Binding of calcium to SP-A, a surfactant-associated protein.
SP-A is a lung-specific pulmonary surfactant-associated protein containing a calcium-dependent carbohydrate recognition domain and collagen-like sequence. The protein is a major component of the extracellular form of surfactant known as tubular myelin. SP-A is thought to influence the surface properties of surfactant lipids and regulate the turnover of extracellular surfactant through interaction with a specific cell-surface receptor. These properties of SP-A are dependent on the presence of calcium. We have estimated calcium binding parameters for SP-A from binding data obtained by equilibrium dialysis and gel permeation chromatography. Our results suggest that each SP-A monomer binds two to three calcium ions in conditions chosen as similar to those found in the alveolar lumen. The binding data are best fit to a model incorporating two calcium binding sites with different affinities. Studies with a fragment of SP-A generated by limited proteolysis suggest the higher affinity site for calcium is located in the noncollagenous carboxy-terminal end of SP-A. This region of SP-A contains a carbohydrate recognition domain homologous to other C-type lectins. The binding of calcium to this region of SP-A causes a conformational change as assessed by a small change in the intrinsic fluorescence spectrum and a marked change in the susceptibility to proteolysis. At physiological calcium concentrations, intact SP-A aggregates in a reversible fashion, a property that may be relevant to the formation of tubular myelin.
Haagsman
H P
HP
Cardiovascular Research Institute, University of California, San Francisco 94143.
Sargeant
T
T
Hauschka
P V
PV
Benson
B J
BJ
Hawgood
S
S
D000818:Animals; D000890:Anti-Infective Agents / Q000737:chemistry / Q000378:metabolism*; D001665:Binding Sites; D001992:Bronchoalveolar Lavage Fluid / Q000737:chemistry; D002118:Calcium / Q000378:metabolism*; D002918:Chymotrypsin; D004285:Dogs; D005453:Fluorescence; D006023:Glycoproteins / Q000737:chemistry* / Q000378:metabolism; D006801:Humans; D006868:Hydrolysis; D011487:Protein Conformation; D011510:Proteolipids / Q000737:chemistry / Q000378:metabolism*; D037662:Pulmonary Surfactant-Associated Protein A; D037661:Pulmonary Surfactant-Associated Proteins; D011663:Pulmonary Surfactants / Q000737:chemistry / Q000378:metabolism*; D013329:Structure-Activity Relationship; D013499:Surface Properties
D000890:Anti-Infective Agents; D006023:Glycoproteins; D011510:Proteolipids; D037662:Pulmonary Surfactant-Associated Protein A; D037661:Pulmonary Surfactant-Associated Proteins; D011663:Pulmonary Surfactants; D002918:Chymotrypsin; D002118:Calcium