IRFP: Bacterial Paleophysiology: Investigating The Physiology of Hopanoids and the Implications of Their Legacy in the Geologic Record

Saenz James P, Woods Hole MA

Investigators

Abstract

The International Research Fellowship Program enables U.S. scientists and engineers to conduct nine to twenty-four months of research abroad. The program's awards provide opportunities for joint research, and the use of unique or complementary facilities, expertise and experimental conditions abroad. This award is co-funded by the Office of International Science and Engineering and by the Division of Molecular and Cellular Biology's Cellular Processes Program and Networks and Regulation Program. This award will support a twenty-four-month research fellowship by Dr. James P. Sáenz to work with Dr. Kai Simons at the Max Planck Institute of Molecular Cell Biology and Genetics in Dresden, Germany. The cell membrane provides a dual role as a barrier between life and death and as a platform for orchestrating biochemical activity. In order for the membrane to coordinate the multitude of biochemical reactions that are necessary for life, there must be a mechanism for subcompartmentalizing and trafficking the bioactive components of the cell membrane. The lipid raft hypothesis holds that the cell membrane is not homogenous, but that it consists of coexisting immiscible liquid phases that can self-organize into lipid rafts, or lipid domains. Such rafts are thought to serve as platforms that are essential in orchestrating signal transduction, cell polarization, and protein trafficking. In eukaryotes, sterols play an essential role in the formation of a liquid-ordered (lo) "raft" phase, which is immiscible with the surrounding phospholipids in a liquid-disordered phase (ld). While it is likely the ability to coordinate biochemical activity through membrane subcompartmentalization is essential to all life, the presence of lipid rafts and their molecular basis is unknown in the Bacterial domain. Hopanoids are membrane lipids produced by bacteria that have been dubbed bacterial "sterol surrogates." Understanding the physiology of hopanoids lies at the heart of two long-standing riddles in biology and Earth history: What function do hopanoids serve in bacteria, and what does their presence in rocks deposited more than 2.5 billion years ago tell us about early life on Earth? The work I am conducting seeks to understand how hopanoids behave in membranes at the molecular level through the application of biophysical and physiological methodologies that have successfully provided significant insight into the role of sterols. Data currently emerging from my work suggests that hopanoids can serve a similar role to sterols in the formation of liquid ordered "raft" domains. The identification of lipid rafts in bacteria and the discovery of hopanoids as an agent of membrane lipid partitioning would alter our understanding of prokaryotic physiology and provide a new model for understanding membrane subcompartmentalization. This work will actively promote the exchange of ideas and expertise across research fields and bridge geographical barriers between working groups; both of these activities are essential for promoting innovation in science. The results stemming from this work are expected to attract interest from microbial biologists, Earth scientists, and membrane physiologists world-wide. The scientific impact of the proposed work will be high due to the present strong interest in discovering the role of hopanoids in bacteria and their novel implications for understanding the evolution and physiology of membrane subcompartmentalization.