Presents the Friday Keck Center Teleconference*
Domain Structure of RyR1 channel at Subnanometer Resolution
Irina I. Serysheva, Ph.D.,
Assistant Professor, National Center for macromolecular Imaging,
Baylor College of Medicine
4:00 pm Friday
26th Jan. , 2007
(Refreshments at 3:45)
5.521 Levin Hall
Abstract: The ryanodine receptor (RyR) is the intracellular Ca2+ release channel that mediates ligand-gated release of Ca2+ from the sarcoplasmic reticulum (SR) into the cytoplasm. RyR1 is the predominant type in the skeletal muscle SR membrane where it forms homotetramers with a Mr over 2.2 MDa. The RyR1 cytoplasmic region contains approximately 80% of the protein mass and accommodates multiple regulatory sites for a wide variety of endogenous molecules and pharmacological modifiers. The remaining RyR1 mass accounts for the membrane-spanning region comprising the Ca2+ conduction pathway across the SR membrane. Using single particle electron cryomicroscopy we determined the structure of RyR1 in the closed conformation at a 9.6 \u01fa-resolution. The subnanometer structure of RyR1 offers considerably more structural details than any previously published structures, and for the first time, secondary structure elements can be clearly distinguished. Using a combination of visualization and computational tools, we have unambiguously determined five α helices in the membrane-spanning region, including a bent inner helix and a short pore helix that form the ion-conduction pathway across the membrane. A helix running parallel to the membrane/cytoplasmic interface could conceivably form a part of a gating mechanism of the channel. Comparison of the arrangement of the pore-forming helices in RyR1 with known channel structures reveals a striking similarity to the open MthK channel, although the RyR1 structure is determined under conditions (Ca2+ < 10 nM) favoring its closed conformation. This observation suggests that bending of the inner helix is not essential for RyR1 opening. The cytoplasmic region architecture comprises multiple structural domains with well defined boundaries at subnanometer resolution. Secondary structure elements were clearly identified in several domains including several α-helices and beta-sheets in the clamp-shaped regions at the four corners of the cytoplasmic assembly. Several other alpha-helices with various inclination angles to the membrane plane were found in the stem region and could be involved in maintaining the connection between the membrane-spanning and the cytoplasmic regions. Using sequence-based fold recognition, we found that residues 12-207 and 216-565 of the N-terminal region of RyR1 have significant structural similarity with the ligand binding suppressor domain (1XZZ) and the IP3-binding core region (1N4K) of the IP3R1 channel, respectively. Molecular models for both aforesaid RyR1 sequences were constructed using iterative comparative protein structure modeling combined with fitting into cryoEM density map. Pseudo-atomic models of the N-terminal region of RyR1 were computationally localized to adjacent areas within the clamp-shaped region of the 9.6 Å-structure of RyR1. While the molecular function of the N-terminal portion of RyR1 remains obscure, the presented structural analysis sheds light onto the structure-function elucidation of the receptor and its relationship to the closely related IP3R channel. ( http://ncmi.bcm.tmc.edu/homes/irina )
