Crystal structure of the C2 fragment of streptococcal protein G in complex with the Fc domain of human IgG.

BACKGROUND: Streptococcal protein G comprises two or three domains that bind to the constant Fc region of most mammalian immunoglobulin Gs (IgGs). Protein G is functionally related to staphylococcal protein A, with which it shares neither sequence nor structural homology. RESULTS: To understand the competitive binding of these two proteins to the Fc region, the crystal structure of a single Ig-binding domain of streptococcal protein G was determined at 3.5 A resolution in complex with the Fc fragment of human IgG and compared with the structures of protein A:Fc and protein G:Fab complexes. Protein G binds to the interface between the second and third heavy chain constant domains of Fc, which is roughly the same binding site used by protein A. Protein G comprises one alpha-helix packed onto a four-stranded beta-sheet. Residues from protein G that are involved in binding are situated within the C-terminal part of the alpha-helix, the N-terminal part of the third beta-strand and the loop region connecting these two structural elements. The identified Fc-binding region of protein G agrees well with both biochemical and NMR spectroscopic data. However, the Fc-binding helices of protein G and protein A are not superimposable. CONCLUSIONS: Protein G and protein A have developed different strategies for binding to Fc. The protein G:Fc complex involves mainly charged and polar contacts, whereas protein A and Fc are held together through non-specific hydrophobic interactions and a few polar interactions. Several residues of Fc are involved in both the protein G:Fc and the protein A:Fc interaction, which explains the competitive binding of the two proteins. The apparent differences in their Fc-binding activities result from additional unique interactions.

A comparative analysis of 23 structures of the amyloidogenic protein transthyretin.

Self-assembly of the human plasma protein transthyretin (TTR) into unbranched insoluble amyloid fibrils occurs as a result of point mutations that destabilize the molecule, leading to conformational changes. The tertiary structure of native soluble TTR and many of its disease-causing mutants have been determined. Several independent studies by X-ray crystallography have suggested structural differences between TTR variants which are claimed to be of significance for amyloid formation. As these changes are minor and not consistent between the studies, we have compared all TTR structures available at the protein data bank including three wild-types, three non-amyloidogenic mutants, seven amyloidogenic mutants and nine complexes. The reference for this study is a new 1.5 A resolution structure of human wild-type TTR refined to an R-factor/R-free of 18.6 %/21.6 %. The present findings are discussed in the light of the previous structural studies of TTR variants, and show the reported structural differences to be non-significant.

Structural distribution of mutations associated with familial amyloidotic polyneuropathy in human transthyretin.

The human plasma protein transthyretin (TTR) is a highly stable soluble homotetrameric protein. Still, conformational changes in the wild type protein can lead to self-assembly into insoluble amyloid fibrils. In addition, 74 point mutations are known to enhance amyloid formation causing familial amyloidotic polyneuropathy (PAP). Alignment of TTR sequences from twenty different species shows that only six of these mutations occur as natural amino acids in other organisms. In this paper we analyse the distribution of FAP mutations within the three-dimensional structure of TTR. Contradictory to what might be expected from protein stability studies, the mutations are not restricted to structurally rigid parts of the molecule, nor are they concentrated at the monomer interaction sites.

The beta-slip: a novel concept in transthyretin amyloidosis.

Transthyretin is a tetrameric plasma protein associated with two forms of amyloid disease. The structure of the highly amyloidogenic transthyretin triple mutant TTRG53S/E54D/L55S determined at 2.3 A resolution reveals a novel conformation: the beta-slip. A three-residue shift in beta strand D places Leu-58 at the position normally occupied by Leu-55 now mutated to serine. The beta-slip is best defined in two of the four monomers, where it makes new protein-protein interactions to an area normally involved in complex formation with retinol-binding protein. This interaction creates unique packing arrangements, where two protein helices combine to form a double helix in agreement with fiber diffraction and electron microscopy data. Based on these findings, a novel model for transthyretin amyloid formation is presented.