ROLE OF THE C-TERMINAL DOMAIN OF PULMONARY SURFACTANT PROTEIN-A IN BINDING TO ALVEOLAR TYPE-II CELLS AND REGULATION OF PHOSPHOLIPID SECRETION

Surfactant protein A (SP-A), with a reduced denatured molecular mass o f 26-38 kDa, is characterized by a collagen-like sequence in the N-ter minal half of the protein. This protein forms an oligomeric structure which is dependent upon this collagenous domain. SP-A has been demonst rated to function as an inhibitor of phospholipid secretion by primary cultures of alveolar type II cells via a cell surface receptor for th e protein. However, the receptor-binding domain of SP-A has not been i dentified. The purpose of the present study was to investigate the rol e of the C-terminal domain of SP-A in binding to type II cells and reg ulation of phospholipid secretion. A monoclonal antibody to human SP-A , whose epitope was localized at the C-terminal domain of the protein, abolished the inhibitory activity of human SP-A on lipid secretion by type II cells, and attenuated the ability of human SP-A to compete wi th I-125-(rat SP-A) for receptor binding. SP-A was then digested with collagenase and the collagenase-resistant fragment (CRF), which is the C-terminal domain of SP-A (thus lacking the N-terminal domain), was i solated. Gel filtration chromatography revealed that CRF exists as a m onomer in solution containing Ca2+. CRF had the ability to inhibit pho spholipid secretion, although at a higher concentration than for SP-A, and was also able to compete with I-125-(rat SP-A) for binding to typ e II cells. A direct binding study showed that CRF bound to type II ce lls in a concentration-dependent manner. The present study demonstrate s that the non-collagenous, C-terminal, domain of SP-A is responsible for the protein's inhibitory effect on lipid secretion and its binding to type II cells.