Thermal stabilities of brain spectrin and the constituent repeats of subunits

The different genes that encode mammalian spectrins give rise to proteins differing in their apparent stiffness. To explore this, we have compared the thermal stabilities of the structural repeats of brain spectrin subunits (αII and βII) with those of erythrocyte spectrin (αI and βI). The unfolding transition midpoints (Tm) of the 36 αII- and βII-spectrin repeats extend between 24 and 82 °C, with an average higher by some 10 °C than that of the αI- and βI-spectrin repeats. This difference is reflected in the Tm values of the intact brain and erythrocyte spectrins. Two of three tandem-repeat constructs from brain spectrin exhibited strong cooperative coupling, with elevation of the Tm of the less stable partner corresponding to coupling free energies of approximately −4.4 and −3.5 kcal/mol. The third tandem-repeat construct, by contrast, exhibited negligible cooperativity. Tandem-repeat mutants, in which a part of the “linker” helix that connects the two domains was replaced with a corresponding helical se...