SITE-DIRECTED MUTAGENESIS OF SURFACTANT PROTEIN-A REVEALS DISSOCIATION OF LIPID AGGREGATION AND LIPID UPTAKE BY ALVEOLAR TYPE-II CELLS

Surfactant protein A (SP-A) binds to dipalmitoylphosphatidylcholine (D PPC) and induces phospholipid vesicle aggregation. It also regulates t he uptake and secretion of surfactant lipids by alveolar type II cells . We introduced the single mutations Glu195 --> Gln (rE195Q), Lys201 - -> Ala (rK201A) and Lys203 --> Ala (rK203A) for rat SP-A, Arg199 --> A la (hR199A) and Lys201-->Ala (hK201A) for human SP-A, and the triple m utations Arg197, Lys201 and Lys203-->Ala (rR197A/K201A/K203A) for rat SP-A, into cDNAs for SP-A, and expressed the recombinant proteins usin g baculovirus vectors. All recombinant proteins avidly bound to DPPC l iposomes. rE195Q, rK201A, rK203A, hR199A and hK201A function with acti vity comparable to wild type SP-A. Although rR197A/K201A/K203A was a p otent inducer of phospholipid vesicle aggregation, it failed to stimul ate lipid uptake. rR197A/K201A/K203A was a weak inhibitor for lipid se cretion and did not competed with rat [I-125]SP-A for receptor occupan cy. From these results, we conclude that Lys201 and Lys203 of rat SP-A , and Arg199 and Lys201 of human SP-A are not individually critical fo r the interaction with lipids and type II cells, and that Glu195 of ra t SP-A can be replaced with Gin without loss of SP-A functions. This s tudy also demonstrates that the SP-A-mediated lipid uptake is not dire ctly correlated with phospholipid vesicle aggregation, and that specif ic interactions of SP-A with type II cells are involved in the lipid u ptake process. (C) 1998 Elsevier Science B.V. All rights reserved.