Structure and function of Plasmodium actin II in the parasite mosquito stages

Abstract

Actins are filament-forming, highly-conserved proteins in eukaryotes. They are involved in essential processes in the cytoplasm and also have nuclear functions. Malaria parasites (Plasmodium spp.) have two actin isoforms that differ from each other and from canonical actins in structure and filament-forming properties. Actin I has an essential role in motility and is fairly well characterized. The structure and function of actin II are not as well understood, but mutational analyses have revealed two essential functions in male gametogenesis and in the oocyst. Here, we present expression analysis, high-resolution filament structures, and biochemical characterization of Plasmodium actin II. We confirm expression in male gametocytes and zygotes and show that actin II is associated with the nucleus in both stages in filament-like structures. Unlike actin I, actin II readily forms long filaments in vitro, and near-atomic structures in the presence or absence of jasplakinolide reveal very similar structures. Small but significant differences compared to other actins in the openness and twist, the active site, the D-loop, and the plug region contribute to filament stability. The function of actin II was investigated through mutational analysis, suggesting that long and stable filaments are necessary for male gametogenesis, while a second function in the oocyst stage also requires fine-tuned regulation by methylation of histidine 73. Actin II polymerizes via the classical nucleation-elongation mechanism and has a critical concentration of ~0.1 μM at the steady-state, like actin I and canonical actins. Similarly to actin I, dimers are a stable form of actin II at equilibrium.

Toxoplasma gondii actin filaments are tuned for rapid disassembly and turnover

“…Neither actin isoform from P. falciparum shows large differences in the positioning of the d -loop when compared to skeletal actin filaments (Fig. 5d, f ) 25 , 52 , 54 . For P. falciparum Act2, the substitution of asparagine for the glycine at position 42 might facilitate a more stable filament.…”

Toxoplasma gondiiactin filaments are tuned for rapid disassembly and turnover

“…Neither actin isoform from P. falciparum shows large differences in the positioning of the D-loop when compared to skeletal actin filaments (24, 49, 50). For P. falciparum Act2, the substitution of an asparagine for the glycine at position 42 might facilitate a more stable filament.…”

A non-canonical Arp2/3 complex is essential for Plasmodium DNA segregation and transmission of malaria.

“…Plasmodium encodes for two actin isoforms, the ubiquitously expressed actin 1 and the actin 2, which is expressed in mosquito stages. Actin 2 is essential for male gametogenesis, localises both to the cytoplasm and the nuclear spindle of the male gametocyte and in its absence, male gametocytes do not exflagellate 41,45 . While complementation of actin 2 with actin 1 restores gamete formation and exflagellation, parasites still arrest in early oocysts, suggesting that actin 1 complements cytoplasmic, but not nuclear functions of actin 2 46 .…”