
<ArticleTitle>Expression and characterisation of the thrombospondin type I repeats of human <prot>properdin</prot>.</ArticleTitle>
<AbstractText><prot>Properdin</prot>, an upregulator of the alternative complement pathway, is central to deposition of the activated complement fragment <prot>C3b</prot> on the surfaces of the pathogens, which it achieves by preventing the dissociation of the <prot>Bb</prot> catalytic subunit from the inherently labile <prot>C3bBb</prot> complexes. It is also known to bind sulphated glycoconjugates, such as sulphatides. <prot>Properdin</prot> has an unusual structure formed by oligomerisation of a rod-like monomer into cyclic dimers, trimers and tetramers. The monomer (approximately 53 kDa) contains an N-terminal region of no known homology, followed by six non-identical repeats of 60 amino acids (based on exon/intron boundaries), called 'thrombospondin type I repeats' or TSR modules. We have expressed and purified the N-terminal region and each of the individual TSR repeats in Escherichia coli. Although the individual recombinant TSRs, after a denaturation-renaturation cycle, appeared to be correctly folded modules, as judged by the one-dimensional (1D)- and 2D-nuclear magnetic resonance spectra of <prot>TSR3</prot>, they did not show binding to either <prot>C3b</prot> or sulphatide. Polyclonal antibodies were raised against each TSR and were found to be module-specific. The <prot>anti-TSR5</prot> polyclonal antibody was found to inhibit binding of native human <prot>properdin</prot> to solid-phase <prot>C3b</prot>, or sulphatides. It could also block <prot>properdin</prot>-dependent haemolysis of rabbit erythrocytes. These results are consistent with the view that the <prot>TSR5</prot> contains the major site in properdin which is involved in both <prot>C3b</prot> and sulphatide binding. It also suggests that a co-operative intramolecular interaction between TSRs, as found in the native molecule, is required for <prot>TSR5</prot> to bind either <prot>C3b</prot> or sulphatides.</AbstractText>
