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Description

Pulmonary surfactant-associated protein D (UniProt P35247, also known as Collectin-7, Lung surfactant protein D, PSP-D, SP-D) is encoded by the SFTPD (also known as COLEC7, PSPD, SFTP4) gene (Gene ID 6441) in human. Surfactant, produced by alveolar type II cells and airway Clara cells, resides at the air-liquid interface of the lungs and is most commonly known for its role in reducing surface tension. Surfactant is a lipoprotein complex composed of approximately 10% proteins, including surfactant protein A (SP-A), SP-B, SP-C, and SP-D. SP-A and SP-D belong to the collectin family of proteins and can modulate innate immunity. SP-D is known to enhance pulmonary clearance of pathogens, such as Pseudomonous aerginosa, Klebsiella pneumonia, respiratory syncytial virus (RSV) and Influenza virus. SP-D is also shown to modify allergic responsee in the lungs by binding common allergens, including house dust mite and pollen particles. SP-A-deficient and SP-D-deficient mice are susceptible to bleomycin-induced lung injury and display increased cellular inflammation, more severe lung fibrosis, and reduced survival. In addition to lung, SP-D is also reported to be detected in extrapulmonary tissues, such as kidney and pancreas. Serum levels of either SP-A or SP-D in human are correlated with mortality in patients with pulmonary fibrosis. Human SP-D is produced with a signal peptide sequence (a.a. 1-20), which is removed posttranslationally to yield the mature protein (a.a. 21-375) that contains a collagen-like domain (a.a. 46-222), a coiled coil region (a.a. 223-252), and a C-type lectin domain (a.a. 260-375).