RING Finger Proteins Mediators of Ubiquitin Ligase Activity

ing that a small noncanonical RING finger protein, Rbx1 (also referred to as ROC1 and HRT1), is an essential component of SCF (Skp1/cullin-1/F-box protein) E3 complexes and that a complex containing Rbx1 and Until recently no specific function had been ascribed to Cdc53/cullin-1 (CUL-1) is sufficient to mediate ubiqui-the RING finger beyond a role in dimerization of several tination in vitro. Rbx1 was also determined to be an proteins. Over the last year and a half, however, reports integral component of VHL-containing complexes, which from a number of laboratories studying diverse biologi-have subsequently been shown to have E3 activity (re-cal processes have led to the realization that RING finger viewed in Deshaies, 1999; Tyers and Jorgensen, 2000). proteins play critical roles in mediating the transfer of The activity of the yeast N-end rule E3, Ubr1, was also ubiquitin (Ub) both to heterologous substrates as well dependent on its RING finger (Xie and Varshavsky, as to the RING finger proteins themselves. As RING 1999). c-Cbl was shown to promote the ubiquitination fingers are found in hundreds of proteins, the number of activated receptor protein tyrosine kinases (RPTKs) of candidate Ub protein ligases (E3s) has now increased in a RING finger–dependent manner (Waterman et al., dramatically. 1999 and references therein) and subsequently deter-Protein ubiquitination begins with the formation of a mined to have the intrinsic capacity to ubiquitinate thiol-ester linkage between the C terminus of Ub and RPTKs in vitro in an SH2-and RING finger–dependent the active site cysteine (Cys) of the Ub activating enzyme manner. Additionally, the bacterially expressed c-Cbl (E1). Ub is then transferred to an Ub conjugating enzyme RING alone bound E2s and mediated ubiquitination (Ubc or E2), again through a thiol-ester linkage. E3s, Eukaryotic genomes encode a single or at most a few E1s. Substantially more E2s exist, at least 11 in yeast and over 20 in mammals. The diversity and number of proteins that are regulated by ubiquitination predicts the existence of a large number of E3s. However, until recently relatively few E3s were known. The discovery of E6-AP as an E3 responsible for human papilloma virus E6-dependent ubiquitination of p53 led to the identification of proteins containing an ‫053ف‬ amino acid region of homology to the E6-AP carboxyl terminus (HECT domain), several of which have now been shown to be The first suggestion that RING fingers are associated on human papilloma virus E6 protein and formation …