Role of the C-terminal domain of pulmonary surfactant protein A in binding to alveolar type II cells and regulation of phospholipid secretion.

Abstract

Surfactant protein A (SP-A), with a reduced denatured molecular mass of 26-38 kDa, is characterized by a collagen-like sequence in the N-terminal half of the protein. This protein forms an oligomeric structure which is dependent upon this collagenous domain. SP-A has been demonstrated to function as an inhibitor of phospholipid secretion by primary cultures of alveolar type II cells via a cell surface receptor for the protein. However, the receptor-binding domain of SP-A has not been identified. The purpose of the present study was to investigate the role of the C-terminal domain of SP-A in binding to type II cells and regulation 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 with 125I-(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 isolated. Gel filtration chromatography revealed that CRF exists as a monomer in solution containing Ca2+. CRF had the ability to inhibit phospholipid secretion, although at a higher concentration than for SP-A, and was also able to compete with 125I-(rat SP-A) for binding to type II cells. A direct binding study showed that CRF bound to type II cells in a concentration-dependent manner. The present study demonstrates 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.