Structure and influence on stability and activity of the N-terminal propeptide part of lung surfactant protein C

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Mature lung surfactant protein C (SP-C) corresponds to residues 24–58 of the 21 kDa proSP-C. A late processing intermediate, SP-Ci, corresponding to residues 12–58 of proSP-C, lacks the surface activity of SP-C, and the SP-Ciα-helical structure does not unfold in contrast to the metastable nature of the SP-C helix. The NMR structure of an analogue of SP-Ci, SP-Ci(1–31), with two palmitoylCys replaced by Phe and four Val replaced by Leu, in dodecylphosphocholine micelles and in ethanol shows that its α-helix vs. that of SP-C is extended N-terminally. The Arg-Phe part in SP-Ci that is cleaved to generate SP-C is localized in a turn structure, which is followed by a short segment in extended conformation. Circular dichroism spectroscopy of SP-Ci(1–31) in microsomal or surfactant lipids shows a mixture of helical and extended conformation at pH 6, and a shift to more unordered structure at pH 5. Replacement of the N-terminal hexapeptide segment SPPDYS (known to constitute a signal in intracellular targeting) of SP-Ci with AAAAAA results in a peptide that is mainly unstructured, independent of pH, in microsomal and surfactant lipids. Addition of a synthetic dodecapeptide, corresponding to the propeptide part of SP-Ci, to mature SP-C results in slower aggregation kinetics and altered amyloid fibril formation, and reduces the surface activity of phospholipid-bound SP-C. These data suggest that the propeptide part of SP-Ci prevents unfolding by locking the N-terminal part of the helix, and that acidic pH results in structural disordering of the region that is proteolytically cleaved to generate SP-C.

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