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Free keywords:
Amino Acid Sequence; Bacterial Outer Membrane Proteins/ch [Chemistry]; *Bacterial Outer Membrane Proteins/ge [Genetics]; Bacterial Outer Membrane Proteins/ip [Isolation & Purification]; Bacterial Outer Membrane Proteins/ul [Ultrastructure]; Base Sequence; Cloning, Molecular; DNA, Bacterial; Electrophoresis, Polyacrylamide Gel; Freeze Fracturing; Gram-Negative Anaerobic Bacteria/ch [Chemistry]; *Gram-Negative Anaerobic Bacteria/ge [Genetics]; Gram-Negative Anaerobic Bacteria/ul [Ultrastructure]; Image Processing, Computer-Assisted; Microscopy, Electron; Molecular Sequence Data; Polymerase Chain Reaction; Porins; Protein Structure, Secondary
Abstract:
We have discovered a new oligomeric protein component associated with the outer membrane of the ancestral eubacterium Thermotoga maritima. In electron micrographs, the protein, Omp alpha, appears as a rod-shaped spacer that spans the periplasm, connecting the outer membrane to the inner cell body. Purification, biochemical characterization and sequencing of Omp alpha suggest that it is a homodimer composed of two subunits of 380 amino acids with a calculated M(r) of 43,000 and a pI of 4.54. The sequence of the omp alpha gene indicates a tripartite organization of the protein with a globular NH2-terminal domain of 64 residues followed by a putative coiled-coil segment of 300 residues and a COOH-terminal, membrane-spanning segment. The predicted length of the coiled-coil segment (45 nm) correlates closely with the spacing between the inner and outer membranes. Despite sequence similarity to a large number of coiled-coil proteins and high scores in a coiled-coil prediction algorithm, the sequence of the central rod-shaped domain of Omp alpha does not have the typical 3.5 periodicity of coiled-coil proteins but rather has a periodicity of 3.58 residues. Such a periodicity was also found in the central domain of staphylococcal M protein and beta-giardin and might be indicative of a subclass of fibrous proteins with packing interactions that are distinct from the ones seen in other two-stranded coiled-coils.
