RH5 (PfRH5, Q8IFM5, PF3D7_0424100) — curation notes

Provenance note: automated deep research was not available in this environment
(Falcon authentication failed; the OpenAI key was invalid). No -deep-research-*.md
file was generated. What follows is a manual literature review compiled from the
UniProt record (RH5-uniprot.txt), the cached publications in publications/, and GO
term definitions checked against OLS. Quotes are verbatim from the cited cached source.

1. Identity and architecture

• Reticulocyte-binding protein homolog 5, 526 aa precursor, signal peptide 1–24, chain
  25–526; no transmembrane domain and no GPI anchor (UniProt Q8IFM5; KW Signal,
  FT SIGNAL 1..24).
• An atypical member of the PfRh (reticulocyte-binding-like homolog) family: much
  smaller than the other members and lacking the C-terminal TM/cytoplasmic region
  PMID:19000690.
  Pfam PF18515 (Rh5); the fold is a kite-shaped, largely helical structure solved to
  2.18 Å PMID:25296023.
• Proteolytically processed: cleaved at 140/141 to a ~45 kDa form during invasion
  (UniProt PTM; PMID:21909261, PMID:28186186).

2. Expression and localization

• Blood-stage specific: schizonts and free merozoites (UniProt DEVELOPMENTAL STAGE,
  at protein level).
• Stored in the rhoptries (a genuine apicomplexan secretory organelle, not a
  generic vesicle) of developing merozoites, shown by IFA against a rhoptry marker and
  by immuno-EM PMID:18827878.
• Released at invasion and found at the merozoite–erythrocyte interface, and shed into
  the medium PMID:21909261.
• Colocalizes with CyRPA and PfRipr at the junction with the host cell
  PMID:27374406.
• The complex inserts into the host membrane PMID:30542156.

Protist-specific caution. The invasion literature calls the merozoite–erythrocyte
interface a "tight junction" (the apicomplexan moving junction). This is not the
metazoan occluding junction that GO:0005923 bicellular tight junction describes
("An occluding cell-cell junction that is composed of a branching network of sealing
strands that completely encircles the apical end of each cell in an epithelial sheet",
OLS). The GOA row carrying GO:0005923 for RH5 is an animal-specific term imported on
the strength of that shared word. The underlying IDA observation is nevertheless real, so
the row is reviewed as MODIFY to a proposed new moving junction term (parent
GO:0030054 cell junction, confirmed in OLS as "a specialized region of connection
between two or more cells"), matching the term proposed in the PLAF7 AMA1 review, rather
than as REMOVE, which would discard a genuine localisation.

3. Molecular function: host receptor binding

• The host receptor is basigin (BSG/CD147, Ok blood group antigen)
  PMID:22080952. Invasion is blocked by soluble
  BSG, BSG knockdown and anti-BSG antibodies across all strains tested
  PMID:22080952.
• The RH5–BSG interaction is a host-tropism determinant: PfRH5 binds chimpanzee BSG
  weakly and gorilla BSG not at all PMID:24297912 — this is the basis
  for the IPI rows with the Pan troglodytes BSG accession A0A0M5L610, which are
  cross-species binding measurements, not an assertion that RH5 acts in chimpanzees.
• Earlier work, before BSG was identified, reported binding to an unknown glycosylated
  erythrocyte receptor PMID:19000690.

4. The RH5–CyRPA–PfRipr invasion complex and its membrane tether

• PfRipr (O97302, PF3D7_0323400) forms a complex with PfRh5 PMID:21909261.
• CyRPA (Q8IFM8, PF3D7_0423800) is the GPI-linked member that holds the trimer together
  PMID:25583518; a
  direct CyRPA–PfRh5 contact is demonstrated biochemically PMID:28195530 and structurally PMID:30542156.
• P113 (Q8ILP3) binds the RH5 N-terminus and anchors it to the merozoite surface
  PMID:28186186, a releasable
  tether PMID:28186186.
• GO has no term for this complex; a proposed_new_terms entry is made for it.

5. Biological process: erythrocyte invasion, and what RH5 itself does

• RH5 is the only essential member of the PfRh family: all other dbl/rbl genes can be
  knocked out PMID:24212193.
• RH5 is needed for irreversible attachment, not for initial contact
  PMID:28195038 — i.e. the gene genuinely participates in adhesion as well as entry.
• Downstream of binding, RH5–BSG engagement itself triggers a host Ca²⁺ signal and
  cytoskeletal remodelling PMID:28409866, and the
  complex is required for Ca²⁺ release and junction formation PMID:27374406. This is RH5 acting on the host: the
  appropriate framing is a symbiont-mediated activation of host signalling via agonism of
  a host cell-surface receptor (GO:0141134, verified in OLS), not a parasite-intrinsic
  signalling process. Note the mechanism is still described cautiously in the field
  PMID:31204103.

Host-vs-symbiont term use. Every process claim here is written from the parasite's
side: GO:0085017 entry into host cell by a symbiont-containing vacuole (the specific
term), GO:0044650 adhesion of symbiont to host cell, GO:0141134 for the action on host
signalling. The broader GO:0044409 symbiont entry into host rows, and the intermediate
GO:0046718 symbiont entry into host cell rows the gene already carries, are correct but
less specific than the evidence supports, so all are MODIFYed to GO:0085017. OLS
confirms GO:0085017 is a descendant of both, and its definition is invasion that forms
"a vacuole in which the symbiont resides", beginning with attachment to a host membrane
that invaginates and closing when the host membrane seals behind the new vacuole -- i.e.
exactly moving-junction merozoite invasion and formation of the parasitophorous vacuole.
GO:0046718 is instead framed around breaching the membrane and releasing the symbiont or
its genome into the cytoplasm (its definition still carries a virus-specific comment), so
it is a poorer fit. GO:0085017 is also what GOA itself uses for the other P. falciparum
invasion proteins in this project (MSP1 by IDA, CSP by IMP), so this aligns RH5 with
MSP1, CSP and MIC2. Locations
are likewise split correctly between parasite-side (GO:0005576, GO:0020008,
GO:0034591) and host-side (GO:0043655, GO:0020002) terms; note that
GO:0043655 host extracellular region sits under GO:0018995 host cellular component
(checked in OLS), not under GO:0005576, so the two are independent claims and both are
retained.

6. Heparin binding

PfRH5 binds heparin with very high affinity in a pull-down/elution assay
PMID:24212193, and
heparin disrupts its erythrocyte binding PMID:24212193. The observation is real, but heparin/heparan sulfate is not a mature
erythrocyte surface component, so this is treated as a genuine but non-core in vitro
binding property (likely reflecting the basic surface that also engages BSG), not as the
physiological ligand interaction. Kept, marked non-core.

7. Vaccine relevance (context only, not annotated)

RH5 is the leading blood-stage vaccine target; antibodies neutralize invasion
PMID:31204103, and engineered stabilized variants exist PMID:28096331. Vaccine candidacy is
not itself a GO-annotatable function and is recorded here only as context.

8. Curation decisions taken (summary)

• GO:0005923 bicellular tight junction → MODIFY to a proposed moving junction term
  (animal epithelial term; the merozoite "tight junction" is the apicomplexan moving
  junction, but the IDA localisation itself stands).
• GO:0044409 symbiont entry into host (9 rows) and GO:0046718 symbiont entry into host cell (2 EXP rows) → MODIFY to GO:0085017 entry into host cell by a symbiont-containing vacuole, the descendant that describes vacuole-mediated apicomplexan invasion and the
  term GOA uses for MSP1 and CSP.
• GO:0033644 host cell membrane → MODIFY to GO:0020002 host cell plasma membrane
  (the gene already carries the descendant with EXP evidence).
• GO:0005515 protein binding rows naming basigin → MODIFY to GO:0046789 host cell surface receptor binding; rows naming the parasite partners CyRPA, PfRipr and P113 →
  REMOVE. The interactions are real and specific, but the bare term carries no functional
  information and, the partners being parasite proteins, there is no informative MF child
  to refine to. What those rows encode — RH5's place in the trimeric invasion complex — is
  carried instead by the proposed complex term below and by the core functions, so nothing
  is lost. (Repo policy: a protein-binding row is either MODIFYed or REMOVEd, never kept.)
• GO:0032991 protein-containing complex → MODIFY to a proposed new term for the
  RH5–CyRPA–PfRipr invasion complex.
• GO:0008201 heparin binding → kept, non-core (see §6).
• One NEW process annotation, GO:0141134, for the RH5→basigin agonism that triggers
  host Ca²⁺ signalling (§5). Participation test: RH5 itself performs the receptor-binding
  step that activates the pathway, and two independent studies attribute the trigger to
  the RH5–BSG interaction specifically.