Solution conformation of human apolipoprotein C-1 inferred from proline mutagenesis: far- and near-UV CD study

Solution structure of lipid-free apolipoprotein C-1 (apoC-1, 6.6 kD) was analyzed by circular dichroism (CD) of 15 mutants containing single Pro or Ala substitutions in predicted alpha-helical regions. While the majority of Pro substitutions induce complete (L11P, L18P, R23P, I29P, M38P, W41P, T45P) or partial (G15P, L34P) helical unfolding, similar substitutions at other sites (A7P, Q31P, V49P, L53P) do not cause large changes in the secondary structure or stability. The results suggest that lipid-free apoC-1 is comprised of two dynamic helices that are stabilized by interhelical interactions and are connected by a short linker containing residues 30-33. We propose that the minimal folding unit in the lipid-free state of this and other exchangeable apolipoproteins comprises the helix-turn-helix motif formed of four 11-mer sequence repeats. Comparison of the helical content in lipid-free and lipid-bound apoC-1 suggests that lipid binding shifts the conformational equilibrium toward preexisting highly helical conformation. Remarkably, near-UV CD spectra of wild type and mutant apoC-1 are not significantly altered upon thermal or chemical unfolding and thus result from residual aromatic clustering that is retained in the unfolded state. Correlation of far- and near-UV CD of the mutant peptides suggests that the hydrophobic cluster containing W41 is essential for the helical stability and may form a helix nucleation site in apoC-1.