Evidence for a COP-I-independent transport route from the Golgi complex tothe endoplasmic reticulum

The cytosolic coat-protein complex COP-I interacts with cytoplasmic 'retrie val' signals present in membrane proteins that cycle between the endoplasmi c reticulum (ER) and the Golgi complex, and is required for both anterograd e and retrograde transport in the secretory pathway, Here we study the role of COP-I in Golgi-to-ER transport of several distinct marker molecules. Mi croinjection of anti-COP-I antibodies inhibits retrieval of the lectin-like molecule ERGIC-53 and of the KDEL receptor from the Golgi to the ER, Trans port to the ER of protein toxins, which contain a sequence that is recogniz ed by the KDEL receptor, is also inhibited, In contrast, microinjection of anti-COP-I antibodies or expression of a GTP-restricted Arf-l mutant does n ot interfere with Golgi-to-ER transport of Shiga toxin/Shiga-like toxin-1 o r with the apparent recycling to the ER of Golgi-resident glycosylation enz ymes. Overexpression of a GDP-restricted mutant of Rab6 blocks transport to the ER of Shiga toxin/Shiga-like toxin-1 and glycosylation enzymes, but no t of ERGIC-53, the KDEL receptor or KDEL-containing toxins. These data indi cate the existence of at least two distinct pathways for Golgi-to-ER transp ort, one COP-I dependent and the other COP-I independent. The COP-I-indepen dent pathway is specifically regulated by Rab6 and is used by Golgi glycosy lation enzymes and Shiga toxin/Shiga-like toxin-1.