Structural and functional differences among human surfactant proteins SP-A1, SP-A2 and coexpressed SP-A1/SP-A2: role of supratrimeric oligomerization

• Published in: Biochemical journal 2007-06, Vol.406 (3), p.479-489
• Main Authors: Sánchez-Barbero, Fernando, Rivas, Germán, Steinhilber, Wolfram, Casals, Cristina
• Format: Article
• Language: English
• ISSN: 0264-6021; 1470-8728
• DOI: 10.1042/BJ20070275

Surfactant protein A (SP-A) is a membrane-associated surfactant protein 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 this 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 analyzed 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, differential scanning calorimetry, circular dichroism, 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. Coexpressed 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 Kd for binding to bacterial rough-lipopolysaccharide (Re-LPS), 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.