A conformational study of human spectrin.

Urea denaturation profiles of spectrin dimer, measured by circular dichroism in the regions of the peptide and aromatic Cotton effects, reflect the existence of several independently unfolding domains, as well as the presence of flexible, non-globular structure. As shown by sedimentation velocity and cross-linking experiments, dissociation of the two subunits largely precedes unfolding. The flexible, segmentally mobile structure reveals itself further in the appearance of sharp signals in the high-resolution proton magnetic resonance spectrum. These spectra reveal that some 20% of the chain is in the segmentally mobile form, regardless of ionic strength, and that its composition is highly hydrophobic, with few polar side chains. This suggests the possibility that this part of the molecule may penetrate into the lipid bilayer. Conformational stability of the spectrin dimer, as measured by circular dichroism, is substantially unaffected by the state of phosphorylation and by the ionic strength, even though the latter is known to affect the size or shape of the molecule.