MOSMO

MOSMO is a transmembrane tetraspan protein that functions as a negative regulator of Hedgehog signaling by promoting the removal of the GPCR Smoothened (SMO) from the cell surface and primary cilium (PMID:29290584). It is a core subunit of the MEGF8-MOSMO-MGRN1 (MMM) membrane E3 ubiquitin ligase complex, in which MOSMO and MEGF8 serve as the transmembrane components; a long helix engages SMO through an intramembrane degron and extends into the cytoplasm to position the MGRN1 RING domain for ubiquitylation of SMO's cytoplasmic surface, driving SMO degradation and attenuating pathway output (PMID:34486668, PMID:42190653). Loss of MOSMO stabilizes SMO at the plasma membrane and ciliary membrane, hyperactivating Hedgehog signaling and quantitatively raising cellular sensitivity to Sonic Hedgehog, an effect reversible by pharmacological SMO inhibition (PMID:29290584, PMID:34486668). Through this control of signaling strength, MOSMO localizes to the plasma membrane, cytoplasmic vesicles, and primary cilium (PMID:34746155) and is required for vertebrate craniofacial patterning, neural crest cell specification and migration, and prevention of multi-organ birth defects (PMID:34486668, PMID:34746155, PMID:35401697).

1. 2017 High

   Established MOSMO as a previously unannotated negative regulator of Hedgehog signaling, answering whether an unknown tetraspan protein controls SMO localization and SHH responsiveness.

   Evidence Genome-wide CRISPR screen with SMO localization and SHH-guided neural differentiation readouts in two cell types

   PMID:29290584

   Open questions at the time

   • Did not define the molecular mechanism of SMO removal
   • No binding partners or complex membership identified
   • Did not establish in vivo developmental consequences
2. 2021 High

   Placed MOSMO within the MEGF8-MOSMO-MGRN1 complex and showed that SMO abundance is the quantitative effector linking MOSMO loss to developmental defects.

   Evidence Mosmo-/- mouse knockout with birth-defect phenotyping and pharmacological SMO-inhibitor rescue in utero, plus biochemical dissection of the MMM complex

   PMID:34486668

   Open questions at the time

   • Did not resolve the structural basis of SMO recognition by the complex
   • Stoichiometry and assembly of MMM left undefined
   • Direct ubiquitylation mechanism not yet visualized
3. 2021 Medium

   Defined the subcellular distribution of MOSMO and confirmed its role in craniofacial development across a second vertebrate model.

   Evidence CRISPR/Cas9 knockout of both zebrafish paralogs with fluorescence localization in zebrafish and chick embryos and craniofacial phenotyping

   PMID:34746155

   Open questions at the time

   • Localization based on imaging in single lab without complex co-localization
   • Did not biochemically link localization to SMO degradation
4. 2021 Medium

   Tested whether MOSMO acts within a genetic interaction network, revealing synergistic phenotypes with SETD5 relevant to neurodevelopmental defects.

   Evidence Pairwise RNAi knockdown epistasis in Drosophila and X. laevis with brain morphology and axon outgrowth readouts

   PMID:33819264

   Open questions at the time

   • Genetic interaction not connected to a molecular mechanism
   • Relationship to Hedgehog/SMO pathway not established
   • Relies on knockdown rather than null alleles
5. 2022 Medium

   Resolved the cellular basis of MOSMO craniofacial requirement by linking it to neural crest cell specification and migration.

   Evidence Morpholino knockdown in X. laevis with readouts of craniofacial structure, pharyngeal arch migration, and neural crest specification/motility

   PMID:35401697

   Open questions at the time

   • Single-organism, single-lab morpholino study without rescue
   • Mechanistic link from SMO regulation to neural crest behavior not directly tested
6. 2026 High

   Defined the molecular mechanism of SMO ubiquitylation, answering how MOSMO and MEGF8 anchor the complex and orient MGRN1 toward SMO.

   Evidence Cryo-EM structure of the human MEGF8-MOSMO-MGRN1 complex integrated with biophysical and functional ubiquitylation assays

   PMID:42190653

   Open questions at the time

   • Dynamics of SMO capture and release in cilia not visualized
   • Regulation of MMM assembly/activity in vivo unresolved
   • Substrate range beyond SMO not addressed

• How MMM complex activity is regulated spatially and temporally during development, and whether MOSMO has substrates or roles beyond SMO, remains open.
• No regulatory inputs controlling MMM activity identified
• No substrates other than SMO characterized
• Connection between neural crest phenotypes and SMO regulation not mechanistically closed