title: Crystal structure of the translocation ATPase SecA from Thermus thermophilus reveals a parallel, head-to-head dimer.
abstract: The mechanism of pre-protein export through the bacterial cytoplasmic membrane, in which the SecA ATPase plays a crucial role as an "energy supplier", is poorly understood. In particular, biochemical and structural studies provide contradictory data as to the oligomeric state of SecA when it is integrated into the active trans-membrane translocase. Here, we report the 2.8 A resolution crystal structure of the Thermus thermophilus SecA protein (TtSecA). Whereas the structure of the TtSecA monomer closely resembles that from other bacteria, the oligomeric state of TtSecA is strikingly distinct. In contrast to the antiparallel (head-to-tail) dimerization reported previously for the other bacterial systems, TtSecA forms parallel (head-to-head) dimers that are reminiscent of open scissors. The dimer interface is abundant in bulky Arg and Lys side-chains from both subunits, which stack on one another to form an unusual "basic zipper" that is highly conserved, as revealed by homology modeling and sequence analysis. The basic zipper is sealed on both ends by two pairs of the salt bridges formed between the basic side-chains from the zipper and two invariant acidic residues. The organization of the dimers, in which the two pre-protein binding domains are located proximal to each other at the tip of the "scissors", might allow a concerted mode of substrate recognition while the opening/closing of the scissors might facilitate translocation.
authors.affiliation: Department of Biochemistry and Molecular Genetics, University of Alabama at Birmingham, 434 Kaul Genetics Building, 720 20(th) Street South, Birmingham, AL 35294, USA. dmitry@uab.edu
authors.forename: Dmitry G
authors.initials: DG
authors.lastname: Vassylyev
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Mori
authors.forename: Marina N
authors.initials: MN
authors.lastname: Vassylyeva
authors.forename: Tomoya
authors.initials: T
authors.lastname: Tsukazaki
authors.forename: Yoshiaki
authors.initials: Y
authors.lastname: Kimura
authors.forename: Tahir H
authors.initials: TH
authors.lastname: Tahirov
authors.forename: Koreaki
authors.initials: K
authors.lastname: Ito
mesh_terms: D000251:Adenosine Triphosphatases / Q000737:chemistry*; D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry*; D017124:Conserved Sequence; D018360:Crystallography, X-Ray; D019281:Dimerization; D019143:Evolution, Molecular; D026901:Membrane Transport Proteins / Q000737:chemistry*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D021122:Protein Subunits / Q000737:chemistry; D000069816:SEC Translocation Channels; D000081416:SecA Proteins; D016963:Thermus thermophilus / Q000201:enzymology*
chemical_list: D001426:Bacterial Proteins; D026901:Membrane Transport Proteins; D021122:Protein Subunits; D000069816:SEC Translocation Channels; D000251:Adenosine Triphosphatases; D000081416:SecA Proteins