Protein import into chloroplasts and its regulation by the ubiquitin-proteasome system Event

Abstract: Chloroplasts belong to a family of related organelles called plastids, which are a defining feature of plants. The plastid family includes a range of structurally and functionally diverse variants (e.g., chromoplasts, amyloplasts, etioplasts) in addition to chloroplasts, although the latter are by far the best known members owing to their abundance and vital role in photosynthesis. Chloroplasts may contain up to 3000 different proteins, most of which are encoded in the nucleus and synthesized on cytosolic ribosomes. Such proteins are made in precursor form, each one with an amino-terminal targeting signal, or transit peptide. The transit peptide directs the protein through a post-translational targeting pathway, and is cleaved upon arrival inside the chloroplast. This targeting or import process is mediated by the coordinate action of two proteinaceous import machines, one in each of the organelle’s envelope membranes. The import machinery of the outer envelope membrane is called the TOC complex, and that in the inner membrane is called the TIC complex. Components of the TOC and TIC complexes have been identified through biochemical analyses. Interestingly, genome sequence information has revealed that many of these components (particularly receptor components of the TOC complex) exist in multiple homologous forms. We and others have used genetic approaches to dissect the functional significance of these different TOC protein isoforms in Arabidopsis. Results indicate that the different isoforms function in different import pathways with distinct client specificities, the regulated operation of which may control the organelle’s proteome and functions, and influence the differentiation of different plastid types. Our recent work has shown that the TOC apparatus is regulated by direct action of the ubiquitin-proteasome system, and that such control is mediated by a ubiquitin E3 ligase in the chloroplast outer envelope membrane called SP1. We have shown that regulation by SP1 has profound effects on plastid developmental processes, and is additionally important during plant responses to abiotic stress.