Published online by Cambridge University Press: 01 February 2016
Biological systems routinely produce nanoscopic molecular structures withconsiderably less dispersion in size and shape than encountered in mostmanufactured materials. Indeed, Biological structures are frequently andessentially monodisperse. An example of this uniformity, combined with anintriguing geometry, is the nanometer-scale protein nanorings produced byinteraction of the protein tubulin with certain hydrophobic tri-, tetra- andpentapeptides originally extracted as natural products from marine biosystems.Different peptides produce different sized nanorings, but we focus on thoseproduced by binding to tubulin of the cyclic depsipeptide cryptophycin. Thenanorings that form upon binding of this ligand show a sharp mass distributionindicating that the nanorings are made of 8 tubulin dimers of 100 kDa.
In this submission, we demonstrate how a combination of fluorescence correlationspectroscopy, dynamic light scattering, electron microscopy, analyticalultracentrifugation, small-angle neutron scattering, and modeling is applied toreveal interactions of tubulin and cryptophycin in solution and to characterizetheir structures. We find that the cryptophycin-tubulin nanorings(∼25 nm diameter) are single-walled, appear rigid, are composed of 8tubulin dimers in a single closed ring, and are stable upon dilution tonanomolar concentrations.
Similar studies with a different peptide, the linear pentapeptide dolastatin 10,demonstrated that binding of this peptide to tubulin produces larger nanorings(14 tubulin dimers, ∼45 nm diameter rings), with slightly differentproperties. The ability to adjust the ring size with different peptides, andproduce uniform nanorings with properties that differ slightly between sizeclasses, makes the tubulin-peptide ring structures an appealing structuralsystem.