Crystal Structure of a Major Outer Membrane Protein from Thermus thermophilus HB27

• Published in: Journal of molecular biology 2009-02, Vol.385 (5), p.1445-1455
• Main Authors: Brosig, Alexander, Nesper, Jutta, Boos, Winfried, Welte, Wolfram, Diederichs, Kay
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Bacterial Outer Membrane Proteins - chemistry; Cations, Divalent - chemistry; Crystallography, X-Ray; Eubacterium; Models, Molecular; Molecular Sequence Data; outer membrane biogenesis; Protein Structure, Secondary; T. thermophilus; thermophile; Thermus; Thermus thermophilus; Thermus thermophilus - chemistry; TTC0834; TTC1475
• ISSN: 0022-2836; 1089-8638
• DOI: 10.1016/j.jmb.2008.12.003

The thermophilic eubacterium Thermus thermophilus belongs to one of the oldest branches of evolution and has a multilayered cell envelope that differs from that of modern Gram-negative bacteria. Its outer membrane contains integral outer membrane proteins (OMPs), of which only a few are characterized. TtoA, a new β-barrel OMP, was identified by searching the genome sequence of strain HB27 for the presence of a C-terminal signature sequence. The structure of TtoA was determined to a resolution of 2.8 Å, representing the first crystal structure of an OMP from a thermophilic bacterium. TtoA consists of an eight-stranded β-barrel with a large extracellular part to which a divalent cation is bound. A five-stranded extracellular β-sheet protrudes out of the membrane-embedded transmembrane barrel and is stabilized by a disulfide bridge. The edge of this β-sheet forms crystal contacts that could mimic interactions with other proteins. In modern Gram-negative bacteria, the C-terminal signature sequence of OMPs is required for binding to an Omp85 family protein as a prerequisite for its assembly. We present hints that a similar assembly pathway exists in T. thermophilus by an in vitro binding assay, where unfolded TtoA binds to the Thermus Omp85 family protein TtOmp85, while a mutant without the signature sequence does not.