Elucidating the role of SP2 and the SP1-SP2 machinery in chloroplast protein degradation

点击翻译按钮获取中文摘要

The human population is growing rapidly and set to reach 9bn by 2050, and there is ever increasing pressure on natural resources. Thus, the drivers for increased crop yields and resilience to climate change and sub-optimal growing conditions are stronger than ever. To meet these demands it will be essential to develop improved crop varieties. Through research on the model plant thale cress, we recently made a significant breakthrough: We discovered a gene called SP1 that controls important aspects of plant growth, including plant responses to adverse environmental conditions such as water stress and high salinity (collectively, abiotic stresses). Thale cress plants can be made more tolerant of such stresses by modifying SP1 expression. Recently, we identified another gene called SP2 that functions in the same regulatory pathway as SP1. In this project, we will study the SP2 gene in detail, to elucidate its function, to understand how it works together with SP1, and to investigate its potential use for crop improvement by conducting studies in rice.The SP1/2 genes regulate the development of structures inside plant cells called chloroplasts. Chloroplasts are normal cellular constituents (i.e., they are organelles), and in many ways they define plants. They contain the green pigment chlorophyll and are responsible for photosynthesis, capturing sunlight energy and using it to power the activities of the cell. As photosynthesis is the only significant mechanism of energy-input into the living world, chloroplasts are of huge importance, not just to plants but to all life on Earth. Chloroplasts also have critical roles in plant responses to abiotic stress, and so are ideal targets for engineering stress tolerance in crops.Chloroplasts are composed of thousands of different proteins, and most of these are encoded by genes in the cell nucleus and so are synthesized outside of the organelle in the cellular matrix known as the cytosol. As chloroplasts are each surrounded by a double-membrane envelope, sophisticated machinery is needed to bring about the import of these proteins into the organelle. This comprises two molecular machines, one in each membrane, called TOC (for "Translocon at the Outer membrane of Chloroplasts") and TIC. Each machine is composed of several different proteins that work cooperatively.The SP1 gene encodes a regulatory factor called a "ubiquitin E3 ligase". Such regulators work by labelling-up unwanted proteins to target them for removal. The SP1 E3 ligase mediates the removal of TOC components, and thereby controls TOC functions so that only the desired proteins are imported by chloroplasts. Such control enables major functional changes of chloroplasts during development and in adaptation to stress. But TOC proteins are deeply embedded in the chloroplast outer membrane, presenting a physical obstacle to their removal following labelling by SP1. Our discovery of SP2 provides a clue as to how this obstacle is overcome. The SP2 gene encodes a channel across the chloroplast outer membrane, and our evidence suggests that it forms the exit gate for the removal of unwanted TOC proteins. In fact, we believe that the SP1 and SP2 factors are stably associated in a complex to enable coordinated protein labelling and removal. We will study this SP1-SP2 machinery to understand more clearly how unwanted chloroplast proteins are removed.Moreover, the role of SP2 in environmental stress tolerance will be studied. In particular, we will manipulate activity of the SP1-SP2 pathway with the aim of improving stress tolerance in rice. The SP1-SP2 pathway appears to operate in many different plant species, including major crops, and so our results have the potential to see broad application. Drought and salinity are among the most significant factors affecting crop yields, with annual global losses due to drought alone estimated at $10bn. We believe that our work with SP1/2 may help to alleviate such losses.