STRUCTURE OF THE N-LINKED OLIGOSACCHARIDE OF THE MAIN DIAGNOSTIC ANTIGEN OF THE PATHOGENIC FUNGUS PARACOCCIDIOIDES-BRASILIENSIS

The major diagnostic antigen of Paracoccinioides brasiliensis is the e xocellularly secreted 43,000 Da glycoprotein (gp43) which contains a s ingle N-linked oligosaccharide chain, This oligosaccharide, although p oorly immunogenic in man, is responsible for the cross-reactivity of t he gp43 with sera from patients with histoplasmosis, and may have a ro le in fungal virulence, It contains a neutral high-mannose core (Man(7 )GlcNAc(2)) to which a (1-->6)-linked alpha-D-Manp chain of variable l ength, substituted at the 2-O positions by single alpha-D-Manp residue s, is attached, A terminal unit of beta-D-galactofuranose is (1-->6)-l inked to one of the (1-->2)-linked mannosyl residues, either in the C or in the A arm of the oligosaccharide, The heterogeneity of the oligo saccharide is determined by the different sizes of the A arm and the s ites of insertion of the beta-galactofuranosyl unit. The complete stru cture was determined hy methylation analysis, H-1-NMR, mass spectromet ry, acetolysis and mannosidase degradation, Electrospray mass spectrom etry showed that the oligosaccharide comprises several subtypes rangin g from Hex(18)GlcNAc(2) to Hex(10)GlcNAc(2) which accounts for the dif fuse migration of the gp43 in polyacrylamide gels, The average size of the most frequent subtype is Hex(13.6)GlcNAc(2). Dilute acid treatmen t to remove beta-D-Galf reduced the molecular masses of the majority o f the subtypes by a single sugar unit.