Protein filaments and actin polymerization

The processes of polymerization of protein monomers into filaments, such as actin and tubulin, can be investigated based on a simple kinetic model. To develop a model, we let Ai represent the monomer and At represent the filaments made up of l subunits. Our simple kinetic model has two distinct steps  

4 In the twisting double-helical actin chain, the nucleation step involves only one monomer-monomer interaction while each monomer placed on the chain following nucleation involves two monomer-monomer bonds. In the helix-coil theory, the formation of two consecutive helical states represents the nucleation step that allows for a third helical state and hydrogen bond formation. 

---