The SNARE-associated protein Sft2 functions in Imh1-mediated SNARE recycling transport upon ER stress.

Vesicular trafficking involving SNARE proteins play a crucial role in the delivery of cargo to the target membrane. Arf-like protein 1 (Arl1) is an important regulator of the endosomal trans-Golgi network (TGN) and secretory trafficking. In yeast, ER stress-enhances Arl1 activation and Golgin Imh1 recruitment to the late-Golgi. Although Arl1 and Imh1 are critical for GARP-mediated endosomal SNARE-recycling transport in response to ER stress, their downstream effectors are unknown. Here, we report that the SNARE-associated protein Sft2 acts downstream of the Arl1-Imh1 axis to regulate SNARE recycling upon ER stress. We first demonstrated that Sft2 is required for Tlg1/Snc1 SNARE-recycling transport under tunicamycin-induced ER stress. Interestingly, we found that Imh1 regulates Tlg2 retrograde transport to the late-Golgi under ER stress, which in turn is required for Sft2 targeting to the late-Golgi. We further showed that Sft2 with 40 amino acids deleted from the N-terminus exhibits defective mediation of SNARE recycling and decreased association with Tlg1 under ER stress. Finally, we demonstrated that Sft2 is required for GARP-dependent endosome-to-Golgi transport in the absence of Rab protein Ypt6. This study highlights Sft2 as a critical downstream effector of the Arl1-Imh1 axis, mediating the endosome-to-Golgi transport of SNAREs.

Golgin Imh1 and GARP complex cooperate to restore the impaired SNARE recycling transport induced by ER stress.

The accumulation of misfolded proteins in the endoplasmic reticulum (ER) induces the unfolded protein response (UPR), which acts through various mechanisms to reduce ER stress. While the UPR has been well studied for its effects on the ER, its impact on the Golgi is less understood. The Golgi complex receives transport vesicles from the endosome through two types of tethering factors: long coiled-coil golgin and the multisubunit Golgi-associated retrograde protein (GARP) complex. Here, we report that ER stress increases the phosphorylation of golgin Imh1 to maintain the GARP-mediated recycling of the SNAREs Snc1 and Tlg1. We also identify a specific function of the Golgi affected by ER stress and elucidate a homeostatic response to restore this function, which involves both an Ire1-dependent and a MAP kinase Slt2/ERK2-dependent mechanism. Furthermore, our findings advance a general understanding of how two different types of tethers act cooperatively to mediate a transport pathway.

Action of Arl1 GTPase and golgin Imh1 in Ypt6-independent retrograde transport from endosomes to the trans-Golgi network.

The Arf and Rab/Ypt GTPases coordinately regulate membrane traffic and organelle structure by regulating vesicle formation and fusion. Ample evidence has indicated that proteins in these two families may function in parallel or complementarily; however, the manner in which Arf and Rab/Ypt proteins perform interchangeable functions remains unclear. In this study, we report that a Golgi-localized Arf, Arl1, could suppress Ypt6 dysfunction via its effector golgin, Imh1, but not via the lipid flippase Drs2. Ypt6 is critical for the retrograde transport of vesicles from endosomes to the trans-Golgi network (TGN), and its mutation leads to severe protein mislocalization and growth defects. We first overexpress the components of the Arl3-Syt1-Arl1-Imh1 cascade and show that only Arl1 and Imh1 can restore endosome-to-TGN trafficking in ypt6-deleted cells. Interestingly, increased abundance of Arl1 or Imh1 restores localization of the tethering factor Golgi associated retrograde-protein (GARP) complex to the TGN in the absence of Ypt6. We further show that the N-terminal domain of Imh1 is critical for restoring GARP localization and endosome-to-TGN transport in ypt6-deleted cells. Together, our results reveal the mechanism by which Arl1-Imh1 facilitates the recruitment of GARP to the TGN and compensates for the endosome-to-TGN trafficking defects in dysfunctional Ypt6 conditions.