Disassembly and degradation of Photosystem I

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DESCRIPTION (provided by applicant): The long-term goal of this project is to understand how membrane protein complexes are disassembled and degraded. Many proteins are degraded in a regulated and energy-consuming fashion, and proteins recognized by the cell as aberrant are targeted by these "quality control" systems preferentially. Although the cellular mechanisms used to degrade soluble proteins are becoming well understood, knowledge of the corresponding process for membrane proteins has lagged, despite the key role many membrane proteins play in important processes, such as signal transduction, bioenergetics, and cellular homeostasis. Several human diseases, such as cystic fibrosis and retinitis pigmentosa, are caused by degradation of membrane proteins with point mutations. The model system proposed here is the photosystem I complex in the unicellular green alga, Chlamydomonas reinhardtii. This protein is an essential part of the photosynthetic electron transport chain and uses light absorbed by its associated chlorophyll molecules to drive electron transfer across the thylakoid membrane. It should serve as an excellent model substrate for membrane protein degradation, because the functional portion of the protein is in the plane of the membrane, where it has many spectroscopic probes built into it. The project comprises the complementary approaches of suppressor genetics and biochemistry. The specific aims of the project are 1) to isolate mutations that slow the degradation of a mutant photosystem I protein that is normally targeted by the quality control system; 2) to characterize the specificity of the mutants' phenotypes; 3) to create an in vitro degradation system that faithfully mimics the in vivo process; and 4) to use this system to characterize the process and identify some of the factors involved in it.