The Conformation of Clathrin Triskelia in Solution

A principal component in the protein coat of certain post-golgi and endocytic vesicles is clathrin, a three-legged heteropolymer (known as a triskelion) that assembles into polyhedral cages principally made up of pentagonal and hexagonal faces. In vitro, this assembly depends on the pH, with cages forming more readily at low pH and less readily at high pH. We have developed procedures, based on static and dynamic light scattering, to determine the radius of gyration, R$_{g}$, and hydrodynamic radius, R$_{H,}$ of isolated triskelia, under conditions where cage assembly occurs. Calculations based on rigid molecular bead models of a clathrin triskelion show that the measured values can be accounted for by bending of the legs and a puckering at the vertex. We also show that the values of R$_{g}$ and R$_{H}$ measured for clathrin triskelia in solution are qualitatively consistent with the conformation of clathrin in a ``D$_{6}$ barrel'' cage assembly measured by cryoEM tomography.