The FERA Domain Is a Membrane-Binding Four-Helix Bundle Embedded within Ferlin Membrane Fusion Proteins

Abstract

The ferlins are long, string-like proteins that bind to phospholipid membranes in the presence of Ca2+, and have been implicated in membrane fusion. Dysferlin, myoferlin, and otoferlin in particular have received considerable attention due to their impact on human health. Dysferlin is involved in membrane repair, and lack of dysferlin function in skeletal muscle could lead to progressive damage to muscle cells. Mutations in the dysferlin gene are linked to Limb-Girdle Muscular Dystrophy in humans. Myoferlin is involved in the initial formation of muscles by fusing myotubes into myocytes early in development. Otoferlin is found exclusively in the ear, and may play a synaptotagmin-like role in exocytosis. Mutations in otoferlin are linked to non-syndromic deafness in humans. We have discovered that the FerA domain, which is located at the center of each of the ferlin proteins, is a four-helix bundle that can bind to lipids in a Ca2+-dependent manner. Although the net charge of each FerA domain varies from highly negative to highly positive, each that we have studied shows a Ca2+-dependent lipid association, suggesting a role in membrane fusion. Additionally, thermodynamic analysis of the dysferlin FerA domain and two clinically-identified mutations, L705M and P731R, shows that domain stability is compromised in these mutants. Decreased stability and enhanced membrane binding of the FerA mutants suggest that an open, membrane bound form exists in addition to the closed, soluble form.