The Pneumocystis carinii major surface glycoprotein (MSG): its potential involvement in the pathophysiology of pneumocystosis.

Abstract

The major antigens of rat Pneumocystis carinii are 45, 60 and 116 kDa, and those of human P. carinii 20, 40, 66, and 95 kDa [1]. The predominant band on acrylamide gels is a surface antigen, which varies in size (95–140 kDa) depending on the host and method of preparation. This antigen is referred to by a variety of names, including gpA, gp95, gp116, gp120 and MSG. In the following the latter term will be used for this antigen. MSG is encoded by a family of approximately 100 genes, each of which is responsible for the expression of a different isoform of MSG [2, 3]. Human MSG is 95 kDa, slightly smaller than the corresponding rat antigen [4,5]. Polyclonal and monoclonal antibodies directed against rat and human P. carinii MSG show that species-specific as well as antigenic determinants are shared and that individual isolates of human and rat P. carinii are antigenically different [4–8]. Biochemical studies indicate that MSG is an acidic glycoprotein composed of a protein core and a carbohydrate portion. N -Linked carbohydrates rich in mannose, glucose, and N -acetylglucosamine residues represent one tenth of the MSG mass [4,9]. Expression of recombinant MSG has largely been unsuccessful. Rat P. carinii MSG has been recombinantly expressed using the Escherichia coli or baculovirus expression system, but differs in glycosylation from native MSG [10]. The difficulty of expressing human recombinant MSG is believed to rely on critical upstream conserved sequences not yet understood. Native MSG preparations from all species are obtained by solubilization of P. carinii organisms with zymolyase and purification utilizing high performance liquid chromatography [11]. Zymolyase hydrolyzes linear glucose polymers with β-1,3-linkages [12]. Studies of the cyst wall have demonstrated that zymolyase treatment removes …