Structural and functional differences among human surfactant proteins SP-A1, SP-A2 and co-expressed SP-A1/SP-A2: role of supratrimeric oligomerization.
SP-A (surfactant protein A) is a membrane-associated SP that helps to maintain the lung in a sterile and non-inflamed state. Unlike SP-As from other mammalian species, human SP-A consists of two functional gene products: SP-A1 and SP-A2. In all the functions examined, recombinant human SP-A1 invariably exhibits lower biological activity than SP-A2. The objective of the present study was to investigate why SP-A2 possesses greater biological activity than SP-A1 and what advantage accrues to having two polypeptide chains instead of one. We analysed structural and functional characteristics of recombinant baculovirus-derived SP-A1, SP-A2 and co-expressed SP-A1/SP-A2 using a wide array of experimental approaches such as analytical ultracentrifugation, DSC (differential scanning calorimetry) and fluorescence. We found that the extent of supratrimeric assembly is much lower in SP-A1 than SP-A2. However, the resistance to proteolysis is greater for SP-A1 than for SP-A2. Co-expressed SP-A1/SP-A2 had greater thermal stability than SP-A1 and SP-A2 and exhibited properties of each protein. On the one hand, SP-A1/SP-A2, like SP-A2, had a higher degree of oligomerization than SP-A1, and consequently had lower K(d) for binding to bacterial Re-LPS (rough lipopolysaccharide), higher self-association in the presence of calcium and greater capability to aggregate Re-LPS and phospholipids than SP-A1. On the other hand, SP-A1/SP-A2, like SP-A1, was more resistant to trypsin degradation than SP-A2. Finally, the importance of the supratrimeric assembly for SP-A immunomodulatory function is discussed.
Sánchez-Barbero
Fernando
F
Departamento de Bioquímica y Biología Molecular I and CIBER (Centro Investigación Biomédica en Red) Enfermedades Respiratorias, Universidad Complutense de Madrid, 28040 Madrid, Spain.
Rivas
Germán
G
Steinhilber
Wolfram
W
Casals
Cristina
C
D002118:Calcium / Q000378:metabolism; D002942:Circular Dichroism; D019281:Dimerization; D006801:Humans; D007376:Interleukin-2 / Q000378:metabolism; D007700:Kinetics; D008070:Lipopolysaccharides / Q000494:pharmacology; D008561:Membrane Fusion; D010743:Phospholipids / Q000737:chemistry; D011485:Protein Binding; D037662:Pulmonary Surfactant-Associated Protein A / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D011994:Recombinant Proteins / Q000737:chemistry; D014357:Trypsin / Q000494:pharmacology
D007376:Interleukin-2; D008070:Lipopolysaccharides; D010743:Phospholipids; D037662:Pulmonary Surfactant-Associated Protein A; D011994:Recombinant Proteins; C445842:SFTPA1 protein, human; C445843:SFTPA2 protein, human; D014357:Trypsin; D002118:Calcium