surfactant proteins
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
- SFTPA deficiency (SP-A deficiency)
- SFTPB deficiency (SP-B deficiency)
- SFTPC deficiency (SP-C deficiency)
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#