cDNA Sequence Diversity and Genomic Clusters of Major Surface Glycoprotein Genes of Pneumocystis carinii

The major surface glycoprotein (MSG) of Pneumocystis carinii plays a crucial role in the pathobiology of P. carinii, which often causes fatal pneumonia in AIDS patients. The cDNAs encoding MSG antigens were cloned from a λgtl1 expression library of rat-derived P. carinii by immunoscreening. The cloned cDNAs constituted a gene family containing ∼70% amino acid identity between subtypes. The diversity of MSG cDNAs was high and reflected the genomic structure of MSG genes clustered in the P. carinii chromosomes. These multiple genes may account for the high-level expression ofMSG that could generate potential variations in the cell surface. Moreover, the MSG sequences have significant sequence homology to tropomyosins and myosins, suggesting physical or functional association with the membrane cytoskeleton.