Figure 2.

Final, refined class averages of the cylindrically masked SYCP3 fibre subtomograms. (a) 2D classification of projected subvolumes reveals clear evidence of density connecting adjacent striations (marked by an arrow), consistent with the shape of SYCP3's tetrameric core structure [26]. (b) Three-dimensional isosurface rendering of the four refined class averages. The resolution of the final refined classes ranged between 27 and 30 Å as estimated by FSC analysis. The classes had nearly equal distribution and tended to split subvolumes equally regardless of the number of specified classes (electronic supplementary material, figure S5). Isosurface sections along the Y-axis and in the XZ plane are shown next to each isosurface rendering. The Y-axis sections through a three-particle region of the class averages highlight variations in conformation and spacing between individual SYCP3 particles. XZ cross-sections through the inter-striation region of all class averages show an approximately rectangular pattern of SYCP3 particles, but with a high degree of heterogeneity in particle arrangement between classes. (c) Superposition of the crystal structure of SYCP3 66–230 tetramer (PDB ID 4CPC; shown as blue ribbon) into the isosurface rendering of a refined class average. (d) Schematic drawing for the three-dimensional arrangement of SYCP3 molecules in a fibre, based on the result of the cryo-electron tomography. The association between adjacent SYCP3 molecules are mediated exclusively by their intrinsically disordered tails, yielding a highly flexible polymeric assembly.