COPI mediates recycling of an exocytic SNARE by recognition of a ubiquitin sorting signal

Abstract

The COPI coat forms transport vesicles from the Golgi complex and plays a poorly defined role in endocytic trafficking. Here we show that COPI binds K63-linked polyubiquitin and this interaction is crucial for trafficking of a ubiquitinated yeast SNARE (Snc1). Snc1 is a v-SNARE that drives fusion of exocytic vesicles with the plasma membrane, and then recycles through the endocytic pathway to the Golgi for reuse in exocytosis. Removal of ubiquitin from Snc1, or deletion of a β'-COP subunit propeller domain that binds K63-linked polyubiquitin, disrupts Snc1 recycling causing aberrant accumulation in internal compartments. Moreover, replacement of the β'-COP propeller domain with unrelated ubiquitin-binding domains restores Snc1 recycling. These results indicate that ubiquitination, a modification well known to target membrane proteins to the lysosome or vacuole for degradation, can also function as recycling signal to sort a SNARE into COPI vesicles in a non-degradative pathway.

Toolbox: Creating a systematic database of secretory pathway proteins uncovers new cargo for COPI

“…The localization to COPI‐positive structures raises the interesting possibility that Atg9 cycles between the endosomal system and the Golgi thereby connecting different compartments recently implicated in autophagosome formation . It has also recently been published that a fraction of COPI binds polyubiquitinated cargo in endosomes for delivery to late‐Golgi . However, for technical reasons, we could not assay co‐localization of GFP‐Atg9 with the Sec7‐mCherry, Snf7‐mCherry or Vam6‐mCherry markers.…”

“…All scale bars are 5 μm for delivery to late-Golgi. 35 However, for technical reasons, we could not assay co-localization of GFP-Atg9 with the Sec7-mCherry, Snf7-mCherry or Vam6-mCherry markers.…”

A defect in thymic tolerance causes T cell-mediated autoimmunity in a murine model of COPA syndrome

“…First, the cellular mechanisms by which mutant Copa in thymic epithelial cells leads to a defect in negative selection remains unclear. Disruptions in COPI trafficking have been shown to alter endolysosomal function and macroautophagy (1,(37)(38)(39), cellular processes important in the proteolysis and loading of self-proteins onto MHC molecules (40, 41). We previously showed that cell lines derived from COPA syndrome patients demonstrate a constellation of morphological and 14 functional findings that suggest impaired macroautophagy (1).…”

Formation of COPI-coated vesicles at a glance

“…These motifs contact the propeller domain of α-or β'-COP that are ideally positioned adjacent to the surface of the membrane (Eugster et al, 2004;Jackson et al, 2012;Ma and Goldberg, 2013;Schroder-Kohne et al, 1998). A second site in the propeller domain of β'-COP also regulates recycling of the exocytic SNARE Snc1 by recognizing polyubiquitylated Snc1 (Xu et al, 2017). Arginine (R)-based ER retrieval signals (ɸRxR; in which ɸ represents any hydrophobic amino acid) are another class of sorting signals on the cytoplasmic domains of heteromultimeric proteins, such as channels and receptors (Zerangue et al, 1999(Zerangue et al, , 2001.…”