The Ubiquitin–26S Proteasome System—A Versatile Player Worthy of Close Attention in Plants

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In the crowded and confined space of a cell, numerous proteins work collaboratively in various subsystems, such as metabolic pathways, organelle compartments, and complexes, to regulate cell growth and development. Given the estimated concentration range from 20% to 30% (w/v) in terms of the total cellular protein [1], the average size of 52 kDa ascribed to human proteins [2,3], and the median 50 μm in diameter of human cells [4], it can be estimated that there are about 1.5 ~ 2.3 × 1011 protein molecules in one human cell. It has also been estimated that plants, such as Arabidopsis thaliana, have an average protein size and cellular volume similar to those of humans [4,5]. Thus, there are myriad proteins in both human and plant cells. How such a large cohort of proteins is organized and managed for keeping intracellular proteome homeostasis constitutes the central topic in molecular and cellular biology.

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