The association of the surface array and the outer membrane of Deinococcus radiodurans

The hexagonal surface array of Deinococcus radiodurans strain Sark exhibits remarkably strong bonding of its subunits and association with the outer membrane. The array is not dissociated nor is it separated from the outer membrane by reagents that disrupt hydrogen bonds (urea, guanidine hydrochloride), ionic bonds (pH, LiCl, KCl, NaCl), disulphide bonds (β-mercaptoethanol, dithiothreitol), or chelating agents (ethylenediaminetetraacetic acid). Only disruption of the outer membrane with sodium dodecyl sulphate at room temperature separated the two layers. The isolated array was composed of a major protein of 115 000 molecular weight, a minor protein of 108 000 molecular weight, and no other polymers. Their amino acid profile was indicative of a protein, lacking proline, with a high proportion (51.2%, mole/mole) of polar amino acids. The electron-transparent layer between the hexagonal surface array and the outer membrane was still associated with the purified array, suggesting that the electron-transparent "layer" is a component of the protein array and impenetrable to stains so that the reactive groups are inaccessible. The observations suggest that hydrophobic amino acids make up a major domain of the array molecule and that nonspecific hydrophobic interactions are a large component of the forces involved in the tight association of the array within itself and with the underlying outer membrane. The surface of a mutant lacking the hexagonal surface array was compared with the surface of the wild type that possesses the hexagonal surface array using the two-phase droplet system of Schürch et al. (1981. Biochim. Biophys. Acta, 640: 557–571) to estimate the relative surface hydration. The contact angle for the mutant lacking the array was 128.8 ± 0.8 (SEM), n = 39, and the contact angle for the wild type was 92.6 ± 1.4 (SEM), n = 39, indicating that the surface of the outer membrane is relatively more hydrophobic than the outer aspect of the hexagonal surface array.