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

Abstract

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.