STRUCTURE OF A SOLUBLE, GLYCOSYLATED FORM OF THE HUMAN-COMPLEMENT REGULATORY PROTEIN CD59

Background: CD59 is a cell-surface glycoprotein that protects host cel ls from complement-mediated lysis by binding to and preventing the nor mal functioning of the complement proteins C8 and/or C9 which form par t of a membrane penetrating assembly called the membrane attack comple x. 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. Res ults: CD59 was purified from human urine, retaining the N-glycan and a t least some of the non-lipid component of the glycosylphosphatidylino sitol membrane anchor. The three-dimensional structure of the protein component has been determined in the presence of the carbohydrate grou ps using two-dimensional NMR spectroscopy. The protein structure is we ll defined by the NMR data (root mean square deviation from the mean s tructure of 0.65 angstrom for backbone atoms and no distance constrain t violations greater than 0.4 angstrom). Structure calculations were a lso carried out to model the orientation of the N-acetylglucosamine re sidue that is directly linked to Asn18. Conclusions: The main features of die protein structure are two antiparallel beta-sheets (a central one with three strands and another with two), a short helix that packs against die three-stranded beta-sheet, and a carboxy-terminal region that, although lacking regular secondary structure, is well defined an d packs against the three-stranded beta-sheet, on the opposite face to the helix. we have used die structure, in combination with existing b iochemical data, to identify residues that may be involved in C8 bindi ng.