Two crystal structures of the B1 immunoglobulin-binding domain of streptococcal protein G and comparison with NMR.

The structure of the 56-residue B1 immunoglobulin-binding domain from streptococcal protein G has been determined in two different crystal forms. The crystal structures were deduced by molecular replacement, based on the structure of the B2 domain (Brookhaven accession code 1 PGX). Final R values are 0, 174 and 0. 198 for orthorhombic and trigonal forms, for diffraction data from 6.0 to 2.07 A and from 6 to 1.92 A, respectively. The orthorhombic crystals have an unusually high packing density for protein crystals, with V m =1.66 and a solvent content of 26%. The protein structure is found to be very similar (rms deviation 0.25 A for 56 Cα's) in the two crystal forms, with an efficiently packed hydrophobic core between a four-stranded β-sheet and a four-turn α-helix