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Summary
Essential cellular processes, such as cell division, rely on the coordinated destruction of proteins. The predominant means of accomplishing this involves a large cellular machine, the proteasome (1). Proteasomal degradation ensues when proteins are modified with ubiquitin, a small protein, that has many different roles (2). This tagging involves a carrier protein (an E2 ubiquitin-conjugating enzyme) and a substrate-determining protein (an E3 ligase). For example, during the cell division cycle, a large multiprotein E3 ligase, the anaphase-promoting complex/cyclosome (APC/C), utilizes two E2 enzymes, UBE2C and UBE2S, to target proteins for destruction (3). On pages 199 and 200 of this issue, two Research Articles by Lu et al. focus on these reactions and illuminate, at the single-molecule level, the process of ubiquitination by APC/C (4), as well as the recognition and subsequent destruction of APC/C substrates by proteasomes (5). Both studies substantially enrich our knowledge of ubiquitination and degradation, reveal new properties of APC/C and the proteasome, and challenge established concepts about the ubiquitin-proteasome system.
