No surface binding or secondary nucleation takes place in this model. In addition to elongation, Model B also allows monomers to bind to the surface when no disordered flanks are present. In Model C, surface binding enables secondary nucleation and subsequent growth. After every proteinase K degradation step new nucleation-viable binding sites become available. Model D assumes that protofilaments can be proteolytically fragmented by Proteinase K degradation. Formal descriptions of the models are provided in the SI.