Fig. 1: CryoEM structure of ex vivo cannulae from the hyperthermophilic archaeon Pyrodictium abyssi AV2.

a cryoEM volume revealing a two-start (C2) helical filament with a pitch p and diameter of 94.6 Å and 260 Å, and a rise and twist of 3.18 Å and 12.1°, respectively; b local interaction network between neighboring cannula protofilaments in which subunits interlock through donor strand complementation along the axial (e.g., i → j → k) direction and partake in further intermolecular contacts mediated by calcium coordination along the axial and radial (e.g., j-1→j → j + 1) direction; c stick representation of three types of calcium pockets: (i) axial stacking of subunits via type I and II calcium binding centers, (ii) radial contacts are mediated via type III Ca²⁺ binding sites; d raw cryoEM image of a cannulae filaments loaded with a helically winding thinner filament (2 nm; cargo). White arrows denote winding cargo and dangling cargo associated with the cannulae filaments; e surface electrostatics showing a predominantly negatively charged surface and a positively charged lumen for the cannula model. The representative image in this figure (d) is derived from a total of 2365 cryo-EM movies from two technical replicates.