Nano-Scale Morphology of Melanosomes Revealed by Small-Angle X-Ray Scattering

Melanosomes are highly specialized organelles that produce and store the pigment melanin, thereby fulfilling essentialfunctions within their host organism. Besides having obvious cosmetic consequences – determining the color of skin, hairand the iris – they contribute to photochemical protection from ultraviolet radiation, as well as to vision (by defining howmuch light enters the eye). Though melanosomes can be beneficial for health, abnormalities in their structure can lead toadverse effects. Knowledge of their ultrastructure will be crucial to gaining insight into the mechanisms that ultimately leadto melanosome-related diseases. However, due to their small size and electron-dense content, physiologically intactmelanosomes are recalcitrant to study by common imaging techniques such as light and transmission electron microscopy.In contrast, X-ray-based methodologies offer both high spatial resolution and powerful penetrating capabilities, and thusare well suited to study the ultrastructure of electron-dense organelles in their natural, hydrated form. Here, we report onthe application of small-angle X-ray scattering – a method effective in determining the three-dimensional structures ofbiomolecules – to whole, hydrated murine melanosomes. The use of complementary information from the scattering signalof a large ensemble of suspended organelles and from single, vitrified specimens revealed a melanosomal sub-structurewhose surface and bulk properties differ in two commonly used inbred strains of laboratory mice. Whereas melanosomes inC57BL/6J mice have a well-defined surface and are densely packed with 40-nm units, their counterparts in DBA/2J micefeature a rough surface, are more granular and consist of 60-nm building blocks. The fact that these strains have differentcoat colors and distinct susceptibilities to pigment-related eye disease suggest that these differences in size and packing areof biological significance