title: A structure for the signal sequence binding protein SRP54: 3D reconstruction from STEM images of single molecules.
abstract: The 54-kDa subunit SRP54 of the signal recognition particle in eukaryotic cells is responsible for the recognition of nascent proteins destined for secretion or membrane integration. The three-dimensional structure of this protein was determined using computational techniques applied to images of the molecule obtained via high-resolution, low-dose, scanning transmission electron microscopy at low temperature. The reconstructions at spatial resolutions between 12 and 15 A feature two unequal domains joined by a slender linker. The two-domain structure is in agreement with genetic and biochemical data indicating organization of SRP54 into a larger N-terminal GTP-binding region and a smaller C-terminal peptide-binding region. The structure has similarities to other protein domains with related functions and similar amino acid sequences. The larger domain of the 3D reconstruction is consistent in shape and size with the GTP-binding domains of EF-Tu and p21-RAS, while the smaller domain is compatible in structure with part of the peptide-binding protein calmodulin. The overall shape of SRP54 and the deduced location of critical functional regions of the molecule provide a structural framework for its known biochemical properties in the targeting cycle of the signal recognition particle.
authors.affiliation: Ontario Cancer Institute and Department of Medical Biophysics, University of Toronto, Canada.
authors.forename: G J
authors.initials: GJ
authors.lastname: Czarnota
authors.forename: D W
authors.initials: DW
authors.lastname: Andrews
authors.forename: N A
authors.initials: NA
authors.lastname: Farrow
authors.forename: F P
authors.initials: FP
authors.lastname: Ottensmeyer
mesh_terms: D000595:Amino Acid Sequence; D002147:Calmodulin / Q000737:chemistry; D003198:Computer Simulation; D019204:GTP-Binding Proteins / Q000737:chemistry / Q000648:ultrastructure; D017348:Microscopy, Electron, Scanning Transmission / Q000379:methods; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010444:Peptide Elongation Factor Tu / Q000737:chemistry / Q000648:ultrastructure; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D018271:Signal Recognition Particle / Q000737:chemistry* / Q000648:ultrastructure*; D013329:Structure-Activity Relationship
chemical_list: D002147:Calmodulin; D018271:Signal Recognition Particle; D019204:GTP-Binding Proteins; D010444:Peptide Elongation Factor Tu