FOLDING PROPERTIES OF AN ANNEXIN-I DOMAIN - A H-1-N-15 NMR AND CD STUDY

The annexin fold consists of four 70-residue domains with markedly hom ologous sequences and nearly identical structures. Each domain contain s five helices designated A to E. Domain 2 of annexin I was obtained b y chemical synthesis including ten specifically labeled residues and s tudied by H-1-N-15 NMR and circular dichroism (CD). In pure aqueous so lution this annexin domain presents, at most, 25% of residual helix se condary structure compared to 75%-85% for the native helix content and thus does not constitute an autonomous folding unit. Dodecylphosphoch oline (DPC) micelles were used to provide the annexin domain with non- specific hydrophobic interactions. The structuring effect of micelles was thoroughly investigated by CD and H-1-N-15 NMR. Most, but not all, of the native helix secondary structure was recovered at DPC saturati on. NMR data made it possible to determine the intrinsic helix propens ity hierarchy of the different helix segments of the domain: A similar to B similar to E > C, D. This hierarchy is remarkably well correlate d with the location of the helices in the native protein since A, B, a nd E helices are those in contact with the remaining parts of the prot ein. This result tends to support the view that, for large proteins li ke annexins (35 kDa), high intrinsic secondary structure propensities, at least helix propensity, in selected protein segments is necessary for a correct folding process. As a consequence this also indicates th at important information concerning the folding pathway is encoded in the protein sequence.