Atomic force microscopy studies of protein interactions with lipid membranes

The behaviour of biological components in cellular membranes is vital to the function of cells however many vital phenomena associated with membrane functions are not yet fully understood. Supported lipid bilayers provide a model of real cellular membranes. This thesis examines how increasing the complexity of model lipid bilayers through changes in lipid composition, membrane protein content and cytoskeletal interactions can be used to extract significant biological information with biophysical techniques and analysis. The atomic force microscope (AFM) is a powerful tool in the study of biological systems allowing both three dimensional sub-nanometer resolution and mechanical interrogation under physiological conditions. The recent arrival of high-speed atomic force microscopy has transformed the information and processes which can be obtained, enabling direct imaging of biomolecular processes in real time. The work in this thesis shows that the AFM cannot only be used to investigate membranes but also deposit them in situ at lateral scales comparable to their height. Studies of confining lipid and protein diffusion in these quasi-one dimensional systems shows confinement reduces mobility of lipid with important implications on the behaviour of pores and defects cellular membranes. Studies of lipid phase behaviour of compositions thought be simplified models of the cell membrane lipid content show evidence that the small “raft” domains detected in real cells are not stable equilibrium phase separated domains, but non-equilibrium compositional fluctuations. Actin polymerisation induced at positively charged bilayers in non-polymerising conditions provides new insight into polymerisation processes whilst also describing a simple novel method to create “synthetic” robust actin-membrane scaffolds with controllable coverage. This polymerisation process was then applied to coating of lipid microbubbles for combined ultrasound imaging and drug delivery applications. The addition of the actin coating increased bubbles lifetimes, stability, elasticity and stiffness whilst allowing the attachment of model drug carriers.