SORTLR

SORTLR (PF3D7_1451800) is a Golgi-localized sortilin-family sorting receptor of Plasmodium falciparum that governs delivery of cargo to the apical secretory organelles required for merozoite invasion (PMID:24587025, PMID:29582546). Positioned at the Golgi apparatus, SORTLR acts as a transmembrane escorter that physically engages rhoptry-destined cargo, interacting with regions of the GPI-anchored rhoptry-associated membrane antigen (RAMA) that are themselves sufficient to direct a reporter to the rhoptries (PMID:29299530). Its trafficking function is essential: conditional knockdown blocks formation of the inner membrane complex, abolishes production of new merozoites, and mislocalizes rhoptry, microneme, and dense granule cargo, which instead accumulate in the endoplasmic reticulum and parasitophorous vacuole, while protein export to the erythrocyte and the constitutive secretory pathway remain intact (PMID:29582546). SORTLR operates downstream of the AP-1 adaptor complex, serving as the multi-ligand receptor through which AP-1 supports invasion-organelle biogenesis and formation of invasive progeny (PMID:41082687). Localization data are also consistent with retromer-mediated retrograde retrieval of SORTLR from endosomes back to the Golgi (PMID:24587025).

1. 2014 Medium

   Establishing where SORTLR resides was the first step toward assigning it a sorting function, since a Golgi receptor implies a role at the trafficking hub of the secretory pathway.

   Evidence Fluorescence microscopy with brefeldin A treatment and co-localization with retromer and Rab7 markers in P. falciparum asexual stages

   PMID:24587025

   Open questions at the time

   • No loss-of-function or interaction assay performed for SORTLR itself
   • Retromer-mediated retrieval inferred from marker co-localization, not directly demonstrated
2. 2018 Medium

   Identifying a physical cargo ligand converted the inferred sorting role into a defined receptor-cargo relationship, showing SORTLR escorts rhoptry-destined proteins.

   Evidence Co-immunoprecipitation between SORTLR and RAMA domains plus a fluorescent reporter targeting assay in P. falciparum

   PMID:29299530

   Open questions at the time

   • No mutagenesis to map the interacting residues
   • No reconstitution to prove sufficiency of the SORTLR-RAMA interaction for transport
   • Whether SORTLR binds other apical cargoes directly not resolved
3. 2018 High

   Conditional depletion demonstrated that SORTLR is not merely a sorting receptor but is essential for invasion-organelle biogenesis and merozoite formation, defining the specificity of its trafficking role.

   Evidence Conditional knockdown of SORTLR with immunofluorescence readouts of IMC formation, merozoite generation, and organelle-specific cargo trafficking in P. falciparum

   PMID:29582546

   Open questions at the time

   • Molecular basis by which loss of SORTLR blocks IMC formation not defined
   • Full repertoire of mistrafficked cargoes not enumerated
   • Order of events (trafficking defect vs. organelle failure) not dissected
4. 2025 Medium

   Placing SORTLR downstream of the AP-1 adaptor complex situated it within an ordered trafficking module, identifying the upstream machinery that directs SORTLR-dependent cargo sorting.

   Evidence Genetic disruption of AP-1 with proximity proteomics from live cells and functional epistasis between AP-1 and SORTLR in P. falciparum

   PMID:41082687

   Open questions at the time

   • Direct physical AP-1-SORTLR interaction mechanism not detailed
   • Sorting signals recognized on SORTLR by AP-1 unknown
   • Single lab, mechanistic detail limited in the report

• The structural basis of ligand recognition and the full ligand repertoire that SORTLR sorts to rhoptries, micronemes, and dense granules remain undefined.
• No structural model of SORTLR or its cargo-binding interface
• Beyond RAMA, no other direct SORTLR ligand mapped
• Mechanism linking SORTLR trafficking to IMC assembly unresolved