Production of surfactant protein C in the baculovirus expression system: the information required for correct folding and palmitoylation of SP-C is contained within the mature sequence

Surfactant protein C (SP-C) is synthesized in the alveolar type II cells of the lung as a 21 kDa propeptide which is proteolytically processed to a 4. 2 kDa mature active form. The main function of this extremely hydrophobic p rotein is to enhance lipid insertion into the air/liquid interface in the l ung upon inhalation. This is necessary to maintain a relatively low surface tension at this interface during breathing. In this report we describe the production of mature human SP-C in the baculovirus expression system. The recombinant protein contains a secondary structure with a high a-helical co ntent (73%), comparable to native SP-C, as determined by circular dichroism and attenuated total reflection Fourier transform infrared analysis. The e xpressed protein is a mixture of dipalmitoylated (15%) and non-palmitoylate d SP-C. This suggests that the information required for palmitoylation is c ontained within the sequence of the mature protein. The activity of the pro tein to insert phospholipids into a preformed monolayer of lipids at an air /liquid interface was determined with a captive bubble surfactometer. Recom binant SP-C significantly reduced the surface tension at the air/liquid int erface during dynamic expansion and compression. We conclude that correctly folded, dipalmitoylated and active SP-C can be expressed in the baculoviru s expression system. Our results may facilitate investigations into the rel ation between structure and function of SP-C and into protein palmitoylatio n in general. (C) 1999 Elsevier Science B.V. All rights reserved.