COLLOIDAL STRUCTURE AND PROTON CONDUCTIVITY OF THE GEL WITHIN THE ELECTROSENSORY ORGANS OF CARTILAGINOUS FISHES

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Highlights • SEM and AFM of the gel from ratfish electrosensory organs reveal a globular structure • Analysis of the gel by SAXS suggests a distinct, colloidal substructure • High proton conductance was observed in the gel, comparable with that of sharks and skates • Proteolysis breaks up large-scale aggregates but has little effect on proton conductance Summary Cartilaginous fishes possess gel-filled tubular sensory organs called Ampullae of Lorenzini (AoL) that are used to detect electric fields. Although recent studies have identified various components of AoL gel, it has remained unclear how the molecules are structurally arranged and how their structure influences the function of the organs. Here we describe the structure of AoL gel by microscopy and small-angle X-ray scattering and infer that the material is colloidal in nature. To assess the relative function of the gel’s protein constituents, we compared the microscopic structure, X-ray scattering, and proton conductivity properties of the gel before and after enzymatic digestion with a protease. We discovered that while proteins were largely responsible for conferring the viscous nature of the gel, their removal did not diminish proton conductivity. The findings lay the groundwork for more detailed studies into the specific interactions of molecules inside AoL gel at the nanoscale.