Date Published: April 17, 2014
Publisher: Public Library of Science
Author(s): Juha Vahokoski, Saligram Prabhakar Bhargav, Ambroise Desfosses, Maria Andreadaki, Esa-Pekka Kumpula, Silvia Muñico Martinez, Alexander Ignatev, Simone Lepper, Friedrich Frischknecht, Inga Sidén-Kiamos, Carsten Sachse, Inari Kursula, Michael J. Blackman.
Actins are highly conserved proteins and key players in central processes in all eukaryotic cells. The two actins of the malaria parasite are among the most divergent eukaryotic actins and also differ from each other more than isoforms in any other species. Microfilaments have not been directly observed in Plasmodium and are presumed to be short and highly dynamic. We show that actin I cannot complement actin II in male gametogenesis, suggesting critical structural differences. Cryo-EM reveals that Plasmodium actin I has a unique filament structure, whereas actin II filaments resemble canonical F-actin. Both Plasmodium actins hydrolyze ATP more efficiently than α-actin, and unlike any other actin, both parasite actins rapidly form short oligomers induced by ADP. Crystal structures of both isoforms pinpoint several structural changes in the monomers causing the unique polymerization properties. Inserting the canonical D-loop to Plasmodium actin I leads to the formation of long filaments in vitro. In vivo, this chimera restores gametogenesis in parasites lacking actin II, suggesting that stable filaments are required for exflagellation. Together, these data underline the divergence of eukaryotic actins and demonstrate how structural differences in the monomers translate into filaments with different properties, implying that even eukaryotic actins have faced different evolutionary pressures and followed different paths for developing their polymerization properties.
Actins are the most abundant and among the most conserved proteins in eukaryotic cells and play indispensable roles in a plethora of key cellular events, including muscle contraction, cell division, shape determination, transport, and cell motility , . Actins are highly conserved in opisthokonts with <10% divergence between yeast and man. The six mammalian actin isoforms differ from each other by a maximum of 6% of the sequence, and are virtually identical across species. Nevertheless, these subtle differences are enough to determine isoform-specific functions . Common to most actins is their capacity to form long filaments. However, in a number of phylogenetically distinct organisms, such as Trypanosoma and Plasmodium spp., actin filaments have not been observed , . Unlike other members of the phylum Apicomplexa, which comprises single-celled eukaryotic intracellular parasites, the malaria parasites have two actin isoforms, which at the sequence level are <80% identical with canonical (opisthokont) actins and each other. This is a remarkable difference, considering the near identity among canonical actins (Fig. S1). An important question is how this divergence at the amino-acid level translates into different structures – and how this, in turn, influences polymerization. An actin cytoskeleton was long thought to be a feature unique to eukaryotic cells, and this view was revisited only two decades ago upon the discovery of the first bacterial actin and tubulin homologs –. The ancient phylum Apicomplexa is likely separated from opisthokonts by an evolutionary distance of a billion years, and the diversion of Plasmodium spp. took place hundreds of millions of years ago . Therefore, looking at the divergent properties of Plasmodium actins provides us with insight into the early stages of actin evolution. The ability of actin to polymerize must have evolved very early – before its involvement as tracks for molecular motors . This may explain some features of both the polymerization propensity and the divergent actin-myosin motor in Apicomplexa. Also the minimal set of actin-binding proteins in Apicomplexa suggests that a common ancestor had a limited polymerization propensity, and the various regulatory proteins in higher eukaryotes have evolved as the polymerization properties of actin itself have been fine-tuned, creating a need for additional regulation. For the biological functions of actin in Apicomplexa, the development of similar polymerization properties has strikingly not been of importance. Source: http://doi.org/10.1371/journal.ppat.1004091