BIOLOGICALLY-ACTIVE 2-DIMENSIONAL CRYSTALS OF AQUAPORIN CHIP

Plasma membranes of several mammalian tissues are highly permeable to water due to the presence of CHIP, the 28-kDa channel-forming integral protein which is the archetypal member of the aquaporin family of wat er channel proteins. To define its native structure, purified red cell CHIP protein was reconstituted into lipid bilayers at a high protein- to-lipid ratio, and the resulting 3-mum diameter membrane vesicles wer e examined by high resolution electron microscopy. The reconstituted m embranes contained highly ordered two-dimensional crystalline lattices of p422(1) symmetry in which each CHIP tetramer contained a central d epression extending from the outer and inner surfaces of the membrane into the transbilayer domain of the molecule. The reconstituted membra nes also exhibited extremely high osmotic water permeability, P(f) = 0 .472 cm/s, corresponding to the sum of activities of all incorporated CHIP molecules. These studies report the first two-dimensional crystal lization of a biologically active water channel and provide direct evi dence of the structure responsible for its pore-like behavior.