Figure 2.

Structures of IP₃R and other tetrameric cation channels. (A and B) Surface-rendered representations of the 3D map of IP₃R viewed along the fourfold axis from the ER lumen (a); viewed along the fourfold axis from the cytoplasm (b); viewed orthogonal to the membrane plane (c); and viewed as in c but sectioned to remove one cytoplasmic petal domain to reveal the cytoplasmic stigma domain (d). In B the dimensions of the 3D map are indicated and the different domains of IP₃R are identified by the following colors: blue, channel domain; pink, cytoplasmic stigma domain; and gray, cytoplasmic petal domains. In Bd the filled arrowhead marks one of the four slender links between the transmembrane domain and the petal subdomain, whereas a corresponding slender link between a petal subdomain and the stigma subdomain is marked with an open arrow. (C) Amino acid sequence of an IP₃R subunit represented linearly (a) and drawn to reveal its overall domain distribution (b). Proposed domains are color-coded as follows: the cytoplasmic stigma domain (pink) containing the IP₃-binding pocket (dark pink), cytoplasmic petal domain (gray), channel domain (blue), and its transmembrane segments (dark blue), and regions of low complexity corresponding to domain boundaries (orange). (D) Comparison of the 3D map of IP₃R, viewed from the ER lumen (Upper) and orthogonal to the membrane plane (Lower), with other cation channels with known 3D structures. (a) Cartoon of the type 1 RyR (39). (b) IP₃R 3D map. (c) Cartoon of the Shaker channel (40). (d) Cartoon of the MthK channel (41).