Curvature and Composition Regulation in Tubular Lipid Membranes: A Biophysical Investigation

University Of Pennsylvania, Philadelphia PA

Investigators

Abstract

Remarkable progress has recently been made in uncovering phenomena that are related to the shape of lipid and lipid/protein membranes. Biological science has recognized the importance of membrane curvature: rather than being a passive matrix that is shaped by external factors, its active participation in processes as important as membrane sorting and trafficking has been realized. Findings from theoretical membrane physics and cell membrane biology have generated an urge to fill the gap between quantitative theories (with insufficient experimental realization) and qualitative descriptions (with missing biophysical measurements). The current impasse is particularly evident in research centered on a recently discovered, potentially ubiquitously important, curvature generating and sensing module, the BAR domain. This banana shaped protein domain is qualitatively observed to cause membranes to assume the shape of cylindrical tubes with varying curvature, but with incompletely understood mechanism and absence of elucidation of the mechanics involved. These and other types of isolated modules will serve as a starting point for systematic investigation of membrane shaping factors. A hiatus of membrane curvature research has been the focus on microscopically resolvable curvature radii of multiple micron sized (giant) vesicles. Technically significantly more challenging (and hence under-explored), but relevant for biological membrane curvatures, is the investigation of mechanical aspects of membrane tubes, where high precision measurement of nanometer scale curvature can be conveniently combined with the measurement of mechanical force/moment equilibria modulated by lipid/protein interactions. Membrane tubes pulled from giant vesicles represent a mechanically controllable membrane system with a steep, precisely adjustable curvature gradient. These tubes will allow addressing a second fundamental problem where theoretical and biological models have not been sufficiently connected through quantitative data. The question to be explored regards to what extent, and based on which molecular details, membrane components are sorted in membrane curvature gradients, to clarify how this biophysical sorting could be involved in intracellular membrane sorting and trafficking. Beginning with curvature dependent lateral sorting of fluorescent lipids, and extending to peripheral and transmembrane protein sorting, the PI will quantify aspects of curvature sorting and characterize and develop curvature sensors that may be used in the cellular context. Synergistically coupled to the PI's research objectives is the involvement and education particularly of undergraduate level students in research projects, enriched by research opportunities for high school students, including minority groups, and teachers, an activity which is harmoniously integrated into long established outreach programs of the NSF funded UPenn MRSEC center, as well as an ACS funded project SEED. Further educational objectives include the integration of the teaching of science ethics into classes directed towards undergraduate and graduate students. This aspect will include discussion of scientific misconduct examples that received media coverage, as well as common gray area cases like opportunities and boundaries of image processing techniques. Furthermore, the PI will integrate methods to use the scientific software Matlab for illustrating both statistical concepts, as well as fundamental aspects of image analysis in teaching and research.