Pneumocystis PCINT1, a molecule with integrin-like features that mediates organism adhesion to fibronectin

Pneumocystis species cause severe pneumonia during chronic immunosuppression, especially in patients with AIDS or malignancy. Adhesion of Pneumocystis to extracellular matrix proteins, particularly fibronectin, associated with alveolar epithelial cell surfaces, triggers organism proliferative pathways. Herein, we report the characterization of a novel Pneumocystis molecule with considerable structural features of an integrin-like extracellular matrix adhesion receptor. A PCINT1115 bp probe was initially identified from partial sequence present within the Pneumocystis genome project database. A full-length 3018 bp cDNA was subsequently obtained with extensive homology to the C-terminal region of Candida albicans INT1 (31% blastx), a gene originally described as encoding an integrin-like molecule implicated in adhesion, growth, and virulence. Sequence analysis of PCINT1 indicated that the Pneumocystis molecule contained both a putative internal RGD motif and four Metal Ion-Dependent Attachment Sites (MIDAS) motifs required for coordination of divalent cations, as well as a specific tyrosine residue found in the cytoplasmic tails of some integrin receptors and C. albicans INT1. Northern, Western and immunofluorescence studies demonstrated that the trophic forms of Pneumocystis, known to be the life cycle forms that tightly adhere to lung epithelium, expressed the molecule to a substantially greater degree than cystic forms. Heterologous expression of PCINT1 in yeast followed by application to human fibronectin-coated surfaces demonstrated these yeast display PCINT1 on their surfaces and subsequently gain the ability to bind fibronectin in a cation dependent fashion. Taken together, these results indicate that Pneumocystis expresses a novel integrin-like PCINT1 molecule sufficient to mediate interactions with extracellular matrix fibronectin, an integral component of host-cell organism interactions during this infection.