ALP5b

ALP5b (PF3D7_0409900) is a structural homolog of human ARP2 that serves as a subunit of an atypical five-subunit Arp2/3 complex operating in the nucleus of Plasmodium male gametocytes (PMID:40514561). Identified in P. berghei by reciprocal co-immunoprecipitation of GFP-tagged ARPC1, ALP5b is predicted by AlphaFold and DALI to occupy the ARP2 position within the complex (PMID:40514561), and it directly interacts with its paralog ALP5a (PMID:41218080). Rather than acting in the cytoplasm, this complex localizes to endomitotic spindles and associates with the kinetochore-associated protein AKiT7; disruption of Arp2/3 subunits causes kinetochore-spindle detachment and formation of subhaploid gametes, placing ALP5b in nuclear actin polymerization at the mitotic spindle to ensure proper kinetochore-spindle attachment (PMID:40514561, PMID:40828051). Loss of ALP5b in P. berghei produces small, non-maturing oocysts, defective DNA segregation during male gametogenesis, and a block in malaria transmission (PMID:41218080). ALP5b is genetically redundant with ALP5a for viability — single deletion is tolerated for blood-stage growth and gametocyte development, whereas simultaneous deletion of both paralogs is lethal (PMID:41218080). All direct evidence derives from the P. berghei ortholog; no biochemical reconstitution or activity data for ALP5b itself has been characterized in the available corpus.

1. 2025 Medium

   Establishing that ALP5b is a bona fide component of a Plasmodium Arp2/3-like complex answered whether this actin-like protein has a defined molecular assembly and position within it.

   Evidence Reciprocal Co-IP of GFP-tagged ARPC1 from activated male gametocytes plus AlphaFold multimer and DALI structural prediction, in P. berghei

   PMID:40514561

   Open questions at the time

   • No in vitro reconstitution or mutagenesis confirming the predicted ARP2 position
   • Structural placement rests on prediction, not experimental structure
   • Actin-nucleating activity of the complex not demonstrated for ALP5b
2. 2025 Medium

   Localization and paralog-interaction studies defined where ALP5b acts and revealed genetic redundancy, answering whether ALP5b is individually essential.

   Evidence Single and double genetic knockouts, fluorescence localization, and Co-IP in P. berghei male gametocytes

   PMID:41218080

   Open questions at the time

   • Molecular basis of ALP5a–ALP5b redundancy not resolved
   • Whether the two paralogs occupy the same complex or distinct complexes is unclear
   • Synthetic lethality mechanism unknown
3. 2025 Medium

   Phenotyping ALP5b deletion connected the complex to a specific cellular process, showing ALP5b is required for DNA segregation and transmission.

   Evidence Genetic knockout with oocyst size measurement, genetic crosses, and microscopy of male gametogenesis in P. berghei

   PMID:41218080

   Open questions at the time

   • Direct molecular link between ALP5b loss and DNA mis-segregation not defined
   • Whether the oocyst defect is cell-autonomous to the male lineage untested at molecular level
4. 2025 Medium

   Spindle localization and the AKiT7 interaction pinned the complex's function to kinetochore-spindle attachment, explaining the DNA segregation defect mechanistically.

   Evidence Co-IP, fluorescence localization to endomitotic spindles, and genetic disruption with kinetochore-spindle association assay in P. berghei

   PMID:40514561 PMID:40828051

   Open questions at the time

   • Whether ALP5b specifically (vs other subunits) contacts AKiT7 not resolved
   • Nuclear actin polymerization at the spindle not directly visualized for ALP5b
   • Mechanism coupling actin dynamics to kinetochore attachment unknown

• Whether the ARP2-position assignment, actin-nucleating activity, and P. falciparum function of ALP5b hold biochemically remains open.
• No in vitro reconstitution of the ALP5b-containing complex
• No experimental structure of ALP5b or its position in the complex
• Function shown only in P. berghei; P. falciparum ALP5b not directly assayed