NATIVE STRUCTURE AND ARRANGEMENT OF INOSITOL-1,4,5-TRISPHOSPHATE RECEPTOR MOLECULES IN BOVINE CEREBELLAR PURKINJE-CELLS AS STUDIED BY QUICK-FREEZE DEEP-ETCH ELECTRON MICROSCOPY

We used quick-freeze deep-etch replica electron microscopy to visualiz e the native structure of inositol-1,4,5-trisphosphate receptor (IP(3) R) in the cell. In the dendrites of Purkinje neurons of bovine cerebel lum there were many vesicular organelles whose surfaces were covered w ith a two-dimensional crystalline array of molecules, Detailed examina tion of the cytoplasmic true surface of such vesicles in replica revea led that the structural unit, identified as IP(3)R by immunocytochemis try and subsequent Fourier analysis, is a square-shaped assembly and i s aligned so that the side of the square is inclined by similar to 20 degrees from the row-line of the lattice, Comparison with the ryanodin e receptor (RyaR), another intracellular Ca2+ channel on the endoplasm ic reticulum, suggested that IP(3)R, unlike RyaR, has a very compact s tructure, potentially reflecting the crucial difference in the functio n of the cytoplasmic portion of the molecule.