Fig. 1. Photoreceptor disc formation is an actin-driven process.

a Schematic of a rod photoreceptor depicts the cell’s inner segment (IS) with the highly modified connecting cilium (CC) extending from its apex, containing a microtubule-based axoneme (Ax). From here an expanse of folded membrane extends, forming the outer segment (OS) discs. Basal discs (BD) are continually added at its base. b A projection from a tomogram of the basal disc of a wild-type photoreceptor. c A slice through a tomographic reconstruction, segmented to highlight the ciliary and disc membranes (green), microtubule-based axoneme (cyan) and microfilaments (purple). Microfilaments extend from the connecting cilium into the basal disc. d Front and back views of a subtomogram averaged map of microfilaments extending into the BD, with helical symmetry-imposed map (e). f Previously published structure of F-actin (EMBD-6448), highlighting 166.67 degree twist per molecule and 27.8 Å rise per molecule. (Scale bars; b, c = 100 nm; d–f = 50 Å) (Similar results were seen in 6 independent experiments, each pooling results from 9 to 32 eyes (8–16 mice) and 4–12 tomograms).