The group imagined that additional binding sites might make Sic1 more likely to rebind at another site than to diffuse away from Cdc4. “It's like seaweed floating in the waves,” says Klein. “Pieces get stuck on a rock. A wave might knock it off, but before it floats away, another arm can get stuck.” Their model shows that the affinity of Cdc4 for Sic1 grows exponentially with each Sic1 phosphorylation, an effect they call allovalency. As long as diffusion is sufficiently slow and rebinding is rapid enough, the model is possible kinetically when biophysically realistic numbers are considered.
Extension of this model is possibly limited by the critical assumption that each binding site must be able to move independently, as is the case for the unstructured Sic1. Lectins and their long-chain carbohydrate ligands might also fit this sort of model. ▪