Spin label EPR structural studies of the N-terminus of alpha-spectrin

Structural and dynamic study of the tetramerization region of non-erythroid alpha-spectrin: a frayed helix revealed by site-directed spin labeling electron paramagnetic resonance

The N-terminal region of alpha-spectrin is responsible for its association with beta-spectrin in a heterodimer, forming functional tetramers. Non-erythroid alpha-spectrin (alphaII-spectrin) has a significantly higher association affinity for beta-spectrin than the homologous erythroid alpha-spectrin (alphaI-spectrin). We have previously determined the solution structure of the N-terminal region of alphaI-spectrin by NMR methods, but currently no structural information is available for alphaII-spectrin. We have used cysteine scanning, spin labeling electron paramagnetic resonance (EPR), and isothermal titration calorimetry (ITC) methods to study the tetramerization region of alphaII-spectrin. EPR data clearly show that, in alphaII-spectrin, the first nine N-terminal residues were unstructured, followed by an irregular helix (helix C'), frayed at the N-terminal end, but rigid at the C-terminal end, which merges into the putative triple-helical structural domain. The region corresponding to the important unstructured junction region linking helix C' to the first structural domain in alphaI-spectrin was clearly structured. On the basis of the published model for aligning helices A', B', and C', important interactions among residues in helix C' of alphaI- and alphaII-spectrin and helices A' and B' of betaI- and betaII-spectrin are identified, suggesting similar coiled coil helical bundling for spectrin I and II in forming tetramers. The differences in affinity are likely due to the differences in the conformation of the junction regions. Equilibrium dissociation constants of spin-labeled alphaII and betaI complexes from ITC measurements indicate that residues 15, 19, 37, and 40 are functionally important residues in alphaII-spectrin. Interestingly, all four corresponding homologous residues in alphaI-spectrin (residues 24, 28, 46, and 49) have been reported to be clinically significant residues involved in hematological diseases.

Structural insight into an ankyrin-sensitive lipid-binding site of erythroid beta-spectrin

It was recently shown that the region within beta-spectrin responsible for interactions with ankyrin includes a lipid-binding site which displayed sensitivity to inhibition by ankyrin. We studied its structure by constructing a series of single and double spin-labeled beta-spectrin-derived peptides and analyzing their spin-spin distances via electron paramagnetic resonance spectroscopy and the Fourier deconvolution method. The results indicate that the whole ankyrin-sensitive lipid-binding site of beta-spectrin exhibits a helical conformation revealing a distinct 3(10)-helix contribution at its N-terminus. The start of the helix was located five residues upstream along the sequence compared to the theoretical predictions. A model based on the obtained data provides direct evidence that the examined lipid-binding site is a highly amphipathic helix, which is correlated with the specific conformation of its N-terminal fragment.

Conformational Studies of the Tetramerization Site of Human Erythroid Spectrin by Cysteine-Scanning Spin-Labeling EPR Methods

We used cysteine-scanning and spin-labeling methods to prepare singly spin labeled recombinant peptides for electron paramagnetic resonance studies of the partial domain regions at the tetramerization site (N-terminal end of alpha and C-terminal end of beta) of erythroid spectrin. The values of the inverse line width parameter (deltaH0(-1)) from a family of Sp alphaI-1-368delta peptides scanning residues 21-30 exhibited a periodicity of approximately 3.5-4. We used molecular dynamics calculations to show that the asymmetric mobility of this helix is not necessarily due to tertiary contacts, but is likely due to intrinsic properties of helix C', a helix with a heptad pattern sequence. The residues with low deltaH0(-1) values (residues at positions 21, 25, and 28/29) were those on the hydrophobic side of this amphipathic helix. Native gel electrophoresis results showed that these residues were functionally important and are involved in the tetramerization process. Thus, EPR results readily identified functionally important residues in the alpha spectrin partial domain region. Mutations at these positions may lead to clinical symptoms. Similarly, the deltaH0(-1) values from a family of spin-labeled Sp betaI-1898-2083delta peptides also exhibited a periodicity of approximately 3.5-4, indicating a helical conformation in the two scanned regions (residues 2008-2018 and residues 2060-2070). However, the region consisting of residues 2071-2076 was in a disordered conformation. Both helical regions include a hydrophilic side with high deltaH0(-1) values and a hydrophobic side with low deltaH0(-1) values, demonstrating the amphipathic nature of the helical regions. Residues 2008, 2011, 2014, and 2018 in the first scanned region and residues 2061, 2065, and 2068 in the second scanned region were on the hydrophobic side. These residues were critical in alphabeta spectrin association at the tetramerization site. Mutations at some of these positions have been reported to be detrimental in clinical studies.

Flexibility of the alpha-spectrin N-terminus by EPR and fluorescence polarization

The structure and flexibility of the biologically important alpha-spectrin amino terminal region was examined by the use of fluorescence and EPR spectroscopy. The region studied has been previously demonstrated to be essential for the alpha-spectrin:beta-spectrin association of the tetramerization site. Appropriate spectroscopic probe moieties were coupled to this region in a recombinant fragment of human erythroid alpha-spectrin. There was good agreement between the EPR and fluorescence techniques in most of this region. Mobility determinations indicated that a portion of the region was relatively immobilized. This is significant, since although predictive methods have indicated that this region should be alpha-helical, previous experimental evidence obtained on smaller synthetic peptides had indicated that this region was disordered. Observed rigidity appears to be incompatible with such a disordered state, and has important ramifications for the flexibility of this molecule that is so integral to its role in stabilizing erythrocyte membranes.