The mechanism of Nedd4-2 has been quantitatively explored for the first time using biochemically defined kinetic assays examining rates of I-polyubiquitin chain assembly as a functional readout. We demonstrate that Nedd4-2 exhibits broad specificity for E2 paralogs of the Ubc4/5 clade to assemble Lys-linked polyubiquitin chains. Full-length Nedd4-2 catalyzes free I-polyubiquitin chain assembly by hyperbolic Michaelis-Menten kinetics with respect to Ubc5B∼ubiquitin thioester concentration ( = 44 ± 6 nm; = 0.020 ± 0.007 s) and substrate inhibition above 0.5 μm ( = 2.5 ± 1.3 μm) that tends to zero velocity， requiring ordered binding at two functionally distinct E2∼ubiquitin-binding sites. The Ubc5BC85A... More
The mechanism of Nedd4-2 has been quantitatively explored for the first time using biochemically defined kinetic assays examining rates of I-polyubiquitin chain assembly as a functional readout. We demonstrate that Nedd4-2 exhibits broad specificity for E2 paralogs of the Ubc4/5 clade to assemble Lys-linked polyubiquitin chains. Full-length Nedd4-2 catalyzes free I-polyubiquitin chain assembly by hyperbolic Michaelis-Menten kinetics with respect to Ubc5B∼ubiquitin thioester concentration ( = 44 ± 6 nm; = 0.020 ± 0.007 s) and substrate inhibition above 0.5 μm ( = 2.5 ± 1.3 μm) that tends to zero velocity， requiring ordered binding at two functionally distinct E2∼ubiquitin-binding sites. The Ubc5BC85A product analog non-competitively inhibits Nedd4-2 ( = 2.0 ± 0.5 μm)， consistent with the presence of the second E2-binding site. In contrast， the isosteric Ubc5BC85S-ubiquitin oxyester substrate analog exhibits competitive inhibition at the high-affinity Site 1 ( = 720 ± 340 nm) and non-essential activation at the lower-affinity Site 2 ( = 750 ± 260 nm). Additional studies utilizing Ubc5BF62A， defective in binding the canonical E2 site， demonstrate that the cryptic Site 1 is associated with thioester formation， whereas binding at the canonical site (Site 2) is associated with polyubiquitin chain elongation. Finally， previously described Ca-dependent C2 domain-mediated autoinhibition of Nedd4-2 is not observed under our reported experimental conditions. These studies collectively demonstrate that Nedd4-2 catalyzes polyubiquitin chain assembly by an ordered two-step mechanism requiring two dynamically linked E2∼ubiquitin-binding sites analogous to that recently reported for E6AP， the founding member of the Hect ligase family.
