COLL 40 |
| Ralph Nossal, Laboratory of Integrative and Medical Biophysics (LIMB), Laboratory of Integrative and Medical Biophysics (LIMB), National Institute of Child Health and Human Development, NIH, Bethesda, MD 20892 |
| The generation of vesicles involved in receptor-mediated endocytosis at a cell plama membrane is mediated by the binding of specific “coat proteins,” the principal component being a heteropolymer known as clathrin which appears in isolated form as a macromolecular complex having the shape of a three-legged pinwheel. Under proper conditions the triskelions spontaneously assemble in vitro into closed polyhedrons that have topological similarity to the seams of soccer balls (like certain carbon nanostructures). We use a simple thermodynamic model to demonstrate how the size distribution of reconstituted clathrin baskets is linked to parameters representing inter-leg association energies and terms related to unfavorable distortion energies. The latter are depicted by the intrinsic curvature and elastic rigidity of individual triskelions, which affect the size of the baskets and their polydispersity. We also show how these factors can regulate assembly through their influence on the critical clathrin concentration Ccr, below which baskets do not form. Comparison between theory and existing data allows us to infer the mechanical role of clathrin-associated “assembly proteins (APs)” in mediating the formation of baskets. Along with results from related analysis of shape fluctuations of individual triskelions, estimates of values of effective elastic moduli lead us to conclude that a principal effect of the binding of APs to clathrin is to prevent legs of neighboring triskelions from sliding with respect to each other when they form the struts of a completed basket. We also show how optical fluctuation methods, such as dynamic light scattering (DLS) and fluorescence correlation spectroscopy (FCS), can be used to illuminate aspects of basket assembly.
|
|
Bio-Colloids
2:00 PM-4:30 PM, Sunday, March 28, 2004 Marriott -- Grand Ballroom J, Oral
Division of Colloid and Surface Chemistry |