9.B.412. The Ccercozoan Amoeba Octotrico Peptide Repeat (CAOPR) Family
The cercozoan amoeba Paulinella chromatophora contains photosynthetic organelles-termed chromatophores, that evolved from a cyanobacterium approximately 100 million years ago, independently from plastids in plants and algae. Despite the more recent origin of the chromatophore, it shows tight integration into the host cell. It imports hundreds of nucleus-encoded proteins, and diverse metabolites are continuously exchanged across the two chromatophore envelope membranes. However, the limited set of chromatophore-encoded solute transporters appears insufficient for supporting metabolic connectivity or protein import. Furthermore, chromatophore-localized biosynthetic pathways as well as multiprotein complexes, include proteins of dual genetic origin, suggest that mechanisms evolved that coordinate gene expression levels between chromatophore and nucleus. Thus, similar to the situation in mitochondria and plastids, nuclear factors in P. chromatophora evolved that control metabolite exchange and gene expression in the chromatophore. Oberleitner et al. 2020 showed that nucleus-encoded transporters are not inserted into the chromatophore inner envelope membrane. Despite the apparent maintenance of its barrier function, canonical metabolite transporters are missing in this membrane, but there are several expanded groups of short chromatophore-targeted orphan proteins. Members of one of these groups are characterized by a single transmembrane helix, and others contain amphipathic helices. Possibly these proteins are involved in modulating membrane permeability. If so, the mechanism generating metabolic connectivity of the chromatophore fundamentally differs from the one for mitochondria and plastids, but likely rather resembles the mechanism in various bacterial endosymbionts in plants and insects. An expanded family of chromatophore-targeted helical repeat proteins show similar domain architectures as known organelle-targeted expression regulators of the octotrico peptide repeat type in algae and plants. Possibly, these chromatophore-targeted proteins evolved convergently to plastid-targeted expression regulators and are involved in gene expression control in the chromatophore (Oberleitner et al. 2020).
Here we list the sequences of some of these proteins in four of these groups: