The binding of Actinia equina L. toxin (EqtII) to membranes
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| Figure 1: Structure of EqtII determined by NMR spectroscopy (PDB entry 1KD6). |
There is much interest in understanding how proteins are able to recognize and target specific biological membranes. For example, sea anemones produce a range of cytolysins which are cytotoxic peptides potentially lethal to vertebrates. An important class of these cytolysins are the actinoporins. Actinoporins, such as the toxin EqtII from Actinia equina L, bind to membranes and then, depending on the composition of the membranes in the target cells, cooperatively assemble into transmembrane pores. In vitro studies show that EqtII has a specific requirement for sphingomyelin. EqtII will bind to a range of model membranes containing phosphatidylcholines but only form pores in the presence of sphingomyeline. In addition, pore formation requires an oligomerization of the toxin on the membrane surface, most probably forming tetramers.
The structure of EqtII has been determined by both NMR spectroscopy and X-ray crystallography (PDB entries 1KD6 and 1IAZ, respectively). It consists of a β-sandwich fold flanked by two α-helices, one on each side (see figure 1), the C-terminal helix is more labile than the N-terminal one. EqtII has attracted much attention in recent years but despite extensive experimental investigations into the nature of its interactions with membranes, little is know in regard to how this toxin induces transmembrane pores. What is known is that certain regions, namely the N-terminal α-helix and some aromatic residues (mainly tryptophanes), play a central role in the interaction of the toxin with membranes such as dipalmitoylphospatidylcholines (DPPC) and DPPC/sphingomyeline bilayers.
To shed more light on this system, Dr David Poger and Professor Alan Mark of the Molecular Dynamics group at UQ are using atomistic molecular dynamics simulations to study in detail the interaction between EqtII and various models of biological membranes and for comparison with the available experimental data. By simulating the interaction of EqtII with membranes containing various concentrations of sphingomyeline they hope to
- observe the toxin in the initial stages of binding and assembly into a pore;
- determine the role of sphingomyeline in binding and pore formation; and
- determine the role of specific amino acids in binding.
The Molecular Dynamics group at relatively new to UQ. The group focuses on predicting the macroscopic (experimentally observable) properties of biomolecular systems such as proteins, nucleic acids and lipid aggregates, in terms of the interactions between atoms. They primarily use and contribute to the molecular dynamics simulation packages GROMACS and GROMOS.
Contacts
Dr David Poger,
Professor Alan E. Mark
Molecular
Dynamics Group, School of Molecular and Microbial Sciences, UQ
Written by Dr D. Poger, September 2006.