Multi-gene family of major surface glycoproteins of Pneumocystis carinii: full-size cDNA cloning and expression.

The major surface glycoprotein (MSG) of Pneumocystis carinii plays a crucial role in the fatal pneumonia caused by this organism in AIDS patients. A cDNA encoding a full-length MSG polypeptide was isolated from a lambda library of rat-derived P. carinii cDNAs. The deduced MSG, referred to as the MSG5 subtype, is a 120,765-Da protein composed of 1,076 amino acids and contains an anchoring hydrophobic sequence at the C-terminus of the protein. Sequence analyses of cloned MSG-cDNAs revealed an MSG-gene family with approximately 70% protein sequence identity between subtypes. P. carinii karyotype hybridization analyses indicated that the MSG gene family members are scattered throughout most of the P. carinii chromosomes. These recombinant MSG proteins reacted with the antiserum from P. carinii-infected rats, as expected, and antiserum generated against P. carinii-infected mice, indicating the existence of common determinants in MSG polypeptides. The family of MSG proteins is rich in cysteine residues and these cysteines are highly conserved in all MSG subtypes regardless of species specificity, suggesting the structural and/or functional importance of these cysteines. The pathobiological significance of the MSG gene family and its sequence diversity in P. carinii is discussed.