VISUALIZATION OF ER-TO-GOLGI TRANSPORT IN LIVING CELLS REVEALS A SEQUENTIAL MODE OF ACTION FOR COPII AND COPI

Exocytic transport from the endoplasmic reticulum (ER) to the Golgi co mplex has been visualized in living cells using a chimera of the tempe rature-sensitive glycoprotein of vesicular stomatitis virus and green fluorescent protein (ts-G-GFP(ct)). Upon shifting to permissive temper ature, ts-G-GFP(ct) concentrates into COPII-positive structures close to the ER, which then build up to form an intermediate compartment or transport complex, containing ERGIC-53 and the KDEL receptor, where CO PII is replaced by COPI. These structures appear heterogenous and move in a microtubule-dependent manner toward the Golgi complex. Our resul ts suggest a sequential mode of COPII and COPI action and indicate tha t the transport complexes are ER-to-Golgi transport intermediates from which COPI may be involved in recycling material to the ER.