GENE FOR AN EXTRACELLULAR-MATRIX RECEPTOR PROTEIN FROM PNEUMOCYSTIS-CARINII

An initial and crucial step in the establishment of many microbial inf ections is the attachment of the pathogen to the host cells. Thus, adh erence of Pneumocystis carinii (Pc) to type I pneumocytes is believed to be important in the induction of Pc pneumonia. Little is known abou t the nature of the attachment of Pc to type I cells, although extrace llular matrix (ECM) proteins, such as fibronectin and laminin, have be en implicated in the process. We report here the isolation of a Pc gen e encoding a receptor protein that binds both fibronectin and laminin in vitro. A cDNA clone encoding the Pc ECM receptor was isolated from a Pc cDNA library and identified on the basis of sequence homology to the human colon carcinoma laminin receptor. Southern blot analysis of Pc genomic DNA confirmed that the cDNA was of Pc origin. Northern blot analysis of Pc total RNA showed a predominant mRNA of approximate to 1400 nucleotides that hybridized to the ECM receptor gene. The ECM rec eptor predicted from the cDNA sequence is 295 amino acid residues long , with a molecular mass of 32.8 kDa. The C-terminal third of the polyp eptide is highly negatively charged, whereas the N-terminal two-thirds contains hydrophobic segments that may play a role in membrane associ ation. Sequence analysis and alignment of the N terminus with the lami nin receptor cDNA sequence of human colon carcinoma support the conclu sion that the Pc ECM receptor cDNA clone is a full length clone. A Wes tern blot of the overexpressed ECM receptor protein bound both laminin and fibronectin in vitro. Antibodies raised to the overexpressed rece ptor protein interacted with a 33-kDa protein in total Pc cell lysates . These findings raise the possibility that the Pc ECM receptor protei n may mediate the organism's attachment to type I pneumocytes and, thu s, may play a crucial role in Pc pathogenesis.