Proteins make up the remaining 10% of surfactant. Half of this 10% is plasma proteins but the rest is formed by the apoproteins SP-A (SFTPA1), B (SFTPB), C (SFTPC) and D (SFTPD). (SP standing for "surfactant protein".)
SP-A and SP-D confer innate immunity as they have carbohydrate recognition domains that allow them to coat bacteria and viruses promoting phagocytosis by macrophages. SP-A is also thought to be involved in a negative feedback mechanism to control the production of surfactant.
SP-B and SP-C are hydrophobic membrane proteins that increase the rate that surfactant spreads over the surface. SP-B and SP-C are required for proper biophysical function of the lung. Humans and animals born with a congenital absence of SP-B suffer from intractable respiratory failure whereas those born lacking SP-C tend to develop progressive interstitial pneumonitis.
The apoproteins are produced by the secretory pathway in type 2 pneumocytes. They undergo much post-translational modification, ending up in the lamellar bodies. These are concentric rings of lipid and protein, about 1 µm in diameter.
Pathology
congenital surfactant deficiency
- mutations of surfactant proteins
deficiency of lamellar bodies in alveolar type II cells with normal surfactant protein (10673207)
References
Alveolar Surfactant Homeostasis and the Pathogenesis of Pulmonary Disease. Whitsett JA, Wert SE, Weaver TE. Annu Rev Med. 2009 Oct 7. PMID: 19824815
Whitsett JA, Weaver TE. Hydrophobic surfactant proteins in lung function and disease. N Engl J Med. 2002 Dec 26;347(26):2141-8. PMID: 12501227