Fig. 2: Cryo-EM structures of SPR phage tail tube.
From: Activation of the bacterial defense-associated sirtuin system
a Upper, representative cryo-EM micrograph of SPR tail tube formed by self-assembled SPR tail tube proteins (TTP); scale bar = 50 nm. Lower, various 2D class averages of SPR tail tube. Red dots indicate the curvature of SPR tail tube. b Cryo-EM reconstructions of SPR tail tube colored by chains; six TTP chains form a hexameric ring and three layers of TTP rings are shown. Top: side view, bottom: top view. c Ribbon model of SPR tail tube; top: side view, bottom: top view. d Cut-away view of SPR tail tube with an ideal B-form DNA double helix (red ribbon) modeled in the central tunnel. Top: side view, bottom: top view. e Cut-away view of SPR tail tube. The surface electrostatic potential is plotted as a red-to-blue gradient from electronegative to electropositive. f Upper, domain organization of SPR tail tube protein (TTP), the outer β-plane of D1 is colored blue, the inner β-plane of D1 is colored red, the a1 helix of D1 is colored cyan; D2 is colored magenta. Lower, ribbon model of SPR TTP with labeled secondary structures. Four important β-harpins of D1 domain (β-h1, β-h2, β-h3 and β-h4) are indicated. g Top view of a TTP hexameric ring colored by inner β-barrel (inner tube layer, red), outer β-planes (blue) and α1 helix (cyan) wedged between them. The six D2 domains (magenta) form decorations that protrude from the tube body. h Top, SPR TTP ribbon model colored by domains, D1 blue, D2 magenta. Key TTP-TTP interactions within the hexameric ring are indicated with black dots. β-sheets involved in the interactions are labeled. Bottom, side view of a SPR tail tube with the central TTP hexameric ring shown in ribbon model; the ring stacked above (Ring +1) and below are shown with semitransparent molecular surface (gray). Four hairpins (β-h1-4) extending from the D1 domain mediate ring-ring interactions.
