Investigating Protein Targeting to the Outer Membrane of Plastids

Abstract

Plastids are plant organelles with specialized functions, such as photosynthesis. The specialized function of each plastid is informed by its distinct and dynamically regulated proteome. The vast majority of plastid proteins are synthesized in the cytosol and are imported into the plastid post-translationally. A variety of receptors and channels embedded within the plastid outer and inner envelope regulate the import of plastid proteins, thus, control the plastid proteome composition. Proteins embedded within the outer envelope membrane of plastids have been generally categorized into four groups which include, B-barrel proteins, tail-anchored proteins, signal-anchored proteins, and CT TP-like proteins. Each group is defined by distinct structural and plastid-targeting characteristics. B-barrel proteins are composed of B-sheets and their plastid-targeting signal and mechanism is not well understood. Tail-anchored and signal-anchored proteins are tethered to the plastid outer envelope membrane by a single transmembrane alpha-helix located at the proteins C-terminus or N-terminus, respectively, and use a variety of physiochemical features for plastid-targeting. Lastly, the only currently defined CT TP-like protein, Toc159, utilizes a C-terminal plastid-targeting signal with transit peptide-like features. In this study, the structure and plastid-targeting signal of the plastid protein Outer Envelope Protein 16-2 (OEP16-2) was investigated. Computational structural analysis showed that OEP16-2 is embedded within the outer envelope membrane by four alpha-helical transmembrane domains and does not share structural similarity with defined categories of outer envelope proteins. Furthermore, three internal transmembrane alpha helical domains were sufficient for plastid-targeting. These internal targeting domains cannot be characterized by currently defined outer envelope protein targeting strategies. Thus, OEP16-2 was classified in a fifth outer envelope protein category, defined by multiple transmembrane alpha helices and internal targeting domains. Future experiments will examine the structure and plastid-targeting signal of other outer envelope proteins with multiple transmembrane helices.