The effect of solution conditions on the conformation of clathrin triskelions

The major component in the protein coat of certain endocytic vesicles is clathrin, a three-legged heteropolymer (known as a ``triskelion'') that assembles into polyhedral cages composed primarily of pentagonal and hexagonal facets. 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 novel techniques to make physical measurements of the clathrin triskelion under conditions where assembly occurs. By sedimentation velocity and laser light scattering, we measure changes in Stokes radius, r$_{H}$ and radius of gyration, r$_{g}$ of the clathrin triskelion as the pH is lowered. Calculations, with the program HYDRO, on a rigid molecular bead model of clathrin show that measured changes may be accounted for by a pH dependent puckering of the arms at the vertex. This is consistent with the idea that a change in clathrin conformation may play a role in clathrin cage assembly.