Inositol 1,4,5-trisphophate receptor : localization and regulation

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1. Localization of inositol 1,4,5-trisphosphate receptor (IP3R) was examined by using monoclonal antibodies raised to mouse cerebellar P400 protein. One of the antibodies, 4C11, reacted with a 240-kD protein in endothelial cells, smooth muscle cells, and epidermal keratinocytes of the mouse. Immunogold labeling for the protein was observed almost exclusively in caveolae, small uncoated inpocketings of the plasma membrane.2. Cultured bovine aortic endothelial cells were treated with a membrane-impermeable biotinylating reagent, and membrane proteins extracted with non-ionic detergent were mixed with streptavidin-agarose. The 240-kD protein was recovered by the streptavidin resin, thus proved to be exposed to the cell surface.3. The 240-kD protein was observed along actin filaments in mouse transformed keratinocytes (PAM212) and bovine aortic endothelial cells.By immunogold electron microscopy using rabbit polyclonal antibody to plasmalemmal Ca^<2+>-pump ATPase, the protein was localized to caveolae of various mouse cells including endothelial cells and smooth muscle cells.5. Functions of the 240-kD protein are not known at present, but considering its similarity to IP3R, the protein may be related to Ca^<2+> influx through the plasma membrane. The results obtained in the present experiments implicate that caveolae are engaged in in and out of Ca^<2+> through the plasma membrane.