Segregation of COPI-rich and anterograde-cargo-rich domains in endoplasmic-reticulum-to-Golgi transport complexes

Membrane traffic between the endoplasmic reticulum (ER) and the Golgi compl ex is regulated by two vesicular coat complexes, COPII and COPI, COPII has been implicated in the selective packaging of anterograde cargo into coated transport vesicles budding from the ER [1]. In mammalian cells, these vesi cles coalesce to form tubulo-vesicular transport complexes (TCs), which shu ttle anterograde cargo from the ER to the Golgi complex [2-4], In contrast, COPI-coated vesicles are proposed to mediate recycling of proteins from th e Golgi complex to the ER [1,5-7], The binding of COPI to COPII-coated TCs [3,8,9], however, has led to the proposal that COPI binds to TCs and specif ically packages recycling proteins into retrograde vesicles for return to t he ER [3,9], To test this hypothesis, we tracked fluorescently tagged COPI and anterograde-transport markers simultaneously in living cells. COPI pred ominated on TCs shuttling anterograde cargo to the Golgi complex and was ra rely observed on structures moving in directions consistent with retrograde transport. Furthermore, a progressive segregation of COPI-rich domains and anterograde-cargo-rich domains was observed in the TCs, This segregation a nd the directed motility of COPI-containing TCs were inhibited by antibodie s that blocked COPI function. These observations, which are consistent with previous biochemical data [2,9], suggest a role for COPI within TCs en rou te to the Golgi complex. By sequestering retrograde cargo in the anterograd e-directed TCs, COPI couples the sorting of ER recycling proteins [10] to t he transport of anterograde cargo.