Structure of a soluble, glycosylated form of the human complement regulatory protein CD59

Background CD59 is a cell-surface glycoprotein that protects host cells from complement-mediated lysis by binding to and preventing the normal functioning of the complement proteins C8 and/or C9 which form part of a membrane penetrating assembly called the membrane attack complex. CD59 has no structural similarity to other complement proteins, but is an example of a plasma protein domain type found also in murine Ly-6 proteins and the urokinase-type plasminogen activator receptor. Results CD59 was purified from human urine, retaining the N - glycan and at least some of the non-lipid component of the glycosylphosphatidylinositol membrane anchor. The three- dimensional structure of the protein component has been determined in the presence of the carbohydrate groups using two-dimensional NMR spectroscopy. The protein structure is well defined by the NMR data (root mean square deviation from the mean structure of 0.65 å for backbone atoms and no distance constraint violations greater than 0.4 å). Structure calculations were also carried out to model the orientation of the N -acetylglucosamine residue that is directly linked to Asn18. Conclusions The main features of the protein structure are two antiparallel β -sheets (a central one with three strands and another with two), a short helix that packs against the three- stranded β -sheet, and a carboxy-terminal region that, although lacking regular secondary structure, is well defined and packs against the three-stranded β -sheet, on the opposite face to the helix. We have used the structure, in combination with existing biochemical data, to identify residues that may be involved in C8 binding.