Peptide-binding specificity of the prosurfactant protein C Brichos domain analyzed by electrospray ionization mass...

The C-terminal domain of lung surfactant protein C (CTC) precursor (proSP-C) is involved in folding of the transmembrane segment of proSP-C. CTC includes a Brichos domain with homologs in cancer- and dementia-associated proteins. Mutations in the Brichos domain cause misfolding of proSP-C and hence amyloid fibril formation in interstitial lung disease. Electrospray ionization mass spectrometry (ESI-MS) with collision-induced dissociation (CID) experiments was applied to study non-covalent interactions between human recombinant CTC or its Brichos domain, and SP-C analogs, homotripeptides and peptides designed to model amyloid fibril formation. The results show that the Brichos domain contains the peptide-binding function of CTC. In titration experiments, apparent dissociation constants (KD) were in the micromolar range where triple-valine showed the lowest KD and triple-tyrosine the highest. Non-hydrophobic peptides failed to form complexes with Brichos. CID revealed that complexes with aromatic peptide ligands are more stable in the gas phase than complexes with non-aromatic ligands. The Brichos domain was also shown to bind fibril-forming peptides containing aromatic/hydrophobic residues.