The human red cell membrane is the model upon which our general understanding of plasma membranes is based. Several membrane transport proteins have first been identified in red cells, and during previous years of this project, a 28kDa protein was discovered, purified, cloned, expressed and functionally defined in Dr. Agre's laboratory. Now designated AQP1, this protein is the first recognized membrane water transport molecule. While multiple homologous aquaporins are now becoming recognized in other tissues, physical studies of human red cell AQP1 are revealing its structure at subnanometer resolution and are providing advanced insight into the biophysical transport function of the molecule. Here Dr. Agre proposes studies to establish the molecular structure of AQP1 at near-atomic resolution and to define the behavior of AQP1 in membranes.
Aim I. Structure and function of purified AQP1 protein. High resolution cryoelectron microscopic analysis of membrane crystals containing red cell AQP1 will be undertaken to elucidate the 3D structure of AQP1 at better than 3 resolution. Yeast and other heterologous systems will be developed to express mutagenized AQP1 molecules with specific epitopes for affinity-purifications, metal binding, definition of the aqueous pore, identification of the sites of ion repulsion and assembly of individual subunits into tetramers. Functional analysis of mutagenized forms of AQP1 will be determined by direct measurement of the water permeability of AQP1 proteins in yeast microsomal vesicles and in reconstituted proteoliposomes.
Aim II. AQP1 protein in cell membranes. The Colton blood group antigens result from a polymorphism in the first extracellular loop of the AQP1 protein. Using fluorescently labeled-anti-Co, Dr. Agre plans to characterize the surface equilibrium distributions of AQP1 on normal red cells by immunofluorescence, immunoprecipitations, and flow sorting. Kinetic distributions of AQP1 in red cell membranes will be undertaken with anti-Co by measurement of fluorescence recovery after photobleaching. Similar studies will be performed on enzymatically modified red cells and red cells from patients with sickle cell anemia and other congenital hemolytic states. These abnormal red cells and AQP1 deficient red cells will also be examined for membrane water permeabilities. To fully define the molecular determinants of the Co antigen, nonerythroid cells will be studied for Co expression by transfection with mutagenized forms of AQP1.
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