Cell wall assembly by Pneumocystis carinii: Evidence for a unique Gsc-1 subunit mediating β-1,3-glucan deposition

Pneumocystis carinii remains a persistent cause of severe pneumonia in immune compromised patients. Recent studies indicate that P. carinii is a fungal species possessing a glucan-rich cyst wall. Pneumocandin antagonists of β-1,3-glucan synthesis rapidly suppress infection in animal models of P. carinii pneumonia. We, therefore, sought to define the molecular mechanisms of β-glucan cell wall assembly by P. carinii. Membrane extracts derived from freshly purified P. carinii incorporate uridine 5′-diphosphoglucose into insoluble carbohydrate, in a manner that was completely inhibited by the pneumocandin L733-560, an antagonist of Gsc-1-type β-glucan synthetases. Using degenerative polymerase chain reaction and library screening, the P. carinii Gsc-1 catalytic subunit of β-1,3-glucan synthetase was cloned and characterized. P. carinii gsc1 exhibited homology to phylogenetically related fungal β-1,3-glucan synthetases, encoding a predicted 214-kDa integral membrane protein with 12 transmembrane domain structure. Immunoprecipitation of P. carinii extracts, with a synthetic peptide anti-Gsc-1 antibody, specifically yielded a protein of 219.4 kDa, which was also capable of incorporating 5′-diphosphoglucose into insoluble glucan carbohydrate. As opposed to other fungi, the expression of gsc-1 mRNA is uniquely regulated over P. carinii's life cycle, having minimal expression in trophic forms, but substantial expression in the thick-walled cystic form of the organism. These results indicate that P. carinii contains a unique catalytic subunit of β-1,3-glucan synthetase utilized in cyst wall formation. Because synthesis of β-1,3-glucan is absent in mammalian cells, inhibition of the P. carinii Gsc-1 represents an attractive molecular target for therapeutic exploitation.