title: AKR2A-mediated import of chloroplast outer membrane proteins is essential for chloroplast biogenesis.
abstract: In plant cells, chloroplasts have essential roles in many biochemical reactions and physiological responses. Chloroplasts require numerous protein components, but only a fraction of these proteins are encoded by the chloroplast genome. Instead, most are encoded by the nuclear genome and imported into chloroplasts from the cytoplasm post-translationally. Membrane proteins located in the chloroplast outer envelope membrane (OEM) have a critical function in the import of proteins into the chloroplast. However, the biogenesis of chloroplast OEM proteins remains poorly understood. Here, we report that an Arabidopsis ankyrin repeat protein, AKR2A, plays an essential role in the biogenesis of the chloroplast OEM proteins. AKR2A binds to chloroplast OEM protein targeting signals, as well as to chloroplasts. It also displays chaperone activity towards chloroplast OEM proteins, and facilitates the targeting of OEP7 to chloroplasts in vitro. AKR2A RNAi in plants with an akr2b knockout background showed greatly reduced levels of chloroplast proteins, including OEM proteins, and chloroplast biogenesis was also defective. Thus, AKR2A functions as a cytosolic mediator for sorting and targeting of nascent chloroplast OEM proteins to the chloroplast.
authors.affiliation: Division of Molecular and Life Sciences, Pohang University of Science and Technology, Pohang, 790-784, Korea.
authors.forename: Wonsil
authors.initials: W
authors.lastname: Bae
authors.forename: Yong Jik
authors.initials: YJ
authors.lastname: Lee
authors.forename: Dae Heon
authors.initials: DH
authors.lastname: Kim
authors.forename: Junho
authors.initials: J
authors.lastname: Lee
authors.forename: Soojin
authors.initials: S
authors.lastname: Kim
authors.forename: Eun Ju
authors.initials: EJ
authors.lastname: Sohn
authors.forename: Inhwan
authors.initials: I
authors.lastname: Hwang
mesh_terms: D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D002736:Chloroplasts / Q000378:metabolism / Q000502:physiology*; D007425:Intracellular Membranes / Q000378:metabolism; D008565:Membrane Proteins / Q000378:metabolism; D018832:Molecular Chaperones; D021381:Protein Transport; D020798:Two-Hybrid System Techniques
chemical_list: C451699:AKR2 protein, Arabidopsis; D029681:Arabidopsis Proteins; D008565:Membrane Proteins; D018832:Molecular Chaperones; C445093:Oep7 protein, Arabidopsis