My group is interested in the structure and dynamics of model and biological membranes, especially upon treatment with toxins or exogeneously added lipids. During the past several years, molecular conformations of several snake cardiotoxins (CTXs) have been determined by us and several other groups. The gross sturctures of these CTXs are found to be similar, they consist of three loops radiating from a central core containing a cluster of four disulfide bonds. The configuration of these loops is characterized by a two-stranded b-sheet near the N-terminal (loop1) and a three-stranded b-sheet next to it (loop 2 and loop 3). Since CTXs with different primary sequences are known to cause depolarization of the muscle cells or fusion of sphingomyelin vesicles within a distint range of concentration, we have focused our current research on the study of the structure/activity relationship of CTXs.
A working hypythesis has been proposed based on the CTX-induced binding and aggregation/fusion activities of zwitlerionic phospholipid dispersions.
In fact, we have established a correlation between the studied activities and the prinary and tertiary structures of CTX at 6-13 and 24-37 amino acid positions located near the tip of loop 1 and loop 2, respectively. These regions are characterized by the coexistence of an hydrophobic surface and a cationic zone. Analogous to the signal sequence for protein import, the region seems well suited for binding to the surface and to the interior of membranes via electrostatic and hydrophobic interactions. Therefore, a combined approach involving site-directed mutegenesis, spectroscopy and electrophysiology is being undertaken to shed light on the interaction of CTXs on membranes.
We are also interested in applying NMR to study the structure and dynamics of model and biological membranes. For instance, we have combined direct surface area measurement of micropipette and relatine bilayer expansion measurement of 13C CP/MAS NMR to provide molecular evidence for explaing the morphological changes of RBC treated with enogeneous phospholipid. We have also determined the conformational changes of cholesterol side chain in lipid bilayers. Our current interest in this research area is to study the dynamics of non freezable water in fully hydrated phospholipid bilayers.