Fig. 4.

Left-handed helical junctions bridge between the right-handed helices and stromal sheets or adjacent right-handed helices. (A, H, and I) Three-dimensional models generated from segmentation of tomographic reconstructions by STEM, showing left-handed helical junctions (purple) connecting between the stroma lamellar sheets (green) and the right-handed helices (light blue) that surround the grana (yellow) (A and I), or between right-handed helices that belong to adjacent grana (H). The left-handed helical structure shown in A is isolated and shown from side and top views (B, Inset). (C–G) Tomographic sequential slices ∼10 nm thick, which were segmented and rendered to obtain the model in A, allow tracking the left-handed helical junction from front to back (a portion is marked with white arrowheads). White arrows in B, E, and H mark the elongated channel formed by the left-handed helical junction. (A and I, Inset) Contact sites of the bifurcations with the helices (marked with multiple white arrows) are vertically displaced from one another by ∼50 nm. One such bifurcation is segmented orange in I, Inset. All elements in A and I, aside from the left-handed helical junctions, are shown in transparency. Additional modeled assemblies from TEM and FIB-SEM are provided in SI Appendix, Fig. S5. (Scale bars, 250 nm.)