Fig. 3: Flublok structural analysis by cryo-electron tomography and molecular modeling.

a–f Example of six serial slices from top to bottom of a cryo-electron tomogram with a field of HA complexes. a, b White arrows indicate the ___location of three individual starfish complexes made of constituent HA molecules. c View in which the slice of the central complex (white brackets) presents a starfish arrangement of its HA molecules. g–h Two different molecular models of specific complexes with trimeric HA ectodomain coordinates, with red denoting the head and blue denoting the stem, docked into the 3D tomogram shown as a transparent isosurface. i A schematic of a HA-starfish structure illustrates how a central hydrophobic core (gray) contains HA transmembrane domains and lipid, and HA proteins point outward exposing the HA head colored in red, and the HA stem colored in blue. j Distance and angular measurements between HA-trimer bases, according to HA ectodomains docked into tomograms of the HA starfish complexes. The plot is segmented into colored regions according to Monte Carlo simulations of HA-stem clashes: no steric clash (blue), all steric clash (yellow), and conditional steric clash depending upon the interdigitation of Fabs (green). k HA-trimer distances between the central axis of head domains in docked complexes were tabulated and plotted as a histogram. The arrow on the x-axis denotes the threshold distance of 16 nm under which bivalent binding is possible. Scale bars are 10 nm for panels a–f, and 5 nm for panels g and h. Flublok cryo samples were collected at least two times and starfish morphology was consistent.