ORC5

PF3D7_0215800 (ORC5) is a functional subunit of the Plasmodium origin recognition complex that couples origin licensing to the parasite's cell-cycle-driven DNA replication program (PMID:18554328). Its identity as a bona fide ORC5 orthologue is established by complementation of S. cerevisiae ORC5, confirming conserved replication-initiation function (PMID:18554328). During active replication in early-to-mid trophozoites, ORC5 assembles into discrete nuclear foci that colocalize with PfORC1 and the replication marker PCNA and co-immunoprecipitates with PCNA; this association is stage-restricted, dissolving at the non-replicating late schizont stage (PMID:18554328). The dependence of these foci on ongoing replication is shown by their disruption upon hydroxyurea treatment, which also arrests parasite growth (PMID:18554328). Unlike PfORC1, ORC5 is excluded from telomeric/subtelomeric heterochromatin (TAREs and var promoters), distinguishing its genomic distribution from the ORC1/Sir2 chromatin-silencing role and placing it at euchromatic replication origins (PMID:22379140). In P. berghei gametocytes, ORC5 is rapidly recruited (within seconds of activation) into a CDPK4-associated pre-replicative complex in a CDPK4-dependent manner, linking origin-complex assembly to signalling at the onset of gametogenesis (PMID:28481199).

1. 2008 Medium

   Whether the Plasmodium ORC5 candidate was a genuine replication-initiation factor was unknown; functional complementation established it as a true ORC5 orthologue rather than merely a sequence homologue.

   Evidence Yeast complementation assay of S. cerevisiae ORC5 plus bioinformatics

   PMID:18554328

   Open questions at the time

   • Complementation shows functional conservation but not the native molecular activity in the parasite
   • Does not define which origins ORC5 binds in P. falciparum
2. 2008 Medium

   It was unclear when and where ORC5 acts in the intraerythrocytic cycle; imaging and Co-IP placed ORC5 at replication foci with PCNA and ORC1 specifically during active replication.

   Evidence Immunofluorescence colocalization and reciprocal Co-IP with PCNA and ORC1 across trophozoite and schizont stages in infected erythrocytes

   PMID:18554328

   Open questions at the time

   • Co-IP does not distinguish direct binding from complex co-membership
   • Stage separation from PCNA at schizont stage is described but its regulatory mechanism is unresolved
3. 2008 Medium

   To test whether ORC5 foci reflect active replication rather than static chromatin binding, replication was chemically blocked; hydroxyurea disrupted ORC5/PCNA foci and arrested growth.

   Evidence Hydroxyurea treatment with immunofluorescence and growth assay in P. falciparum

   PMID:18554328

   Open questions at the time

   • Hydroxyurea acts broadly on replication and does not prove ORC5-specific involvement
   • Growth arrest cannot be attributed to ORC5 disruption alone
4. 2012 Medium

   Whether ORC5 shares ORC1's heterochromatin/silencing role was open; ChIP showed ORC5 is excluded from telomeric/subtelomeric loci occupied by ORC1, functionally separating the subunits' genomic distributions.

   Evidence ChIP with anti-ORC5 polyclonal antibody at TAREs and var promoters in P. falciparum

   PMID:22379140

   Open questions at the time

   • A negative ChIP result does not map the euchromatic origins ORC5 does occupy
   • Does not resolve whether ORC5 ever transiently associates with heterochromatin
5. 2017 Medium

   The timing and regulation of ORC assembly at replication onset were unknown; ORC5 was found to be recruited into a CDPK4-associated pre-replicative complex within seconds of gametocyte activation, dependent on active CDPK4.

   Evidence CDPK4-3xHA immunoprecipitation with quantitative mass spectrometry comparing non-activated vs activated gametocytes with/without CDPK4 inhibitor in P. berghei

   PMID:28481199

   Open questions at the time

   • Interactome enrichment does not establish direct ORC5-CDPK4 contact
   • Shown in P. berghei gametogenesis, not confirmed for the P. falciparum intraerythrocytic cycle
   • Whether CDPK4 phosphorylates ORC5 is not determined

• The direct DNA-binding origins occupied by ORC5, its stoichiometry within the parasite ORC, and whether it is a CDPK4 substrate remain uncharacterized in the available corpus.
• No genome-wide map of ORC5-bound origins
• No structural or biochemical reconstitution of the parasite ORC containing ORC5
• No test of ORC5 essentiality by genetic knockout