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Results demonstrate that the short consensus/complement repeat 1-2 domains are involved in the primary C3dg ligand binding site of human complement receptor CR2 with additional short domains of CR2 facilitating interaction with the ligand. Data reveal that His residue between the two short domains may be influencing the complex formation.

The human complement factor I is a plasma serine protease that contains five domains consisting of One factor I-membrane attack complex domain, one CD5 domain, two low-density lipoprotein receptor domains and one serine protease domain. Factor one is essential in the regulation of complement. X-ray and neutron scattering were undertaken to determine the arrangement of these domain on complement factor I derived from human plasma.

The structure of the heterodimeric receptor-ligand complex found in human coagulation factor VIIa (FVIIa) has been studied using x-ray and neutron scattering. FVIIa is a soluble four-domain plasma serine protease that binds with the two extracellular domains of the transmembrane tissue factor during blood coagulation. The study gives insight on the domain structure of free FVIIa in solution to illustrate how membrane-bound tissue factors detect the FVIIa as a ligand.

X-ray and neutron scattering methods were utilized to determine the solution structure of AmiC and the occurrence of a ligand-dependent conformational change. Results have shown that AmiC behaved differently from the typical periplasmic proteins such that it formed antiparallel dimers in its crystal structure secondary to the crystal lattice formation. Furthermore, it was shown that amiC participated in a monomer-trimer association in the solution.

There are fourteen domains in human immunoglobulin E (IgE) having specific fold structures. IgE contains two domains more than IgG in the Fc region. This makes the number of domains in the IgE-Fc six against four in the IgG-Fc. The bent models of IgE-Fc, with the additional Fc domain pair rotating by 40-50 degrees, produce the best agreement with X-ray and neutron scattering curves. The bent model is also in line with the hydrodynamic model of IgE.

The overall structure of C9 of human complement was determined by neutron and X-ray solution scattering techniques. It was shown that the solution structure of C9 is more elongated than previously deduced from electron microscopy studies and has a maximum dimension estimated as 12 +/- 2 nm. The scattering data were compatible with a limited range of arrangements of the three domains and perforin-like region in monomeric C9. Moreover, the structure modeling for C9 suggests that these four regions are arranged in a V-shaped structure, with an angle of 10 degrees between the two arms, each of length 11.1 nm.