Structure and conservation of the periplasmic targeting factor Tic22 from plants and cyanobacteria

Abstract

Mitochondria and chloroplasts are of endosymbiotic origin. Their integration into cells entailed the development of protein translocons, partially by recycling bacterial proteins. We demonstrate the evolutionary conservation of the translocon component Tic22 between cyanobacteria and chloroplasts. Tic22 in Anabaena sp. PCC 7120 is essential. The protein is localized in the thylakoids and in the periplasm and can be functionally replaced by a plant orthologue. Tic22 physically interacts with the outer envelope biogenesis factor Omp85 in vitro and in vivo, the latter exemplified by immunoprecipitation after chemical crosslinking. The physical interaction together with the phenotype of a tic22 mutant comparable to the one of the omp85 mutant indicates a concerted function of both proteins. The 3D structure allows the definition of conserved hydrophobic pockets comparable to those of ClpS or BamB. The results presented suggest a function of Tic22 in outer membrane biogenesis.

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ORCID for Ivo Tews: orcid.org/0000-0002-4704-1139

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