Biological systems routinely produce nanoscopic molecular structures with
considerably less dispersion in size and shape than encountered in most
manufactured materials. Indeed, Biological structures are frequently and
essentially monodisperse. An example of this uniformity, combined with an
intriguing geometry, is the nanometer-scale protein nanorings produced by
interaction of the protein tubulin with certain hydrophobic tri-, tetra- and
pentapeptides originally extracted as natural products from marine biosystems.
Different peptides produce different sized nanorings, but we focus on those
produced by binding to tubulin of the cyclic depsipeptide cryptophycin. The
nanorings that form upon binding of this ligand show a sharp mass distribution
indicating that the nanorings are made of 8 tubulin dimers of 100 kDa.
In this submission, we demonstrate how a combination of fluorescence correlation
spectroscopy, dynamic light scattering, electron microscopy, analytical
ultracentrifugation, small-angle neutron scattering, and modeling is applied to
reveal interactions of tubulin and cryptophycin in solution and to characterize
their structures. We find that the cryptophycin-tubulin nanorings
(∼25 nm diameter) are single-walled, appear rigid, are composed of 8
tubulin dimers in a single closed ring, and are stable upon dilution to
nanomolar 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 different
properties. The ability to adjust the ring size with different peptides, and
produce uniform nanorings with properties that differ slightly between size
classes, makes the tubulin-peptide ring structures an appealing structural
system.