Proteolytic processes regulate a wide range of functions throughout the life cycle of the cell. Proteases are intimately involved in the control of the cell cycle, gene expression, differentiation, protein targeting and sorting, protein quality control and programmed cell death. Thus, chloroplast proteases are expected to be involved in the regulation of photosynthesis at several different levels as well. However, the role of proteases at most of these levels is still unknown. The development of proplastids and etioplasts into chloroplasts involves not only the synthesis of a new set of proteins, but also massive degradation of the previous population. Gene expression within the chloroplast might require degradation of positive and negative regulators. Limited proteolytic processing is an essential step in the import and sorting of nuclear-encoded chloroplast proteins, and in the maturation of some chloroplast-encoded proteins. Proteases are likely to participate in the biogenetic processes of some protein complexes. Perhaps the most explored aspect of protein degradation in chloroplasts is its role in adapting to changing environmental conditions. Increasing light intensities lead to photodamage, the repair of which requires accelerated removal of damaged proteins. Adaptation to lower light intensities also involves selective proteolytic degradation. Similarly, heat-denatured proteins that fail to refold properly are probably degraded.
Research in chloroplast protein degradation has evolved primarily along two parallel tracks. Detailed characterizations of the degradation of specific proteins, especially in response to changing light intensities, have dominated the field for many years. Attempts have also been made to isolate the proteases responsible for these processes, with only limited success. In recent years, we employed strategies for the identification of chloroplast proteases, independent of their substrates. Using biochemical, molecular and genetic approaches, we identified and characterized a number of chloroplast proteases in all major compartments of the organelle: stroma, thylakoid membrane and lumen. Our current research focuses on chloroplast Clp, FtsH and DegP (HtrA) proteases, and the role they play in the biogenesis and maintenance of the photosynthetic organelle.
