Rigidity of triskelion arms and clathrin nets

Statistical analysis is applied to a set of electron micrographic images (K ocsis, E,, B. L. Trus, C, J. Steer, M. E, Bisher, and A, C. Steven. 1991. J . Struct. Biol. 107:6-14), from which quantitative measures are obtained to support the notion that the three arms of a triskelion have statistically identical properties and exhibit independent structural fluctuations. addit ionally, a study of local contour fluctuations, which indicates that the el astic properties of a triskelion arm are approximately constant over the en tire arm length, is used along with a small deformation statistical mechani cs theory to derive an:effective, average flexural rigidity for the arms. T his result is used to estimate the bending energy necessary to deform a cla thrin patch, and comparison is made with the deformation energy of an equiv alent area of non-clathrin-coated membrane. We estimate that the rigidity o f the clathrin lattice is at least comparable to that of a typical membrane . Hence, the natural curvature of a clathrin cage can stabilize, and perhap s propel, the formation of intracellular coated vesicles.