Figure 11.

Model showing arrangement of tubular networks in relation to RER. The model was adapted from the “priviledged site budding” model by Kuehn and Schekman (1997). Expression of E1 causes the proliferation of a smooth subdomain of the ER from which COPII vesicles are derived. Access of most lumenal ER and membrane proteins into the tubular networks is limited by gating proteins. The concentration of these proteins is highest in the RER denoted by the presence of ribosomes (black circles). Only cargo and proteins that have satisfied the requirements of the ER quality control system are allowed access into this region. Proteins involved in biogenesis of COPII transport vesicles accumulate in these smooth regions of the ER. Although E1 is allowed access into the smooth membranes, it is somehow prevented from entering the transport vesicles. Newly formed transport vesicles are also seen in the vicinity of these membrane nests.