The ADP-ribosylation factor GTPase-activating protein GIo3p is involved in ER retrieval

Retrograde transport of proteins from the Golgi to the endoplasmic reticulum (ER) has been the subject of some interest in the recent past. Here a new thermosensitive yeast mutant defective in retrieval of dilysine-tagged proteins from the Golgi back to the endoplasmic reticulum was characterized. The ret4-1 mutant also exhibited a selective defect in forward ER-to-Golgi transport of some secreted proteins at the nonpermissive temperature. The corresponding RET4 gene was found to encode GIo3p, a GTPase-activating protein (GAP) specific for ADP-ribosylation factor (ARF). In vitro, the Glo3 thermosensitive mutant showed a reduced ARFI-GAP activity. The GIo3 protein belongs to a family of zinc finger proteins that may include additional ARF-GAPs. Gene deletion experiments of other family members showed that only GL03 deletion resulted in impaired retrieval of dilysine-tagged proteins back to the ER. These results demonstrate that Glo3p is the main ARF-GAP specifically involved in ER retrieval.