Lumenal targeted GFP, used as a marker of soluble cargo, visualises rapid ERGIC to Golgi traffic by a tubulo-vesicular network

The mechanism by which soluble proteins without sorting motifs are transpor ted to the cell surface is not clear. Here we show that soluble green fluor escent protein (GFP) targeted to the lumen of the endoplasmic reticulum but lacking any known retrieval, retention or targeting motifs, was accumulate d in the lumen of the ERGIC if cells were kept at reduced temperature. Upon activation of anterograde transport by rewarming of cells, lumenal GFP sta ined a microtubule-dependent, pre-Golgi tubulovesicular network that served as transport structure between peripheral ERGIC-elements and the perinucle ar Golgi complex. Individual examples of these tubular elements up to 20 mu m in length were observed. Time lapse imaging indicated rapid anterograde flow of soluble lumenal GFP through this network. Transport tubules, staine d by lumenal GFP, segregated rapidly from COPI-positive membranes after tra nsport activation. A transmembrane cargo marker, the temperature sensitive glycoprotein of the vesicular stomatitis virus, ts-045 G, is also not prese nt in tubules which contained the soluble cargo marker lum-GFP. These resul ts suggest a role for pre-Golgi vesicular tubular membranes in long distanc e anterograde transport of soluble cargo.