Figure 7. Model for E2 utilization by Hrd1 and Doa10.

Hrd1 uses Ubc7 for both priming and elongation and selects Ubc7 over Ubc6 for priming through higher binding affinity and optimal stimulation through its arginine linchpin (“affinity‐driven pairing”). Doa10 uses Ubc6 for priming. Although it binds the conjugates of the two E2s with similar affinity, its suboptimal histidine linchpin is ineffective at stimulating Ubc7. In contrast, Doa10 pairing with Ubc6 is highly active due to the high basal activity of Ubc6~Ub that does not require substantial stimulation by a linchpin. We call this mode of E2 selection “rate‐driven pairing”. E3‐mediated E2 stimulation is dispensable during elongation, allowing Doa10 to pair productively with Ubc7. For later elongation steps, Cue1 plays an important role in driving reaction rates although Ub chain elongation is generally not rate‐determining for the kinetics of protein degradation. Finally, both RINGs retain the ability to use the non‐preferred E2 for priming (dashed arrows), possibly providing a “backup mechanism” for priming.