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CaaX-like protease of cyanobacterial origin is required for complex plastid biogenesis in malaria parasites

By Thomas R Meister, Yong Tang, Michael J. Pulkoski-Gross, Ellen Yeh

Posted 03 Jun 2020
bioRxiv DOI: 10.1101/2020.06.02.130229

Plasmodium parasites and related apicomplexans contain an essential complex plastid organelle of secondary endosymbiotic origin, the apicoplast. Biogenesis of this complex plastid poses a unique challenge requiring evolution of new cellular machinery. We previously conducted a mutagenesis screen for essential apicoplast biogenesis genes to discover organellar pathways with evolutionary and biomedical significance. Here we validate and characterize a gene candidate from our screen, Pf3D7\_0913500. Using a conditional knockdown strain, we show that Pf3D7\_0913500 depletion causes growth inhibition that is rescued by the sole essential product of the apicoplast, isopentenyl pyrophosphate (IPP), and results in apicoplast loss. Because Pf3D7_0913500 had no previous functional annotation, we name it apicoplast-minus IPP-rescued 4 (AMR4). AMR4 has an annotated CaaX Protease and Bacteriocin Processing (CPBP) domain, which in eukaryotes typically indicates a role in CaaX post-prenylation processing. Indeed, AMR4 is the only CaaX-like protease in Plasmodium parasites which are known to require protein prenylation, and we confirm that the conserved catalytic residue of AMR4 is required for its apicoplast function. However, we unexpectedly find that AMR4 does not act in a CaaX post-prenylation processing pathway in P. falciparum. Instead, we find that AMR4 is imported into the apicoplast and is derived from a cyanobacterial CPBP gene which was retained through both primary and secondary endosymbiosis. Our findings suggest that AMR4 is not a true CaaX protease, but instead acts in a conserved, uncharacterized chloroplast pathway that has been retained for complex plastid biogenesis. ### Competing Interest Statement The authors have declared no competing interest.

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