Postulated arrangement of components that influence COPII assembly and tether ERES to post-ER membranes. (left) Formation of a COPII vesicle by conserved proteins. The ER and ERGIC/Golgi membranes are shown in gray. The COPII coat consists of the purple and green components on the membrane surface. At the base of the budding vesicle, Sec16 suppresses GTP hydrolysis by Sar1 to create a stabilizing ring of Sar1-GTP. In some organisms, Sec16 also binds Sec12 to increase the local concentration of Sar1-GTP. The nascent vesicle is linked to Golgi membranes by conserved tethers. (right) Formation of a large COPII-coated carrier with the aid of TANGO1/cTAGE5. The TANGO1/cTAGE5 complex cooperates with Sec16 to stabilize the COPII lattice and to alter its geometry. cTAGE5 also binds Sec12, whereas TANGO1 binds ERGIC or Golgi membranes. The luminal domain of TANGO1 has additional interactions that are not depicted. Drosophila contains TANGO1 but not cTAGE5. See the text for details.