A Fused α-β “Mini-spectrin” Mimics the Intact Erythrocyte Spectrin Head-to-head Tetramer

Head-to-head assembly of two spectrin heterodimers to form an actin-cross-linking tetramer is a physiologically dynamic interaction that contributes to red cell membrane integrity. Recombinant β-spectrin C-terminal and α-spectrin N-terminal peptides can form tetramer-like univalent complexes, but they cannot evaluate effects of the open-closed dimer interactions or lateral associations of the two-spectrin strands on tetramer formation. In this study we produced and characterized a fused “mini-spectrin dimer” containing the β-spectrin C-terminal region linked to the α-spectrin N-terminal region. This fused mini-spectrin mimics structural and functional properties of intact, full-length dimers and tetramers, including lateral association of the α and β subunits in the dimer and formation of a closed dimer. High performance liquid chromatography gel filtration analyses of this mini-spectrin provide the first direct non-imaging experimental evidence for open and closed spectrin dimers and show that dimer-tetramer-oligomer interconversion is slow at low temperatures and accelerated at 30 °C, analogous to full-length spectrin. This protein exhibits wild type dimer-tetramer dissociation constants of ∼1 μm at 30 °C, independent of initial oligomeric state. Conformational states of the mini-spectrin dimer were probed further using chemical cross-linking, which identified distinct groups of cross-links for “open” and “closed” dimers and confirmed the N-terminal region of α-spectrin remains highly flexible in the complex, exhibiting closely analogous structures to those observed for the isolated α-spectrin N-terminal using NMR (Park, S., Caffrey, M. S., Johnson, M. E., and Fung, L. W. (2003) J. Biol. Chem. 278, 21837–21844). This fusion protein should serve as a useful template for structural and functional studies of the divalent tetramer site. Head-to-head assembly of two spectrin heterodimers to form an actin-cross-linking tetramer is a physiologically dynamic interaction that contributes to red cell membrane integrity. Recombinant β-spectrin C-terminal and α-spectrin N-terminal peptides can form tetramer-like univalent complexes, but they cannot evaluate effects of the open-closed dimer interactions or lateral associations of the two-spectrin strands on tetramer formation. In this study we produced and characterized a fused “mini-spectrin dimer” containing the β-spectrin C-terminal region linked to the α-spectrin N-terminal region. This fused mini-spectrin mimics structural and functional properties of intact, full-length dimers and tetramers, including lateral association of the α and β subunits in the dimer and formation of a closed dimer. High performance liquid chromatography gel filtration analyses of this mini-spectrin provide the first direct non-imaging experimental evidence for open and closed spectrin dimers and show that dimer-tetramer-oligomer interconversion is slow at low temperatures and accelerated at 30 °C, analogous to full-length spectrin. This protein exhibits wild type dimer-tetramer dissociation constants of ∼1 μm at 30 °C, independent of initial oligomeric state. Conformational states of the mini-spectrin dimer were probed further using chemical cross-linking, which identified distinct groups of cross-links for “open” and “closed” dimers and confirmed the N-terminal region of α-spectrin remains highly flexible in the complex, exhibiting closely analogous structures to those observed for the isolated α-spectrin N-terminal using NMR (Park, S., Caffrey, M. S., Johnson, M. E., and Fung, L. W. (2003) J. Biol. Chem. 278, 21837–21844). This fusion protein should serve as a useful template for structural and functional studies of the divalent tetramer site.