Figure 12.

Mixing versus absorption models for Golgi membrane redistribution into the ER in BFA-treated cells. The Poissonian distributed Golgi lifetimes after addition of BFA (Figs. 7 and 8) suggests Golgi redistribution into the ER is initiated by a discrete event, possibly fusion of a single Golgi tubule with the ER. After fusion, redistribution could occur by diffusive mixing of Golgi and ER membranes (bottom left) or by unidirectional absorption of Golgi membrane into the ER (bottom right). Since no significant pool of Golgi lipid or protein remained localized to the Golgi area after Golgi blinkout, the absorption model is favored. Kinetic analysis of Golgi redistribution (Fig. 11) further revealed fluorescent transfer of Golgi protein into and across the ER was too fast to be explained by lateral diffusion within a bilayer and had the characteristics of membrane flow. This suggests Golgi membrane absorption into the ER in BFA-treated cells is mediated by a tension-driven membrane flow. Blue, ER membrane; yellow, Golgi membrane; green, mixed ER–Golgi membranes.