Figure 2.

Alternative manifestations of kinetic proofreading in the spliceosome. In the case of an optimal substrate (left panel), the rate of the forward on-pathway conversion of substrate (S) to product (P) (k_F) is faster than the rate of DExD/H-box ATPase mediated rejection (k_R). Consequently, ATP hydrolysis by the proofreading DExD/H-box ATPase occurs after the conversion and results in transition to a productive conformation (P') at rate k_P. In the case of a suboptimal substrate (middle and right panels), the rate of rejection effectively competes with the forward rate of conversion to antagonize splicing, resulting in formation of a rejected conformation (S^R). Two non-mutually exclusive models can explain specificity for rejection of suboptimal substrates. In proofreading at the conversion step (middle panel), the rate of rejection may be constant for optimal and suboptimal substrates while the conversion rate (boxed) is sufficiently reduced in the case of a suboptimal substrate such that the rate of rejection is faster than the rate of conversion. In proofreading at the rejection step (right panel), the rate of conversion may be constant for optimal and suboptimal substrates while the rate of rejection (boxed) is enhanced for a suboptimal substrate relative to an optimal substrate, similarly leading to a faster rate of rejection than rate of conversion. Such relative enhancement of the rejection rate for a suboptimal substrate might be achieved by stimulation of rejection by a suboptimal substrate or by repression of rejection by an optimal substrate. The sizes of the arrows reflect the magnitude of their rates. Green arrows represent utilization of the productive pathway; red arrows represent utilization of the rejection pathway; black arrows represent unutilized pathways.