Association and folding in legumin oligomers of lupin seed.

We studied the association and conformational behavior under native or denaturing conditions in the 12S in equilibrium with 7S oligomers of lupin legumin and in the modified 7S (m7S) oligomer, which has lost the capacity to make a 12S molecule. Circular dichroism (CD), gel filtration FPLC, and PAGE were used. The native m7S oligomer has more alpha helix and nearly the same amount of beta structure as the 12S in equilibrium with 7S preparation. Conditions that shift the equilibrium in the 12S in equilibrium with 7S system toward the 7S oligomer also make the secondary structure more similar to that of m7S molecules: higher negative ellipticity appears to be a peculiarity of 7S assemblies, whether they contain modified or unmodified monomers. Part of the helical components show low stability and disappear in 1 M urea. The CD and the separation behavior on increasing the urea concentration, and in 6 M guanidine HCl, denote similar multistep unfolding in both preparations. The 12S oligomer disassembles progressively: however, also under highly denaturing conditions, modified and unmodified preparations are mainly present in an associated form. Small amounts of monomer and aggregates were detected at high denaturant concentrations.