Stabilization of the Spectrin-Like Domains of Nesprin-1 alpha by the Evolutionarily Conserved Adaptive Domain

Nesprins are located at the outer and inner membranes of the nuclear envelope and help link the cytoskeleton to the nucleoskeleton. Nesprin-1 alpha, located at the inner nuclear membrane, binds to A-type lamins and emerin and has homology to spectrin-repeat proteins. However, the mechanical and thermodynamic properties of the spectrin-like repeats (SLRs) of nesprin-1 alpha and the potential structural contributions of the unique central domain were untested. In other spectrin superfamily proteins, tandem spectrin-repeat domains undergo cooperatively coupled folding and unfolding. We hypothesized that the large central domain, which interrupts SLRs and is conserved in other nesprin isoforms, might confer unique structural properties. To test this model we measured the thermal unfolding of nesprin-1 alpha fragments using circular dichroism and dynamic light scattering. The SLRs in nesprin-1 alpha were found to have structural and thermodynamic properties typical of spectrins. The central domain had relatively little secondary structure as an isolated fragment, but significantly stabilized larger SLR-containing molecules by increasing their overall helicity, thermal stability and cooperativity of folding. We suggest this domain, now termed the adaptive domain (AD), also strengthens dimerization and inhibits unfolding. Further engineering of the isolated AD, and AD-containing nesprin molecules, may yield new information about the higher-order association of cooperative protein motifs.