The N-terminal propeptide of surfactant protein C adopts an alpha-helical structure, with turn and extended regions. It's main function is the stabilisation of metastable surfactant protein C (SP-C), since the latter can irreversibly transform from its native alpha-helical structure to beta-sheet aggregates and form amyloid-like fibrils. The correct intracellular trafficking of proSP-C has also been reported to depend on the propeptide [1].

1: Structure and influence on stability and activity of the N-terminal propeptide part of lung surfactant protein C. Li J, Liepinsh E, Almlen A, Thyberg J, Curstedt T, Jornvall H, Johansson J; FEBS J. 2006;273:926-935. PMID:16478467