The Paulinella chromatophore transit peptide part2 adopts a structural fold similar to the γ-glutamyl-cyclotransferase fold

The chromatophores of the cercozoan amoeba Paulinella are photosynthetic organelles that evolved from a cyanobacterial endosymbiont. Many nucleus-encoded chromatophore-targeted proteins carry unusual N-terminal targeting signals termed crTPs. The crTPs are bipartite. Whereas crTP _part1 that likely mediates trafficking through the secretory pathway is cleaved off during import, crTP _part2 remains attached to its cargo protein. The function of crTP _part2 is unknown. To contribute to unravel the functional role of crTP _part2 , here we elucidated the structures of crTP _part2 from two different chromatophore-targeted proteins by X-ray crystallography at ∼2.3 Å resolution. Interestingly, the crTP _part2 of both proteins adopts a structural fold. Both structures share a conserved structured core and a flexible N-terminal arm. The structured core resembles proteins of the γ-glutamyl cyclotransferase superfamily within which crTP _part2 structures form a novel protein (sub)-family. The proposed catalytic center typical for proteins with cyclotransferase activity is not conserved in crTP _part2 . A Cys pair that is conserved in crTP _part2 of many chromatophore-targeted proteins has been captured as disulfide bridge. Together, our data suggests that chromatophore-targeted proteins are imported in their folded state and that the fold adopted by crTP _part2 plays a functional role during import. The characterization of its structure and flexibility provides important steps towards elucidating this novel protein translocation mechanism.