Introduction of Mannose Binding Protein-Type Phosphatidylinositol Recognition into Pulmonary Surfactant Protein A

Abstract

Pulmonary surfactant protein A (SP-A) and mannose-binding protein A (MBP-A) are collectins in the C-type lectin superfamily. These collectins exhibit unique lipid binding properties. SP-A binds to dipalmitoyl phosphatidylcholine (DPPC) and galactosylceramide (GalCer) and MBP-A binds to phosphatidylinositol (PI). SP-A also interacts with alveolar type II cells. Monoclonal antibodies (mAbs PE10 and PC6) that recognize human SP-A inhibit the interactions of SP-A with lipids and alveolar type II cells. We mapped the epitopes for anti-human SP-A mAbs by a phage display peptide library. Phage selected by mAbs displayed the consensus peptide sequences that are nearly identical to ¹⁸⁴TPVNYTNWYRG¹⁹⁴ of human SP-A. The synthetic peptide GTPVNYTNWYRG completely blocked the binding of mAbs to human SP-A. Chimeric proteins were generated in which the rat SP-A region Thr¹⁷⁴-Gly¹⁹⁴ or the human SP-A region Ser¹⁷⁴-Gly¹⁹⁴ was replaced with the MBP-A region Thr¹⁶⁴-Asp¹⁸⁴ (rat ama4 or hu ama4, respectively). The mAbs failed to bind hu ama4. Rat ama4 bound to an affinity matrix on mannose-sepharose but lost all of the SP-A functions except carbohydrate binding and Ca²⁺-independent GalCer binding. Strikingly, the rat ama4 chimera acquired the PI binding property that MBP-A exhibits. This study demonstrates that the amino acid residues 174−194 of SP-A and the corresponding region of MBP-A are critical for SP-A-type II cell interaction and Ca²⁺-dependent lipid binding of collectins.