Jeremy P. Derrick

Determination of the solution structures of domains II and III of protein G from Streptococcus by 1H nuclear magnetic resonance

We have used 1H nuclear magnetic resonance spectroscopy to determine the solution structures of two small (61 and 64 residue) immunoglobulin G (IgG)-binding domains from protein G, a cell-surface protein from Streptococcus strain G148. The two domains differ in sequence by four amino acid substitutions, and differ in their affinity for some subclasses of IgG. The structure of domain II was determined using a total of 478 distance restraints, 31 phi and 9 chi 1 dihedral angle restraints; that of domain III was determined using a total of 445 distance restraints, 31 phi and 9 chi 1 dihedral angle restraints. A protocol which involved distance geometry, simulated annealing and restrained molecular dynamics was used to determine ensembles of 40 structures consistent with these restraints. The structures are found to consist of an alpha-helix packed against a four-stranded antiparallel-parallel-antiparallel beta-sheet. The structures of the two domains are compared to each other and to the reported structure of a similar domain from a protein G from a different strain of Streptococcus. We conclude that the difference in affinity of domains II and III for IgG is due to local changes in amino acid side-chains, rather than a more extensive change in conformation, suggesting that one or more of the residues which differ between them are directly involved in interaction with IgG.

The serine acetyltransferase from Escherichia coli: Over-expression, purification and preliminary crystallographic analysis

An expression vector has been constructed which increases the expression of serine acetyltransferase (SAT) from E. coli to 17% of the soluble cell protein. A novel purification procedure, using dye‐affinity chromatography, allows purification of SAT to homogeneity. The enzyme has been crystallised from polyethylene glycol, in the presence of L‐cysteine (an inhibitor of SAT). The crystals which diffract to beyond 3.0 Å resolution are of the tetragonal spacegroup P41212(or P43212) with cell dimensions a = b = 123 Å, c = 79 Å. Since ultracentrifugation and gel‐filtration experiments indicate that purified SAT is a tetramer, there appears to be one‐half tetramer in the asymmetric unit (V m = 2.55 Å3/Da).

Crystallization and preliminary X-ray analysis of the complex between a mouse Fab fragment and a single IgG-binding domain from streptococcal protein G

The Fab fragment of a mouse immunoglobulin G1, complexed with a single IgG-binding domain from streptococcal protein G, has been crystallized in a form suitable for analysis by X-ray diffraction. The needle-shaped crystals were grown from polyethylene glycol 4000 using vapour diffusion methods and diffract to 2.3 A resolution. The space group is P2(1)2(1)2(1) (a = 64.5 A, b = 70.5 A and c = 120.1 A), with one Fab-protein G domain complex in the asymmetric unit. Solution of the three-dimensional structure of the complex will permit a detailed analysis of the molecular interactions between protein G and the Fab portion of IgG.

Identification of the C2-1H histidine NMR resonances in chloramphenicol acetyltransferase by a 13C-1H heteronuclear multiple quantum coherence method.

Chloramphenicol acetyltransferase (CAT) was used to assess the feasibility of study of specific proton resonances in an enzyme of overall molecular mass 75000. [ring2‐13C]Histidine was selectively incorporated into the type III chloramphenicol acetyltransferase (CATIII) using a histidine auxotroph of E. coli. Heteronuclear multiple and single quantum experiments were used to select the C2 protons in the histidyl imidazole ring. One‐ and two‐dimensional spectra revealed six signals out of a total of seven histidine residues in CATIII. pH titration, chemical modification and ligand binding were used to demonstrate that the signal from H195, the histidine at the active site, is not among those observed. Nevertheless, this work demonstrates that selective isotopic enrichment and multiple quantum coherence techniques can be used to distinguish proton resonances in a protein of high molecular mass.