title: The symmetrical structure of structural maintenance of chromosomes (SMC) and MukB proteins: long, antiparallel coiled coils, folded at a flexible hinge.
abstract: Structural maintenance of chromosomes (SMC) proteins function in chromosome condensation and several other aspects of DNA processing. They are large proteins characterized by an NH2-terminal nucleotide triphosphate (NTP)-binding domain, two long segments of coiled coil separated by a hinge, and a COOH-terminal domain. Here, we have visualized by EM the SMC protein from Bacillus subtilis (BsSMC) and MukB from Escherichia coli, which we argue is a divergent SMC protein. Both BsSMC and MukB show two thin rods with globular domains at the ends emerging from the hinge. The hinge appears to be quite flexible: the arms can open up to 180 degrees, separating the terminal domains by 100 nm, or close to near 0 degrees, bringing the terminal globular domains together. A surprising observation is that the approximately 300-amino acid-long coiled coils are in an antiparallel arrangement. Known coiled coils are almost all parallel, and the longest antiparallel coiled coils known previously are 35-45 amino acids long. This antiparallel arrangement produces a symmetrical molecule with both an NH2- and a COOH-terminal domain at each end. The SMC molecule therefore has two complete and identical functional domains at the ends of the long arms. The bifunctional symmetry and a possible scissoring action at the hinge should provide unique biomechanical properties to the SMC proteins.
authors.affiliation: Department of Cell Biology, Duke University Medical Center, Durham, North Carolina 27710-3011, USA.
authors.forename: T E
authors.initials: TE
authors.lastname: Melby
authors.forename: C N
authors.initials: CN
authors.lastname: Ciampaglio
authors.forename: G
authors.initials: G
authors.lastname: Briscoe
authors.forename: H P
authors.initials: HP
authors.lastname: Erickson
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry / Q000648:ultrastructure*; D002868:Chromosomal Proteins, Non-Histone; D002876:Chromosomes, Bacterial / Q000737:chemistry / Q000648:ultrastructure*; D029968:Escherichia coli Proteins; D009687:Nuclear Proteins / Q000737:chemistry / Q000648:ultrastructure; D017510:Protein Folding
chemical_list: D001426:Bacterial Proteins; D002868:Chromosomal Proteins, Non-Histone; D029968:Escherichia coli Proteins; C067035:MukB protein, E coli; D009687:Nuclear Proteins