Structural basis for the high activation energy of spectrin self-association

The association of spectrin hetero-dimer (αβ) to the tetramer (α 2β 2, which predominates in the cell) is marked by an exceptionally high activation energy, so that the reaction does not proceed measurably in the cold. We have tested the hypothesis that this is due to intra-dimer association between the α- and β-chain ends, which must be broken before tetramers can form. Two mutant univalent spectrins with association defects at the α and β ends, respectively, and incapable therefore of intra-dimer bonding, were found to associate rapidly with one another at 4°C. The bimolecular rate constant is greater than for the association of normal dimers by 6 orders of magnitude.