Abstract

The high-resolution three-dimensional structure of a single immunoglobulin binding domain (B1, which comprises 56 residues including the NH 2 -terminal Met) of protein G from group G Streptococcus has been determined in solution by nuclear magnetic resonance spectroscopy on the basis of 1058 experimental restraints. The average atomic root-mean-square distribution about the mean coordinate positions is 0.27 angstrom (Å) for the backbone atoms, 0.65 Å for all atoms, and 0.39 Å for atoms excluding disordered surface side chains. The structure has no disulfide bridges and is composed of a four-stranded β sheet, on top of which lies a long helix. The central two strands (β1 and β4), comprising the NH2- and COOH-termini, are parallel, and the outer two strands (β2 and β3) are connected by the helix in a +3 x crossover. This novel topology (-1, +3 x , -1), coupled with an extensive hydrogen-bonding network and a tightly packed and buried hydrophobic core, is probably responsible for the extreme thermal stability of this small domain (reversible melting at 87°C).