical tweezers force spectroscopy of MIDAS binding lifetimes to Rsa4 and Ytm1 across a force range

   PMID:35147499

   Open questions at the time

   • Force values inferred from isolated domains rather than measured on intact enzyme on particles
   • Single lab
6. 2023 High

   Reconstituted Rea1-driven remodeling in vitro and placed factor removal in temporal context with rRNA restructuring, linking motor action to a defined maturation transition.

   Evidence In vitro maturation assays with purified Rea1, the Rix1 complex, ATP, and pre-60S particles plus cryo-EM of Spb4-enriched intermediates

   PMID:36929751

   Open questions at the time

   • Coordination between Spb4 helicase and Rea1 mechanistically unresolved
   • Single lab
7. 2024 High

   Dissected the linker as the active force-transmitting element, showing ATP hydrolysis sequentially licenses linker-ring and linker-MIDAS engagement.

   Evidence Cryo-EM of Rea1 in multiple nucleotide states with functional assays showing the stem/middle/top linker subdomains are required for assembly factor removal

   PMID:39604383

   Open questions at the time

   • Quantitative force per cycle not measured on the full machine
   • Single lab
8. 2025 Low

   Extended the yeast-defined role to human cells, proposing nucleolar-periphery recruitment via Ki-67 and GNL2 and a role in ribosome-biogenesis transcript levels.

   Evidence siRNA depletion, RNA-seq, confocal microscopy, and co-IP in HEK293T cells (bioRxiv preprint)

   PMID:bio_10.1101_2025.02.13.638155

   Open questions at the time

   • Preprint, single lab with limited mechanistic depth for MDN1
   • Ki-67/GNL2 interaction not validated reciprocally or structurally
   • Direct enzymatic role in human pre-60S not demonstrated

• How the preribosome triggers relief of MIDAS autoinhibition and how the full mechanochemical cycle is coordinated with helicases and the Rix1 complex on native particles remains unresolved.
• Trigger signal coupling particle binding to docking unidentified
• No integrated structure of the actively extracting machine on pre-60S
• Human pathway largely uncharacterized in the corpus