From the red cell to the nucleus: Mechanics and architecture of composite membrane systems

Abstract

Red cells and nuclei are both composite membrane systems: a lipid bilayer or bilayers supported by an underlying membrane skeleton with architectural proteins which connect the lipid membrane to the skeleton. The mechanical properties of red cell bilayers and membrane skeletons have been determined, but the roles of some membrane proteins in red cell architecture are still unknown. In this thesis, numerous biophysical and fluorescence techniques are used to show protein interactions of CD47 and Rh proteins on the red cell surface. Measures of a significant immobile fraction of CD47 suggest protein interactions of CD47 with the red cell membrane skeleton. Further studies suggest this connection is mediated by the cytoplasmic protein 4.2 and is independent of Rh. Biophysical techniques and theories developed for red cells can be applied to nuclear composite membrane systems. Mobility measurements show free diffusion of nuclear pore complexes in yeast, which lack a membrane skeleton. In contrast, metazoan pore complexes are known to be immobile, indicating an important architectural role of the nuclear membrane skeleton in eukaryotic evolution. The nuclear membrane skeleton, known as the lamina network, is known to provide organization and mechanical support to metazoan nuclei. Here, the elasticity of the lamina network is determined by micropipette aspiration to be orders of magnitude stiffer than the red cell membrane skeleton in a model system. Nuclei are able to expand drastically while maintaining their mechanical integrity. Similar analyses with somatic nuclei showed a rigid three-dimensional mechanical character, but the lamina network still provided significant mechanical character