Introduction

In 1975, a small protein of 76 amino acid residues was found to be ubiquitously present in cells and was therefore named ubiquitin (Ub) [1]. The function of this protein remained a mystery until 1980, when Ub was shown to modify protein substrates covalently and target them for degradation in reconstructed reticulocyte by the group of Hershko, Ciechanover, and Rose [2, 3], who won the 2004 Nobel Prize in chemistry for this fundamental discovery. The concept was further substantiated by the lab of Varshavsky to demonstrate the physiological function of Ub during protein turnover in cells [4, 5]. Since then, Ub has risen as a central regulator in almost all cellular processes [6].

Protein modification by Ub is now known to function as a versatile molecular signal in a variety of cellular events, such as proteasome-mediated degradation, protein sorting, apoptosis, DNA repair, and regulation of transcription and translation [6, 7]. Proteins are usually attached by Ub in three steps in an ATP-dependent manner. Ub is first activated by forming a thioester intermediate with a Ub-activating enzyme (E1), then transferred to a Ub-conjugating enzyme (E2) through a thioester bond, and

MonoUb

MonoUb

PolyUb

Ub receptor^ DNA repair

Forked polyUb

PolyUb f Protein degradation Protein trafficking

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