Abstract
BackgroundTail-anchored (TA) proteins are a distinct class of membrane proteins that are sorted post-translationally to various organelles and function in a number of important cellular processes, including redox reactions, vesicular trafficking and protein translocation. While the molecular targeting signals and pathways responsible for sorting TA proteins to their correct intracellular destinations in yeasts and mammals have begun to be characterized, relatively little is known about TA protein biogenesis in plant cells, especially for those sorted to the plastid outer envelope.Methodology/Principal FindingsHere we investigated the biogenesis of three plastid TA proteins, including the 33-kDa and 34-kDa GTPases of the translocon at the outer envelope of chloroplasts (Toc33 and Toc34) and a novel 9-kDa protein of unknown function that we define here as an outer envelope TA protein (OEP9). Using a combination of in vivo and in vitro assays we show that OEP9 utilizes a different sorting pathway than that used by Toc33 and Toc34. For instance, while all three TA proteins interact with the cytosolic OEP chaperone/receptor, AKR2A, the plastid targeting information within OEP9 is distinct from that within Toc33 and Toc34. Toc33 and Toc34 also appear to differ from OEP9 in that their insertion is dependent on themselves and the unique lipid composition of the plastid outer envelope. By contrast, the insertion of OEP9 into the plastid outer envelope occurs in a proteinaceous-dependent, but Toc33/34-independent manner and membrane lipids appear to serve primarily to facilitate normal thermodynamic integration of this TA protein.Conclusions/SignificanceCollectively, the results provide evidence in support of at least two sorting pathways for plastid TA outer envelope proteins and shed light on not only the complex diversity of pathways involved in the targeting and insertion of proteins into plastids, but also the molecular mechanisms that underlie the delivery of TA proteins to their proper intracellular locations in general.
Highlights
Tail-anchored (TA) proteins are a unique class of integral membrane proteins that possess a cytosolic N-terminal functional domain, followed by a single transmembrane domain (TMD) near or at their C terminus, and a short C-terminal hydrophilic tail [1]
We demonstrate that OEP9 is a bona fide TA plastid outer envelope protein and that, like other outer envelope proteins (OEPs), including Toc33 and Toc34, it relies on the ankryin repeatcontaining protein, AKR2A, as a chaperone/receptor for its initial targeting from the cytosol to plastids
According to the information provided for the OEP9 gene locus at GenBank and the Arabidopsis Information Resource (TAIR), the deduced protein is annotated to be of unknown function and possess no putative targeting signal motifs
Summary
Tail-anchored (TA) proteins are a unique class of integral membrane proteins that possess a cytosolic N-terminal functional domain, followed by a single transmembrane domain (TMD) near or at their C terminus, and a short C-terminal hydrophilic tail [1]. Tail-anchored (TA) proteins are a distinct class of membrane proteins that are sorted post-translationally to various organelles and function in a number of important cellular processes, including redox reactions, vesicular trafficking and protein translocation. While the molecular targeting signals and pathways responsible for sorting TA proteins to their correct intracellular destinations in yeasts and mammals have begun to be characterized, relatively little is known about TA protein biogenesis in plant cells, especially for those sorted to the plastid outer envelope
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