Open Access

Joanna Tripp, Alexander Hahn, Patrick König, Nadine Flinner, Daniela Bublak, Eva-Maria Brouwer, Franziska Ertel, Oliver Mirus, Irmgard Sinning, Ivo Tews, Enrico Schleiff

• 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 cross-linking. The physical interaction together with the phenotype of a tic22 mutant comparable with the one of the omp85 mutant indicates a concerted function of both proteins. The three-dimensional structure allows the definition of conserved hydrophobic pockets comparable with those of ClpS or BamB. The results presented suggest a function of Tic22 in outer membrane biogenesis. Background: Although Tic22 is involved in protein import into chloroplasts, the function in cyanobacteria is unknown. Results: Cyanobacterial Tic22 is required for OM biogenesis, shares structural features with chaperones, and can be substituted by plant Tic22. Conclusion: Tic22, involved in outer membrane biogenesis, is functionally conserved in cyanobacteria and plants. Significance: The findings are important for the understanding of periplasmic protein transport.