Regulation of Protein Degradation.

INTRODUCTION The intracellular level of a protein is dependent on both its rate of synthesis and its rate of degradation. Thus, differential regulation of protein stability represents a potential mechanism for modulating gene expression. lncreasing evidence suggests that protein degradation is indeed a regulatory mechanism in vivo. Dramatic differences in the in vivo stability of different proteins have been documented. Multiple proteins have been demonstrated to undergo selective proteolysis only at particular stages in the cell cycle, after certain environmental stimuli, or after specific metabolic or developmental changes. The degradation of many intracellular proteins is ATP dependent, which does not appear necessary based on the energetics of peptide bond hydrolysis but which does provide a potential mechanism for regulation. Proteinase activities are not constant ; some increase or decrease and others appear de novo, resulting in changes in intracellular proteolysis. Selective protein degradation serves a wide variety of roles during the plant life cycle. During seed germination, hydroly-sis of seed storage proteins provides amino acids for protein synthesis in the growing seedling. Seed storage proteins, in turn, are synthesized in part from amino acids produced by the proteolysis of vegetative proteins. In annual plant species, this synthesis is accompanied by leaf senescence. Thus, pro-teolysis plays an important role in reallocating organic nitrogen in the plant. In nonsenescing cells, selective proteolysis serves to reduce the toxic effects of inactivated, denatured, unassem-bled, and abnormal proteins bydegrading them. It also serves to regulate flux through metabolic pathways by regulating the levels of rate-limiting enzymes. Another important cellular function of proteolysis is to modulate the levels of receptors; degradation after signal perception represents a mechanism for signal desensitization. Although it is not directly demonstrated in plants, it is likely that proteolysis of proteins that regulate the activity of cell cycle kinases controls the plant cell cycle transitions as demonstrated for the yeast and animal cell cycles. Indeed, regulated proteolysis is probably directly or indirectly involved in most cellular processes. We are only beginning to elucidate the machineries that degrade proteins and how the activities of those machineries are regulated. Complexities impeding progress include the diversity of intracellular locales in which proteolysis occurs, the diversity of proteolytic activities and pathways present in cells, and the relatively low level of proteinase activities. Our lack of understanding of how to preserve in vivo proteolytic activities in vitro also has interfered with the identification, purification, and analyses of …