A non-canonical Arp2/3 complex is essential for Plasmodium DNA segregation and transmission of malaria
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Abstract
The malaria-causing parasite Plasmodium has a complex life cycle involving both vertebrate and mosquito hosts. Sexual stages or gametocytes are the only stage competent for transmission to mosquitoes. Formation of flagellated male gametes from gametocytes requires rapid rounds of genome replication. Here we discovered a non-canonical Plasmodium actin-related protein 2/3 (Arp2/3) complex essential for DNA segregation during male gametogenesis. Plasmodium Arp2/3 dynamically localizes within the nucleus to the endomitotic spindles and interacts with a kinetochore protein. Deletion of key Arp2/3 subunits or interfering with actin polymerisation leads to the formation of sub-haploid male gametes and a complete block in transmission through delayed developmental arrest at the oocyst stage. Our work identified an evolutionary divergent protein complex in malaria parasites that offers potential targets for transmission-blocking interventions.
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Here we discovered that Plasmodium ARPC1 constitutes part of a highly divergent, non-canonical Arp2/3 complex,
I really loved reading this paper! Although actin and the Arp2/3 complex are well-studied proteins in mammalian cells and common model organisms, this paper really highlights how much we still have to learn about both actin biology and especially the Arp2/3 complex!
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However, we could not identify any orthologues to these proteins in the Plasmodium genomes (51). The mode of Arp2/3 activation thus remains to be determined.
This is also something that we encountered in Chlamydomonas! I wonder if a structural search for some of those NPFs might give you better results than a sequence search.
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While the canonical Arp2/3 complex consists of seven subunits, we have only identified five orthologues.
It might be helpful to discuss the roles of the different subunits in actual complex function. For example, ARPC2 and ARPC4 generally form the primary connection with the mother filament and Arp2 and Arp3 help nucleate the new daughter filament. You identified those essential subunits! It might also be helpful to talk a bit about other non-canonical Arp2/3 complexes throughout the tree of life (for example Chlamydomonas), and to talk about previous studies where people have mutated or removed a subunit and the complex still retained some function.
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We therefore conclude that Plasmodium Arp2/3 nucleates actin, much like canonical Arp2/3 complexes
I'm curious if there are any other known actin-dependent phenotypes that you could probe with your ARPC1 null to support this more in future work.
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Arp2/3 complex is highly diverse and thus not bound by the canonical Arp2/3 inhibitor.
Some docking studies using CK-666 could help support this since you already have the structures available!
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CK-666 drastically impaired overall exflagellation rates
I'm wondering how healthy cells were with this dosage of CK-666? And if you tried any concentrations between 50uM and 250uM?
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Including PbARPC1 itself, we thus identified structural homologues to five out of seven subunits of the Arp2/3 complex, including the core proteins Arp2 and Arp3, which suggests the presence of a non-canonical, minimalistic Arp2/3 complex in Plasmodium.
This section is so cool! I like the model of your putative Arp2 and Arp3 with a couple actin monomers. Did you try modeling a full complex with your identified proteins?
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hile we noted that spindle microtubules appeared longer in
Just pointing out that this sentence appears to be incomplete!
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formed axonemes
I'm curious if there are differences in axoneme formation or structure. I'm not sure about plasmodium cells, but the Arp2/3 complex has been found to be involved in flagellar assembly in other organisms.
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In conclusion, our data demonstrate that PbARPC1 is required for normal oocyst growth and sporozoite development, and deletion of ARPC1 leads to a complete block in transmission.
Very interesting!
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The ARPC1 protein sequence is conserved across the genus Plasmodium but shows less than 20% identity to other ARPC1/ARPC1 proteins from model species
I know that you'll get to structure in your work, but was there anything known about structure conservation previously? I know 20% is super low for sequence identity, but I imagine that the structure could potentially be better conserved.
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Previous phylogenetic studies identified Plasmodium ARPC1/ARC40, from here on named ARPC1 to be consistent with the most common Arp2/3 complex subunit nomenclature, as the sole conserved subunit of the Arp2/3 complex in Plasmodium.
Since you mention phylogenetic studies, I'm curious if this apparent loss of most of the Arp2/3 complex is common in Plasmodium's closest relatives or if this is unique to Plasmodium species.
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