ER-TO-GOLGI TRANSPORT VISUALIZED IN LIVING CELLS

Newly synthesized proteins that leave the endoplasmic reticulum (ER) a re funnelled through the Golgi complex before being sorted for transpo rt to their different final destinations. Traditional approaches have elucidated the biochemical requirements for such transport(1-3) and ha ve established a role for transport intermediates(4-8). New techniques for tagging proteins fluorescently(9,10) have made it possible to fol low the complete life history of single transport intermediates in liv ing cells, including their formation, path and velocity en route to th e Golgi complex. We have now visualized ER-to-Golgi transport using th e viral. glycoprotein ts045 VSVG tagged with green fluorescent protein (VSVG-GFP). Upon export from the ER, VSVG-GFP became concentrated in many differently shaped, rapidly forming pre-Golgi structures, which t ranslocated inwards towards the Golgi complex along microtubules by us ing the microtubule minus-end-directed motor complex of dynein/dynacti n. No loss of fluorescent material from pre-Golgi structures occurred during their translocation to the Golgi complex and they frequently st retched into tubular shapes. Together, our results indicate that these pre-Golgi carrier structures moving unidirectionally along microtubul e tracks are responsible for transporting VSVG-GFP through the cytopla sm to the Golgi complex. This contrasts with the traditional focus on small vesicles as the primary vehicles for ER-to-Golgi transport.