A principal component in the protein coats of certain post-golgi and endocytic 
vesicles is clathrin, which appears as 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 upon the pH, 
with cages forming more readily at low pH and less readily at high pH. We have 
developed procedures, on the basis of 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 triskelion show that the measured 
values can be accounted for by bending 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 a triskelion in a 
"D6 barrel" cage assembly measured by cryoelectron microscopy.