The attachment of ubiquitin and ubiquitin-like polypeptides to intracellular proteins is a key mechanism in regulating many cellular and organismal processes. Assembly of a chain of at least four ubiquitins linked together via their Lys48 residue marks cellular proteins for degradation by the 26S proteasome. In contrast, monoubiquitination or polyubiquitination with chains linked together via Lys63 serve as nonproteolytic signals in intracellular trafficking, DNA repair, and signal transduction pathways. Ubiquitination of proteins is achieved through an enzymatic cascade involving ubiquitin-activating (E1), ubiquitin-conjugating (E2), and ubiquitin-ligating (E3) enzymes (EC 6.3.2.19). Two major types of E3s exist in eukaryotes, defined by the presence of either a HECT or a RING domain[1]. The SCF (Skp1, Cullins, F-box proteins) multisubunit E3 ubiquitin ligase, also known as CRL (Cullin-RING ubiquitin Ligase) is the largest E3 ubiquitin ligase family that promotes the ubiquitination of various regulatory proteins for targeted degradation, thus regulating many biological processes, including cell cycle progression, signal transduction, and DNA replication[2].
The vast majority of p53-regulated genes are induced in response to various stress signals and are responsible for maintaining genetic stability, DNA repair, regulation of crucial cell-cycle check points, and finally induction of apoptosis. The activity of p53 is tightly controlled by two major negative regulators including murine double minute 2 (MDM2; EC 6.3.2.19) and 4 (MDM4 or MDMX) proteins. Human MDM2 and MDMX are structurally related and contain three well-conserved domains: an N-terminal domain (responsible for p53 binding), a zinc-finger domain (function largely unknown) and a C-terminal RING domain (responsible for formation of homo- and heterodimers). Additionally, the RING domain of MDM2 confers E3 ubiquitin ligase activity. Concentration/activity of p53 is kept at low level in unstressed cells. This is accomplished by three parallel mechanisms mediated by MDM2 and/or MDMX. First, MDM2 and MDMX bind the N-terminal transactivation domain (TAD) of p53, preventing thereby its interaction with the transcription machinery and resulting in the inhibition of p53-responsive gene expression. Second, MDM2/X proteins export p53 outside the nucleus into the cytoplasm where it can no longer activate transcription. Finally, MDM2 marks p53 for proteasomal degradation[3]. Many tumors overproduce MDM2 to impair p53 function. Therefore, restoration of p53 activity by inhibiting the p53–MDM2 binding represents an attractive novel approach to cancer therapy[4].