Figure 5

Two-phase origin of the nuclear envelope and trans-envelope transport. (a). Nucleoporins (Nups) forming the octagonal cylindrical scaffold evolved by duplications of coat proteins of COPII secretory vesicles with α-solenoid and/or β-propeller domains, being attached by integral membrane Nups descended from actinobacterial membrane proteins; (b). NPC lumens were narrowed by plugs of FG-repeat-rich Nups, which form a dynamic gel-like meshwork that prevents passive diffusion of macromolecular complexes and mediates active specifically-targeted nucleocytoplasmic exchange by carrier complexes, typically consisting of large karyopherin proteins and their cargo either bound directly or by adaptors. (c). Phase I surface view, showing complete Ran GTPase-mediated fusion of RER cisternae prevented by COPII coat proteins (black blobs) remaining in place to become octagonal NPC scaffolds.