Fig. 7. Models of R-ring/NCS biogenesis from the nuclear envelope. (A) R-rings/NCS form by the invagination of the entire nuclear envelope, the outer and inner nuclear membranes, lined by an electron-dense Nopp140 matrix (gray sprinkled layer). (B) R-rings/NCS form by the exclusive invagination of the inner nuclear membrane. Integral membrane proteins of the rough (1) and the smooth ER (2) and synthesized on the outer nuclear membrane (3) can laterally diffuse through the pore membrane domain (4) and the inner nuclear membrane (5) into the R-rings. The boxed regions show how a close-up of the repeat patterns of Nopp140 matrix and intranuclear membrane cisternae would look for each model.
