Department of Chemical Engineering
Advisor: Atul N. Parikh
9:00 AM, Wednesday, August 24, 2016
1003 Kemper Hall
Equilibrium properties of cellular and amphiphilic membranes, which delimit every living cell, have been extensively studied and are now largely well understood. However, as boundary layers membranes are often driven out of thermodynamic equilibrium in both biological and synthetic settings, because of their interactions with the surrounding milieu. The latter includes a diverse variety of external cues, fields, and triggers, including substrate-adhesion, real-time addition (or removal) of membrane components, and mechanical perturbations. The manners in which membranes respond to this diverse and complex array of external triggers is only beginning to be examined. Such triggers can be of both physical and chemical natures and include substrate induced, concentration gradient induced in the form of osmotic assault, and that of interface interacting and partitioning molecules capable of inducing membrane reorganizations and deformations. Therefore, the full significance that many of these triggers play in the context of biology is a largely unexplored area of research, which underscores the importance of studying and understanding membranes taken out of thermodynamic equilibrium.
In this talk I focus on two examples of non-equilibrium responses of membranes. One is caused by membrane binding of a peptide and another by a real-time insertion of shape imparting molecules within the membrane. An important example in the search for viral therapeutics is the a-helical (AH) domain of the hepatitis C virus nonstructural protein NS5A, which is anchored at the cytoplasmic leaflet of the endoplasmic reticulum, and plays a role in viral replication. However, the peptides derived from this domain also exhibit remarkably broad-spectrum virocidal activity, raising questions about their modes of membrane association. Here, using giant lipid vesicles, we show that the AH peptide discriminates between membrane compositions. In cholesterol-containing membranes, peptide binding induces microdomain formation. By contrast, cholesterol-depleted membranes undergo global softening at elevated peptide concentrations. Furthermore, in mixed populations, the presence of ~100 nm vesicles of viral dimensions suppresses these peptide-induced perturbations in giant unilamellar vesicles, suggesting size-dependent membrane association. These synergistic composition- and size-dependent interactions explain, in part, how the AH domain might on the one hand segregate molecules needed for viral assembly and on the other hand furnish peptides that exhibit broad-spectrum virocidal activity.
Additionally, I examined the membrane activity of a novel class of asymmetric PEGylated lipid molecules. These wedge shaped molecules are capable of producing phase separation and subsequent phase specific curvature generation in multicomponent lipid membranes that are initially optically homogeneous. This demonstrates enhancement in shape-curvature coupling that’s been previously shown to be weak. In each of these cases external perturbations trigger molecular level reorganizations and or mesoscopic symmetry breaking, that produces novel and dynamic membrane shape transformations.