Surfactant protein B in type II pneumocytes and intra-alveolar surfactant forms of human lungs

Am J Respir Cell Mol Biol. 2004 Apr;30(4):449-58. doi: 10.1165/rcmb.2003-0262OC. Epub 2003 Sep 11.

Abstract

Surfactant protein B (SP-B) is synthesized by type II pneumocytes as a proprotein (proSP-B) that is proteolytically processed to an 8-kD protein. In human type II pneumocytes, we identified not only proSP-B, processing intermediates of proSP-B, and mature SP-B, but also fragments of the N-terminal propeptide. By means of immunoelectron microscopy, proSP-B and processing intermediates were localized in the endoplasmic reticulum, Golgi vesicles, and few multivesicular bodies in type II pneumocytes in human lungs. A colocalization of fragments of the N-terminal propeptide and mature SP-B was found in multivesicular, composite, and some lamellar bodies. Mature SP-B was localized over the projection core of lamellar bodies and core-like structures in tubular myelin figures. In line with immunoelectron microscopy and Western blot analysis of human type II pneumocytes, a fragment of the N-terminal propeptide was also detected in isolated rat lamellar bodies. In conclusion, our data indicate that the processing of proSP-B occurs between the Golgi complex and multivesicular bodies and provide evidence that a fragment of the N-terminal propeptide and mature SP-B are transported together to the lamellar bodies. In human lungs, mature SP-B is involved in the structural organization of lamellar bodies and tubular myelin by the formation of core particles.

MeSH terms

  • Adult
  • Animals
  • Bronchoalveolar Lavage Fluid
  • Cells, Cultured
  • Golgi Apparatus / metabolism
  • Humans
  • In Vitro Techniques
  • Lung / cytology*
  • Lung / metabolism*
  • Male
  • Microscopy, Immunoelectron / methods
  • Myelin Sheath / ultrastructure
  • Protein Precursors / metabolism
  • Protein Processing, Post-Translational
  • Proteolipids / metabolism
  • Pulmonary Alveoli / cytology
  • Pulmonary Alveoli / metabolism
  • Pulmonary Alveoli / ultrastructure
  • Pulmonary Surfactant-Associated Protein A / immunology
  • Pulmonary Surfactant-Associated Protein A / metabolism
  • Pulmonary Surfactant-Associated Protein B / immunology
  • Pulmonary Surfactant-Associated Protein B / metabolism*
  • Pulmonary Surfactants / metabolism
  • Rats
  • Rats, Wistar
  • Subcellular Fractions

Substances

  • Protein Precursors
  • Proteolipids
  • Pulmonary Surfactant-Associated Protein A
  • Pulmonary Surfactant-Associated Protein B
  • Pulmonary Surfactants
  • surfactant protein B propeptide