Predictions Suggesting a Participation of β-Sheet Configuration in the M2 Domain of the P2X 7 Receptor: A Novel Conformation?

Abstract

Scanning experiments have shown that the putative TM2 domain of the P2X 7 receptor (P2X 7R) lines the ionic pore. However, none has identified an α-helix structure, the paradigmatic secondary structure of ion channels in mammalian cells. In addition, some researchers have suggested a β-sheet conformation in the TM2 domain of P2X 2. These data led us to investigate a new architecture within the P2X receptor family. P2X 7R is considered an intriguing receptor because its activation induces nonselective large pore formation, in contrast to the majority of other ionic channel proteins in mammals. This receptor has two states: a low-conductance channel (∼10 pS) and a large pore (>400 pS). To our knowledge, one fundamental question remains unanswered: Are the P2X 7R channel and the pore itself the same entity or are they different structures? There are no structural data to help solve this question. Thus, we investigated the hydrophobic M2 domain with the aim of predicting the fitted position and the secondary structure of the TM2 segment from human P2X 7R (hP2X 7R). We provide evidence for a β-sheet conformation, using bioinformatics algorithms and molecular-dynamics simulation in conjunction with circular dichroism in different environments and Fourier transform infrared spectroscopy. In summary, our study suggests the possibility that a segment composed of residues from part of the M2 domain and part of the putative TM2 segment of P2X 7R is partially folded in a β-sheet conformation, and may play an important role in channel/pore formation associated with P2X 7R activation. It is important to note that most nonselective large pores have a transmembrane β-sheet conformation. Thus, this study may lead to a paradigmatic change in the P2X 7R field and/or raise new questions about this issue.