An acidic sequence of a putative yeast Golgi membrane protein binds COPII and facilitates ER export

Open Access
Publisher Website
Google Scholar
Abstract
We previously identified Sys1p as a high copy number suppressor of Ypt6 GTPase‐deficient yeast mutants that are defective in endosome‐to‐Golgi transport. Here, we show that Sys1p is an integral membrane protein that resides on a post‐endoplasmic reticulum (ER) organelle(s). Affinity studies with detergent‐ solubilized yeast proteins showed that the C‐terminal 53 amino acid tail of Sys1p binds effectively to the cytoplasmic Sec23p–Sec24p COPII subcomplex. This binding required a di‐acidic Asp‐Leu‐Glu (DXE) motif, previously shown to mediate efficient ER export of the vesicular stomatitis virus glycoprotein in mammalian cells. In Sys1p, a Glu‐Leu‐Glu (EXE) sequence could not substitute for the (DXE) motif. Mutations of the (DXE) sequence resulted in ER retention of ∼30% of the protein at steady state, whereas addition of the Sys1p tail to an ER‐resident membrane protein led to an intracellular redistribution of the chimeric protein. Our study demonstrates for the first time that, in yeast, a di‐acidic sequence motif can act as a sorting signal for cargo selection during the formation of transport vesicles at the ER by direct binding to COPII component(s).