Abstract
Most plastids proteins are post-translationally imported into organelles through multisubunit translocons. The TIC and TOC complexes perform this role in the two membranes of the plant chloroplast and in the inner two membranes of the apicoplasts of the apicomplexan parasites, Toxoplasma gondii and Plasmodium falciparum. Tic22 is a ubiquitous intermembrane translocon component that interacts with translocating proteins. Here, we demonstrate that T. gondii Tic22 is an apicoplast-localized protein, essential for parasite survival and protein import into the apicoplast stroma. The structure of Tic22 from P. falciparum reveals a fold conserved from cyanobacteria to plants, which displays a non-polar groove on each side of the molecule. We show that these grooves allow Tic22 to act as a chaperone. General chaperones are common components of protein translocation systems where they maintain cargo proteins in an unfolded conformation during transit. Such a chaperone had not been identified in the intermembrane space of plastids and we propose that Tic22 fulfills this role.
Highlights
Tic22 is a core protein import machinery component of chloroplasts and apicoplasts
In this study we use genetic and structural approaches to define the function of Tic22, a molecule with a predicted role in protein import in both plant chloroplasts and apicomplexan apicoplasts
We utilize the robust genetics of T. gondii to demonstrate for the first time for any organism that cells deficient in Tic22 are impaired in plastid protein import
Summary
Tic is a core protein import machinery component of chloroplasts and apicoplasts. Results: Tic is a chaperone, required for parasite survival and for protein import in the apicoplast. The TIC and TOC complexes perform this role in the two membranes of the plant chloroplast and in the inner two membranes of the apicoplasts of the apicomplexan parasites, Toxoplasma gondii and Plasmodium falciparum. Apicomplexan genomes harbor a plastid-targeted Tic homologue and an, as yet, uncharacterized homologue of Toc75 [16, 17] The presence of these homologues suggest that protein translocation across the inner two membranes of apicoplasts is similar to that in plants. Conservation of these TIC and TOC homologues across vast evolutionary distances suggests that they most likely represent “core” components of the plastid import machinery. We establish the structure of Tic in the related parasite P. falciparum and show that Tic from both Toxoplasma and Plasmodium act as chaperone proteins
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