Figure 6.

Implications of the diffusion–reaction model for the stability of the COPI coat during budding. (A) With no lateral diffusion of Arf1 a mature COPI-coated bud should have lost most of its Arf1-GTP molecules through the action of ArfGAP1, which acts in regions of high positive curvature. This could compromise coat stability. (B) Alternatively, new Arf1-GTP molecules diffusing from the Golgi cisternae could constantly replace those that are leaving the bud on GTP hydrolysis, thereby stabilizing the coat. Once membrane fission occurs, the flux of Arf1-GTP necessarily stops and GTP hydrolysis on Arf1 leads to coat disassembly from the vesicle. (C) The black line of the plot reports pair values of k and D that leads to an Arf1 gradient with a characteristic length L=50 nm. The bold coloured lines indicate the values of k and D given by the experiments shown in Figure 5. The yellow area shows the range of reported values for the rate constant (k) of GTP hydrolysis on Arf1 catalysed by ArfGAP1. The blue area describes the range of D-values that would prevent the formation a sharp Arf1-GTP gradient in a curved region of the size of a COPI-coated bud when this region is connected to a flat membrane serving as a reservoir of Arf1-GTP.