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 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.
Murata
Y
Y
Department of Biochemistry, Sapporo Medical College, Japan.
Kuroki
Y
Y
Akino
T
T
D000595:Amino Acid Sequence; D000818:Animals; D000911:Antibodies, Monoclonal; D001665:Binding Sites; D001667:Binding, Competitive; D002478:Cells, Cultured; D002850:Chromatography, Gel; D017364:Collagenases / Q000378:metabolism; D006801:Humans; D015151:Immunoblotting; D008969:Molecular Sequence Data; D008970:Molecular Weight; D010446:Peptide Fragments / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology; D010743:Phospholipids / Q000378:metabolism*; D011510:Proteolipids / Q000737:chemistry* / Q000378:metabolism*; D011650:Pulmonary Alveoli / Q000378:metabolism*; D037662:Pulmonary Surfactant-Associated Protein A; D037661:Pulmonary Surfactant-Associated Proteins; D011663:Pulmonary Surfactants / Q000737:chemistry* / Q000378:metabolism*; D051381:Rats
D000911:Antibodies, Monoclonal; D010446:Peptide Fragments; D010743:Phospholipids; D011510:Proteolipids; D037662:Pulmonary Surfactant-Associated Protein A; D037661:Pulmonary Surfactant-Associated Proteins; D011663:Pulmonary Surfactants; D017364:Collagenases