Recycling and Physiological Roles of Intracellular Proteins

Abstract

Proteins, which are the major functional constituents of the cell, show dynamic turnover facilitated by repeated synthesis and destruction, and an infinite variety of life-spans ranging from several minutes to several months; i.e., approximately 3.0% of all proteins are degraded daily. Proteolysis plays an important role not only in the selective disposal of harmful proteins with aberrant structures, but also in the active breakdown of unnecessary proteins whose accumulation would cause defects of cell homeostasis, and of normal proteins when the cell lacks metabolic energy or acquires amino acids for protein synthesis in nutritionally step-down conditions. Over the past 25 years, studies of proteolysis have led to a marked expansion of knowledge, and are still ongoing at the present time, so that proteolysis is currently a focus of interest in the field of life science. We have been aiming to provide a comprehensive picture of the divergent roles of proteolysis. This review summarizes the physiological importance of the protein renewal/recycling system in cell metabolism.