Measuring rates of ubiquitin chain formation as a functional readout of ligase activity.

Specificity within the pathways of ubiquitin conjugation are defined by protein-binding affinities among the components. Enzyme kinetics provides a facile high-resolution experimental approach for quantitating such protein-binding affinities and yields additional mechanistic insights into the transition state of the enzyme-catalyzed reaction. Most ubiquitin ligases form free polyubiquitin chains at a slow rate in the absence of their cognate target protein as a normal step in their overall catalytic cycle.

Regulating the regulator: Rsp5 ubiquitinates the proteasome.

In this issue of Molecular Cell, Isasa et al. (2010) show that the Rsp5 ubiquitin ligase regulates substrate recruitment to the 26S proteasome by ubiquitinating Rpn10, the proteasome's polyubiquitin degradation signal receptor.