Comparative studies on the moleular architecture of insitol-trisphosphate receptor and ryanodine receptor and their intracellular distribution

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Both inositol-1,4,5-trisphosphate receptor (IP_3R) and ryanodine-receptor (RyaR) form ligand-gated Ca^<2+> -channel and play a crucial role in intracellular signal transduction. Though the total amino-acid sequence was already determined for both, histological studies on these receptors still remain insufficient and nothing substantial has been done on their three-dimensional distribution in cells or tissues. On the structure side, the molecular architecture of tetrameric Rya-R particle has been extensively studied either by negative staining or cryo-electron microscopy, leading to the reconstruction of its three-dimensional structure. Despite many efforts similarly done for IP_3-R,however, nobody has successfully obtained, so far, the concrete information on the molecular architecture of that receptor. The author has been applying his expertised technique of quick-freeze deep-etch replica electron microscopy to the studies of various kinds of protein architecture. With this technique, one obtain very conrasty images of macromolecules in solution or in cells/tissues with very high time-and spacial-resolution, with nicely preserved three-dimensional organization. Thus, we could succesfully obtain the images of IP_3R in situ in the dendrites of cerebellar Purkinje cells. We compared them with those of RyaR and were surprized to find that the physical size of IP_3R is by far smaller than the value calculated from the ratio of molecular weights of two receptors and the actual size of RyaR.We also found that tetrameric IP_3R forms a very regular two-dimensional molecular array in situ in the cell, under certain anoxemic conditions. Further studies must be done to understand the physiological role of such array. It is also of great importance to determine the higher-resolution structure of the receptor molecules to define the sites of interacton with variuos ligands.