A di-acidic (DXE) code directs concentration of cargo during export from the endoplasmic reticulum

Efficient export of vesicular stomatitis virus glycoprotein (VSV-G), a type I transmembrane protein, from the endoplasmic reticulum requires a di-acidic code (DXE) located in the cytosolic carboxyl-terminal tail (Nishimura, N., and Balch, W. E. (1997) Science 277, 556-558). Mutation of the DXE code by mutation to AXA did not prevent VSV-G recruitment to pre-budding complexes formed in the presence of the activated form of the Sar1 and the Sec23/24 complex, components of the COPII budding machinery. However, the signal was required at a subsequent concentration step preceding vesicle fission. By using green fluorescence protein-tagged VSV-G to image movement in a single cell, we found that VSV-G lacking the DXE code fails to be concentrated into COPII vesicles. As a result, the normal 5-10-fold increase in the steady-state concentration of VSV-G in downstream pre-Golgi intermediates and Golgi compartments was lost. These results demonstrate for the first time that inactivation of the DXE signal uncouples early cargo selection steps from concentration into COPII vesicles. We propose that two sequential steps are required for efficient export from the endoplasmic reticulum.