Anterograde flow of cargo across the Golgi stack potentially mediated via bidirectional "percolating" COPI vesicles

How do secretory proteins and other cargo targeted to post-Golgi locations traverse the Golgi stack? We report immunoelectron microscopy experiments e stablishing that a Golgi-restricted SNARE, GOS 28, is present in the same p opulation of COPI vesicles as anterograde cargo marked by vesicular stomati tis virus glycoprotein, but is excluded from the COPI vesicles containing r etrograde-targeted cargo (marked by KDEL receptor), We also report that GOS 28 and its partnering t-SNARE heavy chain, syntaxin 5, reside together in every cisterna of the stack. Taken together, these data raise the possibili ty that the anterograde cargo-laden COPI vesicles, retained locally by mean s of tethers, are inherently capable of fusing with neighboring cisternae o n either side. If so, quanta of exported proteins would transit the stack i n COS 28-COPI vesicles via a bidirectional random walk, entering at the cis face and leaving at the trans face and percolating up and down the stack i n between. Percolating vesicles carrying both post-Golgi cargo and Golgi re sidents up and down the stack would reconcile disparate observations on Gol gi transport in cells and in cell-free systems.