Interdigitating coiled-coil tetramers define the helical architecture of GFAP filaments in astrocytes

GFAP filaments are core elements of the cytoskeleton in astrocytes, shaping cellular processes and influencing central nervous system function and pathology. How these filaments are built at the molecular level has remained unclear. Using cryo-electron microscopy and cryo-electron tomography, we show that GFAP filaments are helical polymers assembled from extended and twisted coiled-coil tetramers that interdigitate to form a complex filament tube. In the filament lumen, the low-complexity head domains aggregate to form a flexible fibre, while the tail domains extend from the surface and facilitate higher-order bundling of the filaments, indicating that both the low-complexity structural regions and the well-ordered coiled-coils of the filament tube contribute to the unique structure of GFAP filaments. Our findings provide molecular insights into the pathogenesis of Alexander Disease, localising multiple deleterious mutations to the critical interlock interaction between successive tetramers emergent in the final filament assembly.