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キーワード:
Amino Acid Sequence ; *Bacteria/en [Enzymology] ; Binding Sites ; Conserved Sequence ; *Cryoelectron Microscopy ; Cytoplasm/me [Metabolism] ; Ions/me [Metabolism] ; Models, Molecular ; Molecular Sequence Data ; Phospholipids/me [Metabolism] ; Protein Structure, Quaternary ; Protein Structure, Secondary ; Protein Subunits ; *Proton-Translocating ATPases/ch [Chemistry] ; Proton-Translocating ATPases/me [Metabolism] ; *Proton-Translocating ATPases/ul [Ultrastructure] ; Sodium/me [Metabolism] ; Support, Non-U.S. Gov't
要旨:
The sodium ion-translocating F(1)F(0) ATP synthase from the bacterium Ilyobacter tartaricus contains a remarkably stable rotor ring composed of 11 c subunits. The rotor ring was isolated, crystallised in two dimensions and analysed by electron cryo-microscopy. Here, we present an alpha-carbon model of the c-subunit ring. Each monomeric c subunit of 89 amino acid residues folds into a helical hairpin consisting of two membrane-spanning helices and a cytoplasmic loop. The 11 N-terminal helices are closely spaced within an inner ring surrounding a cavity of approximately 17A (1.7 nm). The tight helix packing leaves no space for side-chains and is accounted for by a highly conserved motif of four glycine residues in the inner, N-terminal helix. Each inner helix is connected by a clearly visible loop to an outer C-terminal helix. The outer helix has a kink near the position of the ion-binding site residue Glu65 in the centre of the membrane and another kink near the C terminus. Two helices from the outer ring and one from the inner ring form the ion-binding site in the middle of the membrane and a potential access channel from the binding site to the cytoplasmic surface. Three possible inter-subunit ion-bridges are likely to account for the remarkable temperature stability of I.tartaricus c-rings compared to those of other organisms.
