We have continued to use physical and mathematical methods to examine phenomena underlying cell physiologic behaviors. Special attention has been given to determining physical mechanisms of vesicle formation, particularly as related to receptor- mediated endocytosis which is a major process by which eukaryotic cells take up materials from the extracellular milieu. In particular we examined the electric charge imbalance--arising, e.g., from phosphorylation of phosphatidylinositides--necessary to bend the plasma membrane into shapes having the curvatures of endocytic vesicles. We also examined the energetics of protein coat formation, specifically as involving clathrin triskelions. By analyzing published data on size distributions of reconstituted clathrin baskets, we were able to infer values of bending energies for clathrin arms as well as values of the attractive energies associated with arm interactions. We also completed an analysis of an in vivo assay for determining parameters associated with the polymerization of the cortical actin matrix, as manifested through the rigidity of a cell surface. Results were applied to data on the stiffening of PMN neutrophils when the latter are bathed in fMLP and other chemotaxins, from which we were able to infer a linear relationship between the extent of F-actin polymerization and the logarithm of the chemoattractant concentration. We also have used small angle neutron scattering to measure the Q-dependent cross section of microtubules in solution, and have worked out methods for inverting scattering data to determine parameters of microtubule structure. These methods have been used to characterize transformations of S-peptide conformation in taxol stabilized microtubules brought about by variations in solution pH.
