MOSPD2

MOSPD2 is an ER-anchored membrane protein that functions as a general inter-organelle tether, using its Major Sperm Protein (MSP) domain to bind FFAT motifs on organelle-resident partners and thereby form ER-endosome, ER-mitochondria, and ER-Golgi contact sites (PMID:29858488). Recognition is regulated by a phosphorylation switch: phosphorylation of a serine/threonine residue within a non-conventional 'phospho-FFAT' motif controls binding to the MSP domain, a mechanism resolved at atomic resolution by crystal structures of the MSP domain alone and bound to conventional and phospho-FFAT peptides (PMID:33124732). Beyond FFAT-dependent tethering, MOSPD2 also forms ER-lipid droplet contacts through a CRAL-TRIO domain whose amphipathic helix senses lipid packing defects at the droplet surface, and its loss disturbs lipid droplet assembly (PMID:35389430). In a distinct membrane context, MOSPD2 is displayed on the surface of myeloid cells where it controls chemokine-driven directional migration and couples to receptor-downstream signaling (PMID:28137892); mechanistically it binds integrin-β2 (CD18) and holds integrin αLβ2 (LFA-1) in an inactive low-affinity conformation, acting as an adhesion checkpoint (PMID:40312574). This migratory function underlies pro-inflammatory monocyte recruitment in vivo, as MOSPD2 knockout or antibody blockade suppresses experimental autoimmune encephalomyelitis (PMID:32353176) and breast cancer cell chemotaxis and lung metastasis (PMID:29978511). In teleost orthologs MOSPD2 serves as a surface receptor for the antimicrobial peptide LEAP-2, whose binding triggers retromer (VPS35/VPS26/VPS29)-dependent trafficking of MOSPD2 from the ER to the plasma membrane to drive monocyte/macrophage chemotaxis (PMID:33124217, PMID:41017400). MOSPD2 is also exploited at pathogen interfaces, where it and the VAP proteins establish ER-parasitophorous vacuole membrane contact sites in Toxoplasma-infected cells (PMID:37341482, PMID:41073664).

1. 2017 Medium

   Established the first cellular role for MOSPD2, identifying it as a surface protein on myeloid cells required for chemokine-induced migration rather than a purely intracellular factor.

   Evidence siRNA knockdown and neutralizing antibody blockade with chemotaxis and signaling readouts in human monocytes and neutrophils

   PMID:28137892

   Open questions at the time

   • Did not identify the molecular binding partner mediating migration
   • Surface expression mechanism not resolved given ER localization shown later
2. 2018 High

   Defined the core molecular function: MOSPD2 is an ER-anchored MSP-domain protein that tethers the ER to multiple organelles via FFAT-motif partners, placing it in the membrane contact site machinery.

   Evidence Unbiased proteomics, in vitro membrane tethering reconstitution, fractionation, and reciprocal Co-IP

   PMID:29858488

   Open questions at the time

   • Structural basis of FFAT recognition not resolved
   • Reconciliation with reported plasma-membrane/migration role not addressed
3. 2018 Medium

   Extended the migration phenotype to a disease-relevant context, showing MOSPD2 is required for tumor cell chemotaxis and metastasis in vivo.

   Evidence siRNA knockdown across breast cancer lines, chemotaxis, phospho-signaling, and an in vivo lung metastasis mouse model

   PMID:29978511

   Open questions at the time

   • Direct receptor/partner mediating tumor chemotaxis not identified
   • Mechanistic link to ER tethering function unclear
4. 2020 High

   Resolved how FFAT recognition is regulated, revealing a phosphorylation-controlled molecular switch via crystal structures of the MSP domain with conventional and phospho-FFAT ligands.

   Evidence Crystal structures with and without peptides, phosphomimetic/phospho-dead mutagenesis, and sterol transfer assays

   PMID:33124732

   Open questions at the time

   • Kinases/phosphatases controlling the phospho-FFAT switch not identified
   • In vivo regulation of contact-site dynamics not addressed
5. 2020 Medium

   Demonstrated the in vivo physiological consequence of MOSPD2-dependent migration, establishing it as a regulator of inflammatory monocyte recruitment in autoimmune disease.

   Evidence MOSPD2 knockout mice in an EAE model, flow cytometry, cytokine profiling, and monoclonal antibody treatment

   PMID:32353176

   Open questions at the time

   • Surface receptor/effector mediating the phenotype not defined
   • T-cell cytokine shift mechanism unresolved
6. 2020 Medium

   Identified a surface-receptor function for the MOSPD2 ortholog, showing it binds the antimicrobial peptide LEAP-2 to drive macrophage chemotaxis and antibacterial responses.

   Evidence Yeast two-hybrid screen, Co-IP, RNAi knockdown, and chemotaxis/cytokine assays in mudskipper

   PMID:33124217

   Open questions at the time

   • Conservation of LEAP-2 receptor function in mammals not tested
   • Signaling downstream of LEAP-2/MOSPD2 not mapped
7. 2022 High

   Expanded the tethering repertoire to lipid droplets and showed MOSPD2 can engage membranes directly, independent of FFAT proteins, through a lipid-packing-defect-sensing amphipathic helix.

   Evidence Live-cell imaging, in vitro lipid binding, amphipathic helix mutagenesis, and lipid droplet assembly phenotype in KO cells

   PMID:35389430

   Open questions at the time

   • Mechanism linking ER-LD contact to lipid droplet assembly not defined
   • Coordination between CRAL-TRIO and MSP domain functions unclear
8. 2023 Medium

   Showed MOSPD2 is recruited to the Toxoplasma parasitophorous vacuole membrane via its CRAL/TRIO domain and tail anchor, implicating its contact-site machinery at a pathogen interface.

   Evidence IP-LC-MS/MS, domain-deletion mutagenesis, KO growth assays, and immunofluorescence in infected host cells

   PMID:37341482

   Open questions at the time

   • Functional importance is modest (limited growth defect)
   • Direct parasite-side binding partner of MOSPD2 not defined
9. 2025 Medium

   Defined the molecular basis of the myeloid adhesion checkpoint, showing MOSPD2 binds integrin-β2 and locks LFA-1 in a low-affinity state, explaining its control of migration vs. adhesion balance.

   Evidence siRNA, humanized anti-MOSPD2 antibody, Co-IP, integrin conformation and adhesion assays, and in vivo RA/IBD models

   PMID:40312574

   Open questions at the time

   • How an ER-resident protein reaches the cell surface to engage integrins not resolved
   • Structural basis of the MOSPD2-CD18 interaction unknown
10. 2025 Medium

    Resolved how MOSPD2 reaches the surface, showing LEAP2 triggers retromer-dependent trafficking from ER to endosomes to plasma membrane required for chemotaxis.

    Evidence Fractionation, immunofluorescence, retromer-subunit knockdown, Co-IP/MS confirming MOSPD2-VPS35 binding, and chemotaxis assays in teleost cells

    PMID:41017400

    Open questions at the time

    • Whether retromer-dependent surface trafficking operates in mammalian cells untested
    • Trigger linking LEAP2 binding to retromer recruitment not defined
11. 2025 Medium

    Established that ER-PVM contact in Toxoplasma infection is mediated by VAPA/VAPB/MOSPD2 acting redundantly, with a parasite FFAT-like protein engaging the VAPs.

    Evidence Triple genetic knockout, fluorescence microscopy, and FFAT-motif interaction assays

    PMID:41073664

    Open questions at the time

    • Relative contribution of MOSPD2 vs VAPA/VAPB not dissected
    • Consequence of ER-PVM contact for the parasite mechanistically unresolved

• How a single ER-tail-anchored tethering protein is converted into a cell-surface receptor and integrin regulator in myeloid cells, and whether the LEAP-2/retromer surface-trafficking axis is conserved in mammals, remains unresolved.
• No mammalian demonstration of LEAP-2 receptor or retromer surface-trafficking function
• Topological reconciliation of ER-membrane tether vs plasma-membrane receptor roles missing