Caught in the act

Abstract

The crystal structure of a HECT E3 enzyme has been captured as it transfers ubiquitin to a target protein, revealing the dramatic changes in shape that enable it to modify particular residues in its targets.

Keywords: E2 conjugating enzyme; E3 ligase; HECT; NEDD4; Rsp5; S. cerevisiae; ubiquitin.

Figure 1.. Three enzymes are needed to activate ubiquitin and attach it to a target protein.

(A) Ubiquitin (green) is activated by an E1 enzyme and becomes covalently linked to a cysteine residue in the active site of the enzyme (all active sites are marked in yellow). The ubiquitin is then transferred to a cysteine residue in the active site of an E2 enzyme, which delivers it to a cysteine residue in the active site of the E3 enzyme’s HECT domain. In each case ubiquitin is attached to the enzyme by a thioester bond. The E3 then catalyses the transfer of the ubiquitin to a target protein (blue; labelled ‘tar’). (B) Kamadurai et al. have studied the last stage of this process in detail for an E3 enzyme called Rsp5. The HECT domain of the E3 enzyme is composed of an N-lobe (which binds the E2 enzyme) and a C-lobe, which contains the active site (yellow) and also a non-covalent binding site that stabilizes the ubiquitin. The target protein is detected by Rsp5 through an additional domain called the WW3 domain (top panel). The C-lobe then undergoes a major rearrangement that leads to the formation of a composite active site comprising residues of both N- and C-lobes. This rearrangement also places the active site in proximity to the target protein (second panel), which helps the enzyme to select specific lysine residues in the target protein for modification (third panel). The ubiquitylated target protein is then released (bottom panel).