The P2X(2) purinergic receptor permeates cationic ions in response to stimulation by ATP and mediates fast synaptic transmission. Here, we purified the M, receptor using baculovirus-Sf9 cell expression system and observed its structure using electron microscopy. The FLAG-tagged M, receptor, which has intact ion channel function, was purified to be a single peak by affinity purification and gel filtration chromatography. It was confirmed to be a trimer by introducing cross-linking. Negatively stained M, protein images were homogeneous and picked up by automated pick-up programs, aligned, and classified using the modified growing neural gas network method. Similarly oriented projections were averaged to decrease the signal-to-noise ratio. These images demonstrate an inverted three-sided pyramid with the dimensions of 215 A in height and 200 A in side length. It is composed of a high-density trunk and a stain-permeable swollen extracellular domain of a crown-shaped structure. The internal cavities and constituent segments were clearly demonstrated in both the raw images and the averaged images. The threefold symmetrical top view demonstrates the first visual evidence of the trimeric composition of the P2X receptor family. (c) 2005 Elsevier Inc. All rights reserved.