STRUCTURE OF THE OSMO-REGULATED H2O-CHANNEL, AQP-CHIP, IN PROJECTION AT 3.5 ANGSTROM RESOLUTION

An osmo-regulated H2O-channel, aquaporin-CHIP, from bovine red blood c ell membranes was purified and reconstituted with lipids, forming two- dimensional crystalline patches that diffract to about 3.0 Angstrom re solution. 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 Angstrom resolution. The map confi rms that the osmoregulated H2O-channel basic packing unit is a tetrame r 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 r ole 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 the ir axes.