Our research program centers on a critical cellular component: the cell membranes. As a defining feature of a cell and its organelles, the membranes host close to a third of the entire proteome and mediate numerous signaling pathways. We take a multidisciplinary approach to elucidate the key protein-protein, protein-lipid interactions that give rise to the diverse functions of cell membranes. Rational design and chemical synthesis of novel molecular probes is an indispensable part of our research.
1. Synthetic receptors for membrane lipids
The membranes of a cell consist of many different lipids. The exact composition, as well as the distribution of these lipids, can have critical ramifications to biology. For example, phosphatidylserine (PS) is typically confined to the cytosol-facing leaflet of the plasma membrane and its externalization has been recognized as a hallmark event of apoptosis, the programmed cell death pathway. PS externalization has also been reported for the endothelial cells of tumor vasculature. Our group has been interested in developing small molecules that parallel the proteins that nature has evolved for lipid recognition. Specifically we have been exploring the potential of cyclic peptides for the specific recognition of important lipids including PS. Both structure-based design and library screening approaches are used to realize our goals. We believe that synthetic molecules with high lipid selectivity would serve as powerful tools for basic lipid research, as well as biomedical applications such as tumor imaging.
2. Structural determinants of peptide channel formation
Peptides that form cross-membrane channels, like gramicidin A (gA) shown below, hold great promise as novel antibiotics because their membrane toxicity makes bacterial resistance less likely to develop. However, these membrane-embedded structures are usually very hydrophobic and suffer from poor water solubility and lack of bacteria-specificity. We strive to address these problems by understanding the key structural determinants of gA channel formation. The use of strategically designed unnatural amino acids allows us to interrogate gA folding with unprecedented resolution. The new mechanistic understandings make it possible to develop gramicidin mutants that selectively fold into bacterial cells, leading to cell death. The mechanistic understandings of gA channel formation should also provide guidelines for designing membrane embedded structures in general.
3. Fluorinated amino acids in peptide design
Fluorination is becoming a more and more attractive strategy in designing functional proteins and peptides. This is partly due to the fact that fluorine is absent in natural biological systems, which allows the engineered fluorinated proteins/peptides to be conveniently tracked in complex systems like blood serum and live cells, even living organisms. Furthermore, fluorination of a protein, while causing minimal perturbation of sterics, can dramatically change the electronic properties of an amino acid side chain, particularly those of aromatic amino acids. Therefore, fluorinated amino acids serve as ideal probes of aromatic interactions (e.g., cation-pi interactions and pi-pi interactions) in biology. We have been interested in the synthesis of novel fluorinated amino acids and their application to control protein-protein and protein-lipid interactions in membranes. Fundamental research as such often leads to novel functional molecules. For example, incorporating tetrafluorotyrosine into a membrane-lytic toxin affords a "smart" peptide that searches for solid tumors, where the mild acidity turns on its membrane-lytic activity, leading to tumor cell death.