title: Human complement factor I glycosylation: structural and functional characterisation of the N-linked oligosaccharides.
abstract: Factor I (fI) is a key serine protease that modulates the complement cascade by regulating the levels of C3 convertases. Human fI circulates in plasma as a heavily N-glycosylated (25-27% w/w) heterodimer composed of two disulphide linked chains, each carrying three N-linked oligosaccharide chains. It had been suggested that the oligosaccharides may have both structural and functional roles in the interactions with the natural substrate and the cofactor during a catalysis. The N-linked glycans of each fI chain were characterised in detail and the analysis revealed a similar composition of the glycan pools with both chains heavily sialylated. Disialylated structures were in excess over monosialylated ones: 55% over 40% for the heavy chain and 62% over 35% for the light chain. The dominant type of glycan identified on both chains was A(2)G(2)S(2), a biantennary structure with chains terminating in sialic acid linked to galactose. The glycan characterisation facilitated a strategy for the partial deglycosylation of the enzyme. Assessment of the proteolytic activities of the native and partially deglycosylated forms of fI showed that both forms of the enzyme have very similar proteolytic activities against C3(NH(3)) indicating that the charged glycans of fI do not influence the fI-cofactor-substrate interactions.
authors.affiliation: MRC Immunochemistry Unit, Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, England, UK. tsiftsoglou@cbr.med.harvard.edu
authors.forename: Stefanos A
authors.initials: SA
authors.lastname: Tsiftsoglou
authors.forename: James N
authors.initials: JN
authors.lastname: Arnold
authors.forename: Pietro
authors.initials: P
authors.lastname: Roversi
authors.forename: Max D
authors.initials: MD
authors.lastname: Crispin
authors.forename: Catherine
authors.initials: C
authors.lastname: Radcliffe
authors.forename: Susan M
authors.initials: SM
authors.lastname: Lea
authors.forename: Raymond A
authors.initials: RA
authors.lastname: Dwek
authors.forename: Pauline M
authors.initials: PM
authors.lastname: Rudd
authors.forename: Robert B
authors.initials: RB
authors.lastname: Sim
mesh_terms: D000595:Amino Acid Sequence; D017244:Complement Factor I / Q000737:chemistry / Q000378:metabolism*; D004591:Electrophoresis, Polyacrylamide Gel; D006031:Glycosylation; D006801:Humans; D006868:Hydrolysis; D008969:Molecular Sequence Data; D009844:Oligosaccharides / Q000737:chemistry / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid
chemical_list: D009844:Oligosaccharides; D017244:Complement Factor I