Bridging the gap between structural bioinformatics and receptor research: the membrane-embedded, ligand-gated, P2X glycoprotein receptor.

Motivation: No details on P2X receptor architecture had been known at the atomic resolution level. Using comparative homology-based molecular modelling and threading, it was attempted to predict the three-dimensional structure of P2X receptors. This prediction could not be carried out, however, because important properties of the P2X family differ considerably from that of the potential template proteins. This paper reviews an alternative approach consisting of three research fields: bioinformatics, structural modelling, and a variety of the results of biological experiments.

Model: Starting point is the amino acid sequence. Using the sequential data, the first step is a secondary structure prediction. The resulting secondary structure is converted into a three-dimensional geometry. Then, the secondary and tertiary structures are optimized by using the quantum chemistry RHF/3-21G minimal basic set and the all-atom molecular mechanics AMBER96 force field. The fold of the membrane-embedded protein is simulated by a suitable dielectricum. The structure is refined using a conjugate gradient minimizer (Fletcher-Reeves modification of the Polak-Ribiere method). The results of the geometry optimization were checked by a Ramanchandran plot, rotamer analysis, all-atom contact dots, and the C(beta) deviation. As additional tools for the model building, multiple alignment analysis and comparative sequence-function analysis were used. The approach is exemplified on the membrane-embedded, ligand-gated P2X3 receptor subunit, a monovalent-bivalent cation channel-forming glycoprotein that is activated by extracellular adenosine 5'-triphosphate. From these results, a topology of the pore-forming motif of the P2X3 receptor subunit was proposed. It is believed that a fully functional P2X channel requires a precise coupling between (i) two distinct peptide modules, an extracellularly occurring ATP-binding module and a pore module that includes a long transmembrane and short intracellular part, (ii) an interaction surface with membranes, and (iii) hydrogen bonding forces of the residues and hydrated cations. Furthermore, this paper demonstrates the role of quantitative structure-activity relationships (QSARs) in P2X research (calcium ion permeability of the wild-type and after site-directed mutagenesis of the rat P2X2 receptor protein, KN-62 analogs as competitive antagonists of the human P2X7 receptor). EXPERIMENTAL PROOFS: The predictions are experimentally testable and may provide an additional interpretation of experimental observations published in literature. In particular, there is the good agreement of the geometry optimized P2X3 structure with experimentally proposed P2X receptor models obtained by neurophysiological, biochemical, pharmacological, and mutation experiments. Although the rat P2X3 receptor subunit is more complex (397 amino acids) than the KcsA protein (160 amino acids), the overall folds of the peptide backbone atoms are similar.

Limitations: To avoid semantic confusion, it should be noted that "prediction" is defined in a probabilistic sense. Matches to generic rules do not mean "this is true" but rather "this might be true". Only biological and chemical knowledge can determine whether or not these predictions are meaningful. Thus, the results from the computational tools are probabilistic predictions and subject to further experimental verification.

Availability: The geometry optimized P2X3 receptor subunit is freely available for academic researchers on e-mail request (PDB format).