Stabilization of the α-Helical Coiled-coil Domain in Laminin by C-terminal Disulfide Bonds

Abstract

The long arm of laminin in which three polypeptide chains α, β, and γ are assembled in an α-helical coiled-coil structure is stabilized by non-covalent interactions and disulfide bridges. The stabilizing role of the disulfide linkage between the β and γ-chains at the C-terminal region of the assembly domain was investigated with about 100-residue long recombinant fragments. Circular dichroism spectra and electron micrographs were identical for linked and non-linked species and indicated two-stranded coiled-coil structures with about 100% α-helicity at 20 °C. Thermal transition profiles revealed an increase of the melting temperature from 42 °C to 60.4 °C upon disulfide formation at a chain concentration of 25 μM. The enthalpy of interaction was identical for the two species but the negative entropy involved in joining the two chains was reduced by the disulfide bonds. At chain concentrations of 10 μM the Gibbs free energy ΔG was by 17.5 kj/mol more negative for the disulfide-linked than for the unlinked chains. Because of the concentration dependence of the entropy of the non-linked chains, this difference decreased with increasing concentration and, by extrapolation at chain concentrations of 10 mM, the stability of both structures would be the same. As a competing reaction, β-chains associated to four-stranded bundles which probably consist of pairs of two-stranded coiled-coils. After disulfide formation a biphasic transition curve was observed which indicated two different ways of connecting the chains in the bundle.f2f2Abbreviations used: CD, circular dichroism; ER, endoplasmic reticulum.Present address: Per Antonsson, Pharmacia AB, Oncology Immunology, Scheelevägen 22, S-22363 Lund, Sweden.