Cytosolic targeting factor AKR2A captures chloroplast outer membrane-localized client proteins at the ribosome during translation

• Published in: Nature communications Vol. 6; no. 1; p. 6843
• Main Authors: Kim, Dae Heon, Lee, Jae-Eun, Xu, Zheng-Yi, Geem, Kyoung Rok, Kwon, Yun, Park, Joon Won, Hwang, Inhwan
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 16.04.2015; Nature Publishing Group
• Subjects: 631/337/574/1789; 631/449/2675; 631/449/448/1374; 631/80/2023; 82; Antibodies; Arabidopsis; Arabidopsis Proteins - metabolism; Biosynthesis; Chloroplast Proteins - metabolism; Chloroplasts; Chloroplasts - metabolism; Cytosol; Humanities and Social Sciences; Intracellular Membranes - metabolism; Membranes; Molecular Chaperones - metabolism; multidisciplinary; Protein Processing, Post-Translational; Protein Transport; Proteins; Ribonucleic acid; Ribosomal Proteins - metabolism; Ribosomes - metabolism; RNA; Science; Science (multidisciplinary)
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/ncomms7843

In eukaryotic cells, organellar proteome biogenesis is pivotal for cellular function. Chloroplasts contain a complex proteome, the biogenesis of which includes post-translational import of nuclear-encoded proteins. However, the mechanisms determining when and how nascent chloroplast-targeted proteins are sorted in the cytosol are unknown. Here, we establish the timing and mode of interaction between ankyrin repeat-containing protein 2 (AKR2A), the cytosolic targeting factor of chloroplast outer membrane (COM) proteins, and its interacting partners during translation at the single-molecule level. The targeting signal of a nascent AKR2A client protein residing in the ribosomal exit tunnel induces AKR2A binding to ribosomal RPL23A. Subsequently, RPL23A-bound AKR2A binds to the targeting signal when it becomes exposed from ribosomes. Failure of AKR2A binding to RPL23A in planta severely disrupts protein targeting to the COM; thus, AKR2A-mediated targeting of COM proteins is coupled to their translation, which in turn is crucial for biogenesis of the entire chloroplast proteome. Post-translational import of nuclear-encoded proteins shapes the proteome of organelles. Here, Kim et al. show that AKR2A, a critical targeting factor for chloroplast outer membrane proteins, binds to client proteins co-translationally as they exit the ribosome.