Energetics of Clathrin Basket Assembly

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Abstract
A minimal thermodynamic model is used to study the in vitro equilibrium assembly of reconstituted clathrin baskets. The model contains parameters accounting for i) the combined bending and flexing rigidities of triskelion legs and hubs, ii) the intrinsic curvature of an isolated triskelion, and iii) the free energy changes associated with interactions between legs of neighboring triskelions. Analytical expressions for basket size distributions are derived, and published size distribution data (Zaremba S, Keen JH. J Cell Biol 1983;97: 1339–1347) are then used to provide estimates for net total basket assembly energies. Results suggest that energies involved in adding triskelions to partially formed clathrin lattices are small (of the order of kBT), in accord with the notion that lattice remodeling during basket formation occurs as a result of thermodynamic fluctuations. In addition, analysis of data showing the effects of assembly proteins (APs) on basket size indicates that the binding of APs increases the intrinsic curvature of an elemental triskelial subunit, the stabilizing energy of leg interactions, and the effective leg/hub rigidity. Values of effective triskelial rigidity determined in this investigation are similar to those estimated by previous analysis of shape fluctuations of isolated triskelia.