Evolutionary and functional studies of protein H: a surface molecule of Streptococcus pyogenes

University dissertation from Dept. of Cell and Molecular Biology, P.O. Box 94, S-221 00 Lund, Sweden

Abstract: Several bacterial species express surface proteins with affinity for the Fc part of human IgG. This thesis describes evolutionary and functional studies of protein H, an IgGFc-binding surface protein of the human pathogen Streptococcus pyogenes. By using peptide mapping and NMR analysis the IgGFc-binding region of protein G, a surface protein of group C and G streptococci, was identified. Various fragments of protein H localized the IgGFc-binding region to the N-terminal part of this molecule. It was also demonstrated that protein A, an IgGFc-binding protein of Staphylococcus aureus, protein G and protein H bind to the same site on IgGFc, although there is no sequence homology between the IgGFc-binding regions of these proteins. The finding that unrelated bacterial proteins have evolved regions with similar function, represents an example of convergent evolution. Protein H also absorbs albumin from human plasma and by using fragments of protein H, the albumin-binding region was mapped to the C-terminal part of the molecule. Protein G also binds albumin, and the albumin-binding regions of protein H and G show no sequence similarity, demonstrating that these regions also have evolved convergently. Protein H was found to interact with fibronectin type III (FNIII) domains, present in many eukaryotic proteins. The binding site was localized to the N-terminal part of protein H but did not overlap the binding site for IgG. Although the FNIII domain and the Ig-like domain have similar three-dimensional structure, protein H has evolved separate binding sites for these widespread domains. Many S. pyogenes strains aggregate when grown in vitro. Molecular analysis of this property in a strain expressing protein H, shows that protein H contributes to the aggregation. A 19 amino acids sequence was identified in the N-terminal part of protein H as responsible for the aggregation. Aggregating strains adhered to epithelial cells while isogenic mutants devoid of proteins containing the 19 a.a. sequence, did not. Soluble protein H binds to lymphocytes. It was shown that protein H is taken up by the lymphocytes and transported in to the nucleus via a previously unknown route. In the cytosol, protein H interacts with actin and nucleophosmin/B23, a shuttle protein between the cytoplasm and the nucleus. Within the nucleus protein H bind the nuclear proteins SET and hnRNP A2/B1, resulting in a cytostatic effect.

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