ALP5a

ALP5a (PF3D7_0910700) is the Arp3-like subunit of an atypical five-subunit Arp2/3 complex in Plasmodium that supports chromosome segregation during male gametogenesis and downstream mosquito-stage development (PMID:40514561, PMID:40828051). It was identified as an interaction partner in GFP-ARPC1 co-immunoprecipitations from activated P. berghei gametocytes, and AlphaFold prediction with DALI structural comparison assigned it as the Arp3-like (human ARP3-similar) component of the complex (PMID:40514561, PMID:40828051). ALP5a localizes to the nucleus in gametocytes and to endomitotic spindles in male gametocytes, where the complex interacts with the kinetochore-associated protein AKiT7; disruption of Arp2/3 subunits or actin polymerization interferes with kinetochore-spindle association and produces subhaploid gametes (PMID:41218080, PMID:40514561). ALP5a interacts directly with the Arp2-like subunit ALP5b, and while each is individually dispensable for P. berghei blood-stage development, the two are simultaneously indispensable for parasite viability (PMID:41218080). Deletion of ALP5a leaves blood-stage growth, gametocytemia, and ookinete gliding motility intact but impairs DNA segregation during male gametogenesis and yields smaller, fewer oocysts that fail to mature, with severely impaired sporozoite formation, thereby blocking malaria transmission (PMID:41218080, PMID:42629988). Beyond its role as a subunit of this atypical Arp2/3 complex and the transmission phenotypes described, no further biochemical detail specific to ALP5a has been characterized in the available corpus.

1. 2025 High

   Establishing that ALP5a is a bona fide subunit of a Plasmodium Arp2/3-like complex answered whether this actin-like protein assembles into a defined machine rather than acting alone.

   Evidence GFP-ARPC1 co-immunoprecipitation with mass spectrometry from activated P. berghei gametocytes, plus AlphaFold3 prediction and DALI comparison assigning ALP5a as the Arp3-like subunit

   PMID:40514561 PMID:40828051

   Open questions at the time

   • Structural assignment is computational (AlphaFold/DALI), not an experimental structure of the assembled complex
   • Whether ALP5a possesses nucleation-relevant biochemical activity within the complex is not shown
2. 2025 High

   Localization and genetic interaction with ALP5b defined ALP5a as a nuclear factor whose function is co-dependent on its Arp2-like partner, distinguishing essential from redundant roles.

   Evidence Fluorescence localization, co-immunoprecipitation, and single versus double knockout genetics in P. berghei

   PMID:41218080

   Open questions at the time

   • Molecular basis of the synthetic lethality between ALP5a and ALP5b is not resolved
   • Whether ALP5a has functions outside the complex is untested
3. 2025 High

   Placing the complex on endomitotic spindles and linking it to AKiT7 explained how the complex acts mechanistically — coupling kinetochores to spindles during male gametogenesis.

   Evidence Co-immunoprecipitation identifying AKiT7, fluorescence localization to male gametocyte spindles, and genetic disruption with a kinetochore-spindle association readout

   PMID:40514561

   Open questions at the time

   • Direct binding interface between ALP5a and AKiT7 versus complex-level interaction is not dissected
   • How the Arp2/3-like complex physically bridges kinetochores to spindles is not defined
4. 2025 High

   Life-cycle knockout phenotyping established where ALP5a is functionally required, showing dispensability in blood stages but a block at oocyst maturation and transmission.

   Evidence Genetic knockout with blood-stage growth, gametocytemia, mosquito infection, oocyst size, and DNA segregation assays in P. berghei

   PMID:41218080 PMID:42629988

   Open questions at the time

   • Mechanistic link between impaired DNA segregation and downstream oocyst failure is not fully traced
   • Whether the transmission block reflects the spindle role alone or additional functions is unresolved
5. 2026 High

   Independent knockout characterization refined the developmental defect to oocyst development and sporozoite formation while confirming intact ookinete gliding.

   Evidence Genetic knockout with oocyst counts, oocyst size, sporozoite formation, and ookinete gliding assays in P. berghei

   PMID:42629988

   Open questions at the time

   • Stage-specific molecular events downstream of ALP5a loss during sporogony are not identified
   • No rescue/complementation reported to formally exclude second-site effects
6. 2025 Low

   Identification of two phosphorylated serines within seconds of gametogenesis activation raised the possibility of signaling-linked post-translational control of ALP5a.

   Evidence Phosphoproteomic data referenced in the context of ALP5a regulation (P. berghei)

   PMID:40828051

   Open questions at the time

   • Functional role of these phosphosites is not validated by mutagenesis
   • The kinase responsible and the consequence for complex assembly or activity are unknown

• It remains unknown whether ALP5a has any biochemical activity or role independent of the atypical Arp2/3 complex, and how its predicted structure translates into function in an experimentally determined assembly.
• No experimental structure of the assembled complex
• No defined enzymatic or nucleation activity attributed to ALP5a itself
• Human ortholog function not established from this corpus