We subjected solutions of the CR1 15�C25 allotypic variants to AUC and compared their sedimentation coefficients and axial ratios . All had Svedberg values in the narrow range of 3.14�C3.25, and calculated hydrodynamic radii in the range 5.55�C 5.80 nm, with no indication of any loss of folded structure or selfassociation over the concentration range measured. These results are in good agreement with estimates of particles size distribution from dynamic light scattering . Their calculated axial ratios were also similar, within experimental error, falling in the range of 5.6�C6.6. It therefore remains conceivable that these flanking sequences could cooperate in binding to a large ligand such as C3b or C4b, even though in previous work no binding between long-homologous repeat D and C3b/C4b was detected . This possibility, whereby long-homologous repeat D contributes to binding C3b/C4b only after the higher affinity site in long-homologous repeat C had been occupied, was explored through binding assays. To provide independent validation of these results that were based on studies of binding to covalently immobilized C3b and C4b, an ELISA assay was conducted in which C3b or C4b were adsorbed to polystyrene microtitre plates . These experiments also failed to distinguish between the binding properties of CR1 15�C25 variants. We concluded that the variant residues of CCP 25 have neither direct nor indirect roles in engagement of C3b and C4b by site 2. Complement receptor-type 1, a rare example of a protein whose ectodomain is composed entirely from modules of the same type, is a product of gene duplication and exon shuffling . This evolutionary heritage is particularly apparent U0126 MEK inhibitor amongst CR1 sizevariants , which possess different numbers of homologous blocks of seven CCPs arranged in long-homologous repeats . In the Niltubacin 30-CCP ectodomain of the most common size-variant, four tandem N-terminal long-homologous repeats , are followed by two membraneproximal CCPs. Each CCP contains about 60 amino acid residues organized in a b-strand-rich compact unit stabilized by pairs of disulfides between invariant cysteines . We investigated structural and functional consequences of Knops blood group antigenic variations of CR1 likely to be under geographical region-specific selective pressure resulting in non-uniform distribution of allotypes among global populations. These involve loss or gain of charge through substitution of residues predicted to be surface-exposed and thus the E1590/G1601 and K1590/R1601 variants will differ significantly in the electrostatic surfaces presented by their CCPs 24�C25 regions .