Structure of the osmo-regulated H2O-channel, AQP-CHIP, in projection at 3.5 A resolution.

An osmo-regulated H2O-channel, aquaporin-CHIP, from bovine red blood cell membranes was purified and reconstituted with lipids, forming two-dimensional crystalline patches that diffract to about 3.0 A resolution. Electron diffraction patterns and high-resolution images of the crystalline patches embedded in glucose were recorded and used to calculate the projection map at 3.5 A resolution. The map confirms that the osmo-regulated H2O-channel basic packing unit is a tetramer and begins to reveal it's structural design. The basic architecture of the H2O-channel protein consists of a trapezoid-like envelope and a substructure located within the trapezoid that could play a crucial role in the channel structure itself; near this substructure there is a region of very low density, which is the probable site of the channel. The trapezoid-like envelope is composed of high density regions many of which can be interpreted as projections of alpha-helices along their axes.