MON2

MON2 is an evolutionarily conserved Golgi- and endosome-associated scaffold protein that organizes retrograde and recycling membrane traffic by coupling cargo carriers to lipid cues and cytoskeletal motors (PMID:16301316, PMID:31324769). Its core activity is to recruit a DOPEY-family partner: yeast Mon2 binds Dop1 and is required for its Golgi localization and for normal endosome-to-Golgi cycling (PMID:16301316), and in mammals MON2 binds phosphatidic acid while its partner Dopey1 binds phosphatidylinositol-4-phosphate, together forming a dual-lipid-regulated adaptor whose Dopey1 N-terminus engages kinesin-1 to drive microtubule-based transport of secretory and endocytic organelles (PMID:31324769). On endosomes, MON2 assembles with DOPEY2 and the putative aminophospholipid translocase ATP9A into a complex that supports SNX3-retromer-dependent endosome-to-Golgi transport of Wntless and Wnt secretion, with loss of MON2 phenocopying retromer failure and diverting Wntless to lysosomal degradation (PMID:30213940). MON2 further drives the segregation of recycling endosomes from early endosomes and is required for the tubular recycling network and for retrograde Wntless trafficking, recruiting membrane-bound DOPEY2 that engages both kinesin and dynein/dynactin motors (PMID:32404555). In yeast it acts as a negative regulator of Arl1-GTP at the Golgi (PMID:22594927). Beyond core trafficking, MON2 couples endocytic activity to actin remodeling in the Drosophila oocyte through interactions with Cappuccino, Spire, and Rho1 (PMID:21610029), is required for efficient HIV-1 virion production (PMID:21450827), and promotes autophagy by translocating from Golgi to endosomes under starvation and physically activating GABARAPL2/LGG-1 to increase autophagic flux (PMID:34860542, PMID:35188063).

1. 2005 High

   Established MON2 as a Golgi scaffold by showing it recruits the DOPEY orthologue Dop1 and is needed for endosome-to-Golgi cycling, defining the core molecular partnership that anchors all later work.

   Evidence Genetic deletion, co-immunoprecipitation, and fluorescence localization in yeast

   PMID:16301316

   Open questions at the time

   • Mechanism by which Mon2 targets to the trans-Golgi not defined
   • No biochemical reconstitution of the Mon2-Dop1 interaction
   • Direct cargo not identified at this stage
2. 2011 High

   Extended MON2 function beyond canonical trafficking by linking it to actin remodeling, showing it interacts with nucleators Cappuccino and Spire and Rho1 to couple Oskar-induced endocytosis to cortical F-actin in the oocyte.

   Evidence Co-immunoprecipitation, genetic epistasis, and live imaging in Drosophila

   PMID:21610029

   Open questions at the time

   • Whether actin coupling is conserved outside Drosophila germ plasm unknown
   • Direct vs indirect nature of Mon2-Capu/Spire binding not resolved
3. 2011 Medium

   Connected MON2 trafficking function to a disease-relevant process by showing human MON2 is required for efficient HIV-1 virion production.

   Evidence RNAi knockdown with VLP release assay in human cells plus yeast Gag-targeting screen

   PMID:21450827

   Open questions at the time

   • Step in Gag trafficking/assembly that requires MON2 not pinpointed
   • Single-lab study without orthogonal validation of specificity
4. 2012 Medium

   Positioned MON2 within Golgi GTPase regulation by establishing it as a genetic negative regulator of Arl1-GTP.

   Evidence ARL1 allele-specific suppression, synthetic lethality, and CPY secretion assay in yeast

   PMID:22594927

   Open questions at the time

   • No biochemical interaction between Mon2 and Arl1 demonstrated
   • Mechanism of negative regulation (GAP-like vs indirect) unknown
5. 2018 High

   Defined the endosomal effector complex by showing MON2 assembles with DOPEY2 and the flippase ATP9A and works with SNX3-retromer to recycle Wntless from endosome to Golgi, identifying a concrete cargo and pathway.

   Evidence Co-IP, mass spectrometry, C. elegans RNAi epistasis, and ATPase-dead mutant analysis across human and worm

   PMID:30213940

   Open questions at the time

   • Catalytic role of ATP9A flippase activity in the complex not directly resolved
   • How SNX3-retromer engages the complex structurally unknown
6. 2019 High

   Provided the biophysical logic of MON2 carrier function by showing the Dopey1-MON2 complex reads two lipids (PA and PI4P) and recruits kinesin-1, making it a dual-lipid-regulated cargo adaptor for microtubule transport.

   Evidence Co-IP, lipid-binding assays, kinesin-1 pulldown, and transport/live-imaging assays

   PMID:31324769

   Open questions at the time

   • Structure of the adaptor-motor interface not determined
   • How lipid binding is regulated in time/space unknown
7. 2020 High

   Resolved the organelle-level role by showing MON2 drives separation of recycling from early endosomes and is required for the tubular recycling network and retrograde Wntless transport, with DOPEY2 recruited MON2-dependently to engage both kinesin and dynein/dynactin.

   Evidence CRISPR knockouts, live imaging, colocalization, and transferrin/Wntless transport assays

   PMID:32404555

   Open questions at the time

   • Molecular trigger that initiates RE-EE segregation not defined
   • How bidirectional motor engagement is balanced unknown
8. 2021 High

   Linked MON2 to autophagy by showing starvation-induced Golgi-to-endosome translocation lets it bind and activate GABARAPL2/LGG-1 to raise autophagic flux and support longevity in mitochondrial mutants.

   Evidence Co-IP, autophagic flux assays in mammalian cells, C. elegans genetics, proteomics, and translocation imaging

   PMID:34860542 PMID:35188063

   Open questions at the time

   • Signal driving starvation translocation not identified
   • Mechanism by which MON2 activates GABARAPL2 not resolved

• How MON2's distinct activities — retrograde recycling, motor-coupled transport, Arl1 regulation, actin remodeling, and autophagy — are integrated and switched within one protein remains unresolved.
• No structural model of MON2 or its complexes
• Regulatory logic governing Golgi-vs-endosome partitioning unknown
• Whether functions are sequential or parallel not established