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Toxoplasma gondii Toc75 Functions in Import of Stromal but not Peripheral Apicoplast Proteins

Lilach Sheiner1,2, Justin D Fellows1, Jana Ovciarikova2

  • 1Center for Tropical and Emerging Global Diseases & Department of Cellular Biology, University of Georgia, 500 D.W. Brooks Drive, Athens, GA, 30602, USA.

Traffic (Copenhagen, Denmark)
|September 19, 2015
PubMed

Insights

Apicomplexan parasites rely on the apicoplast, a four-membraned organelle. This study identifies Toc75 as crucial for transporting proteins into the apicoplast stroma, essential for parasite survival.

Area of Science:

  • Parasitology
  • Cell Biology
  • Molecular Biology

Background:

  • Apicomplexa are unicellular parasites responsible for significant human and animal diseases like malaria and toxoplasmosis.
  • Most apicomplexan parasites possess an essential relict organelle called the apicoplast, acquired through secondary endosymbiosis, which is enclosed by four membranes.
  • Efficient protein transport into the apicoplast's sub-compartments is vital, requiring specific transport signals and protein channels (translocons) across its membranes.

Purpose of the Study:

  • To investigate the role of apicomplexan Toc75 homologs in protein transport into the apicoplast.
  • To identify and localize genes encoding Toc75 and Sam50, members of the Omp85 protein family, within apicomplexan parasites.
  • To functionally characterize the role of Toxoplasma Toc75 in parasite growth and protein import.

Main Methods:

  • Identification and localization of apicomplexan genes encoding Toc75 and Sam50.
  • Subcellular localization studies of Toc75 and Sam50 proteins in *Toxoplasma* and *Plasmodium*.
  • Functional analysis of *Toxoplasma* Toc75, including gene depletion experiments and assessment of protein import into the apicoplast.

Main Results:

  • Two apicomplexan genes encoding Toc75 and Sam50, both Omp85 family members, were identified.
  • The corresponding proteins were localized to the apicoplast and mitochondrion in *Toxoplasma* and *Plasmodium*.
  • Depletion of *Toxoplasma* Toc75 proved essential for parasite growth and caused a rapid defect in apicoplast stromal protein import, with outer compartment protein import affected secondarily to organelle loss.

Conclusions:

  • The findings provide functional support for the role of an apicomplexan Toc75 homolog in apicoplast protein transport.
  • The data suggest that apicomplexan Toc75 may function in protein translocation across the second innermost membrane of the apicoplast.
  • This study enhances understanding of the complex protein import machinery required for the function of the apicoplast in parasitic organisms.
Keywords:
Omp85PlasmodiumSam50Toc75Toxoplasmaapicomplexaapicoplastcomplex plastidprotein trafficking

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