Structural studies of SCR domains in complement receptor type two
Complement receptor type 2 (CR2 CD21) is a type 1 membrane-bound glycoprotein in the regulators of complement activity (RCA) family. One of its ligands is C3d, a physiological cleavage fragment of the central complement protein C3. The extracellular portion is comprised of 15 or 16 short complement repeat (SCR) domains. Six of the 14 linkers between these SCR domains are atypically long and the implications of this on the solution structure and function of CR2 was unknown. Two previous crystal structures of CR2 SCR 1-2 showed the two SCR domains formed contacts with each other in a closed V-shape. In this thesis, three different recombinant constructs of CR2 were studied in solution using X-ray and neutron scattering, analytical ultracentrifugation, and constrained modelling. The combination of these techniques resulted in medium resolution structures for each of CR2 SCR 1-2, CR2 SCR 1-2 in complex with C3d, a CR2-Ig chimaera, and CR2 SCR 1-15. It was determined that the longest linkers within CR2 are highly flexible. CR2 SCR 1-2 was shown to exist in an open-V shape conformation, both free and in its complex with C3d. Studies on free C3d in solution showed that this forms concentration dependent dimers, although this dimerisation does not occur in the CR2 complex. The modelled complex between C3d and CR2 SCR 1-2 showed that SCR1 as well as SCR 2 is capable of interacting with the ligand. The chimaera of fully glycosylated human CR2 SCR 1-2 with the mouse IgGl Fc domain confirmed the results for the solution structure of free CR2 SCR 1-2 and gives an insight into the therapeutic possibilities of such a molecule. Modelling of glycosylated full length CR2 revealed a flexible semi-extended arrangement of the 15 SCR domains. These results provide an explanation of the molecular basis for the interaction of CR2 SCR 15 with C3d and three other ligands, CD23, gp350 and IFN-ct, in terms of proximity relationships between neighbouring SCR domains. Insight was also obtained into the possible mechanisms by which CR2 SCR 1-15 may activate B cells.