Protein turnover in Arabidopsis

Research project P 13927 Protein turnover in Arabidopsis Andreas BACHMAIR 11.10.1999 Protein turnover is essential for all living cells. Proteins resulting from translational error, or those which are misfolded, denatured, or cannot be incorporated into the multiprotein complex to which they belong, are quickly degraded inside the cell. Furthermore, the concentration of proteins with regulatory function has to be: adjusted in response to cellular needs, which implies fast turnover rates for this type of proteins, as well. The Ubiquitin-dependent protein degradation system is a set of enzymes which carries out most of the intracellular proteolysis. While the process has already been studied for a while in a variety of organisms, there are still gaps in our knowledge, both with regard to basic enzymology, and with regard to the biological consequences of the proteolytic activity. We are studying ubiquitin-dependent protein turnover in the model plant, Arabidopsis thaliana. The goal of our work is to better understand how a plant`s life profits from this proteolytic system. One tool to achieve this goal is the generation of mutant plants with defects in genes of the ubiquitin-dependent protein turnover system. Mutant plants can be characterized by analysis of the gene affected, by physiological studies to assess the consequences of the mutation, and by biochemical analysis.

Proteins, the major constituents of living organisms, can be both synthesized and degraded inside cells. The time during which a protein exists in a cell to fulfill its function can vary between many years and a few minutes. Sophisticated mechanisms help to decide whether a particular protein is degraded quickly, or has a long life. The major degradation pathway for intracellular proteins is the so-called ubiquitin proteasome pathway. This pathway can distinguish between short-lived and long-lived proteins by recognizing so-called degradation signals present in short-lived proteins. These signals lead to the attachment of several copies of the small protein ubiquitin onto the protein substrate. The ubiquitin marks, in turn, are a recognition signal for the proteasome, a protein degrading multienzyme complex. Our interest focuses on the ubiquitin proteasome pathway of plants. Protein degradation plays a major role in plant life, e.g. during fruit development (when protein is degraded in the leaves to supply material for use in the growing fruit), or during leaf yellowing and shedding by trees in fall (proteins are degraded, and components are mobilized out of yellowing leaves for re-utilization in spring). In both cases, parts of the plant die to foster development of other parts. We have characterized an enzyme that recognizes the degradation signal of a substrate protein, a so- called ubiquitin protein ligase. The enzyme was studied by genetic and biochemical means. We have also begun to characterize enzymes with related function. Finally, we have investigated the contribution of the ubiquitin proteasome pathway to cases of so-called programmed cell death in plants. Our investigations contribute to a better understanding of plant metabolism, in particular those processes that enable a plant to use nutrients efficiently. These processes are important to optimize the yield of crop plants.