Figure 7. Inner membrane invaginations and membrane-associated complexes at the chloroplast base-lobe junction.
(A–B) Two sequential slices through a tomogram acquired with a direct electron detector, showing a junction between the base and the side (lobe) of the chloroplast cup. At the bend in the chloroplast envelope, an array of small complexes (arrow) is associated with the cytoplasmic surface of the envelope's outer membrane, between two invaginations in the inner membrane. The cytoplasm (cy), chloroplast envelope (ce), starch granules (sg), and thylakoids (th) are labeled. (C–E) Three sequential slices through the same tomographic volume, showing a magnified view of the chloroplast envelope at the base-lobe junction. D and E correspond to the views in A and B, respectively. Separate halves of lipid bilayers were fairly well resolved along relatively flat membranes. A thylakoid tip (C) bends ∼90° to point towards the chloroplast envelope and (E) nearly contacts a large inner membrane invagination. Small complexes (arrows) are clustered in an array on the outer membrane and are also associated with the inner membrane invagination close to the thylakoid tip. These particles each have a diameter of ∼6 nm. For comparison, the diameter of the F1 ATP synthase subunit is ∼9 nm and the diameter of the fully assembled RuBisCO complex is ∼12 nm (note that the RuBisCO large subunit and six ATP synthase components are encoded by the chloroplast genome and thus, are not imported). The tomogram was 3× binned in A–B and 2× binned in C–E. Unbinned pixel size: 3.4 Å. Scale bars: 100 nm.