Actin-mediated plasma membrane plasticity of the intracellular parasiteTheileria annulata

Abstract

SummaryPathogen-host interactions are modulated at multiple levels by both the pathogen and the host cell. Modulation of host cell functions is particularly intriguing in the case of the intracellular Theileria parasite, which resides as a multinucleated schizont free in the cytosol of the host cell. Direct contact between the schizont plasma membrane and the cytoplasm enables the parasite to affect the function of host cell proteins through direct interaction or through the secretion of regulators. Structure and dynamics of the schizont plasma membrane are poorly understood and whether schizont membrane dynamics contribute to parasite propagation is not known. Here we show that the intracellular Theileria schizont can dynamically change its shape by actively extending filamentous membrane protrusions. We found that isolated schizonts bound monomeric tubulin and in vitro polymerized microtubules, and monomeric tubulin polymerized into dense assemblies at the parasite surface. However, we established that isolated Theileria schizonts free of host cell microtubules maintained a lobular morphology and extended filamentous protrusions, demonstrating that host microtubules are dispensable both for the maintenance of lobular schizont morphology and for the generation of membrane protrusions. These protrusions resemble nanotubes and extend in an actin polymerization-dependent manner; using cryo-electron tomography, we detected thin actin filaments beneath these protrusions, indicating that their extension is driven by schizont actin polymerization. Thus the membrane of the schizont and its underlying actin cytoskeleton possess intrinsic activity for shape control and likely function as a peri-organelle to interact with and manipulate host cell components.

Toxoplasma gondii F-actin forms an extensive filamentous network required for material exchange and parasite maturation

“…This suggests that F-actin is less accessible to exogenously added antibodies after fixation, perhaps due to epitope alterations during fixation or the association of actin binding proteins to filaments. Interestingly, while actin has never been previously associated with this membranous network in T. gondii , it appears very similar to structures recently described for the related parasite Theileria annulata , which was shown to contain F-actin in a similar configuration (Kühni-Boghenbor et al, 2012). …”

Structural Differences Explain Diverse Functions of Plasmodium Actins

“…In addition to the differences we have described in the residues involved in lateral contacts, a candidate responsible for increased stability is residue 200, which is a glycine in Plasmodium actin I and T. gondii actin [69] but serine or threonine in canonical actins as well as Plasmodium actin II and Theileria actin [9], all of which form long filaments. It has been reported that the double mutant G200S/K270M in T. gondii actin leads to an increased filament length when using phalloidin-labeled filaments [69].…”

“…Actin polymerization is indispensable for gliding and likely involved in host cell invasion and egress [6]–[8]. Despite evidence for this crucial role of filamentous actin, long filaments have only been visualized in Theileria [9], which appears not to use actin filaments for host cell invasion [10]. The presence of regular actin filaments in Plasmodium is uncertain [11]–[13].…”

Cryo-electron tomography reveals four-membrane architecture of the Plasmodium apicoplast

“…Tomography was performed essentially as described before [15,21,23,24,28]. Merozoites in RPMI medium mixed with 10 nm colloidal gold particles were transferred onto glow-discharged holey carbon Quantifoil EM grids.…”