  The multidomain protein gelsolin (GSN) is composed of six homologous modules,
sequentially named G1 to G6. Single point substitutions in this protein are
responsible for AGel amyloidosis, a hereditary disease characterized by
progressive corneal lattice dystrophy, cutis laxa, and polyneuropathy. Several
different amyloidogenic variants of GSN have been identified over the years,
but only the most common D187N/Y mutants, in G2, have been thoroughly
characterized, and the underlying functional mechanistic link between mutation,
altered protein structure, susceptibility to aberrant furin cleavage and
aggregative potential resolved. Little is known about the recently identified
mutations A551P, E553K and M517R hosted at the interface between G4 and G5,
whose aggregation process likely follows an alternative pathway. We demonstrate
that these three substitutions impair temperature and pressure stability of GSN
but do not increase its susceptibility to furin cleavage, the first event of
the canonical aggregation pathway. The variants are also characterized by a
higher tendency to aggregate in the unproteolysed forms and show a higher
proteotoxicity in a C. elegans-based assay. Structural studies point to a
destabilization of the interface between G4 and G5 due to three different
structural determinants: beta-strand breaking, steric hindrance and/or charge
repulsion, all implying the impairment of interdomain contacts. All available
evidence suggests that the rearrangement of the protein global architecture
triggers a furin-independent aggregation of the protein, supporting the
existence of a non-canonical pathway of gelsolin amyloidosis pathogenesis.
