Technical: The hollow, spindle-shaped floatation vesicles of aquatic micro-organisms have remarkable physical properties. The 2.0 nm thick proteinaceous wall is impermeable to water but permeable to gas molecules considerably larger than water. Vesicles measuring 75 nm in width and up to 1000 nm in length can withstand as much as 10 atmospheres of hydrostatic pressure. The wall, assembled almost exclusively from repeating units of the highly hydrophobic 7 kDa gas vesicle protein A (gvpA), cannot be solubilized with detergents. While it is difficult to imagine how nature achieves these mechanical, interfacial, and permeability properties, it is also clear, from the strong homology among gvpA's across all organisms, that nature has not found many ways of doing it. Furthermore, the absence of homology with other proteins indicates that the gas vesicle construct is a very special one. The general goal of the project is to understand the structural features of gas vesicles that are responsible for their properties. Two systems will be studied, one adapted to fresh water and the other adapted to highly saline conditions. The work will employ solid state NMR experiments and atomistic modeling. Solid state NMR has the advantage of being able to measure internuclear distances in systems not amenable to either solubilization or crystallization. This information will allow modeling of the complete structure and the interfacial properties of the gas vesicle shell, taking fully into account the periodicity and the asymmetric environment of the vesicle wall. Comparisons between vesicles adapted to fresh water and vesicles adapted to highly saline conditions will help in assigning functional significance to different structural features.
From a practical point of view, the gas vesicles of aquatic micro-organisms are finding direct use in a number of contexts. In particular, they have been used to confer buoyancy in cells that normally do not float, to improve oxygen perfusion in anchorage-dependent cell cultures, and as vaccination vehicles. It is expected that detailed knowledge of gas vesicle structure will allow these direct applications to be fine-tuned with respect to engineering interactions with wild type vesicles or choosing mutant vesicles. In addition, understanding how nature has designed gas vesicles may inspire the design of new materials that make use of the same underlying principles. Of particular interest are the origins of the resistance of the vesicle walls to detergents and proteases, their mechanical strength, their high permeability to gases, and the anticipated extreme dewetting behavior of their inner surface. Education and outreach activities associated with this project will include direct involvement of undergraduates from the Brandeis University Science Posse program during the summer, hands-on activities for students in field trips to campus from underserved high schools in the greater Boston area, and instruction with hands-on activities in the Kroka Expeditions program that serves teens primarily from New Hampshire and Vermont.
