SEQUENCE-SPECIFIC H-1-NMR ASSIGNMENTS AND FOLDING TOPOLOGY OF HUMAN CD59

CD59 is a recently discovered cell-surface glycoprotein that restricts lysis by homologous complement and has limited sequence similarity to snake venom neurotoxins. This paper describes the first results of a two-dimensional NMR study of CD59 prepared from human urine. Nearly complete H-1-NMR assignments were obtained for the 77 amino acid residues and partial assignments for the N-glycan and the glycosylphosphatidylinositol (GPI) anchor. These results together confirm that the C-terminal residue of the mature protein is Asn 77 and that the urine-derived form retains the nonlipid part of the GPI anchor. The data further indicate that the GPI anchor and possibly the N-glycan are structurally inhomogeneous and suggest that the phospholipid present in the intact GPI anchor was removed by phosphatidylinositol-specific phospholipase-D. The folding topology of the protein was determined from NOE enhancements and slowly exchanging backbone amide protons and consists primarily of five extended strands (denoted beta1-beta5 in sequence order), arranged into separate two-stranded (beta1 and beta2) and three-stranded (beta3-beta5) antiparallel beta-sheets. The same folding topology is found in all of the snake venom neurotoxins whose structures have been determined. The region between the beta4 and beta5 strands has helical character, a feature that is not present in the neurotoxins but that is seen in the topologically similar wheat germ agglutinin.