Protein degradation in leaves during senescence has been studied for many years because of its important role in protein turnover and nitrogen recycling (Huffaker, 1990; Vierustra, 1993; Feller and Fisher, 1994; Callis, 1995). Now, it is known that protein degradation is definitely a regulated process. Protein degradation is a complex of several pathways ranging from triggering of degradation to a complete hydrolysis to amino acids. It is known that there exists a number of proteases and their homologues in senescing leaves, and such proteases are thought to be involved in some aspects of protein degradation (see Chapter 4 by Jones and Chapter 7 by Feller). Surprisingly little is known yet about their individual roles in protein degradation in vivo. Activation of existing proteases, change of compartmentation, or change of microenvironment around target proteins in cells or organelles would also be other ways to trigger the degradation. Phosphorylation, ubiquitination, oxidation or reduction of their amino acid residues would modulate the susceptibility of target proteins to proteolysis. Conformational change of target proteins by binding of substrates or effectors might trigger their degradation, too.
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