Recycling of cell surface membrane proteins from yeast endosomes is regulated by ubiquitinated Ist1

Abstract

Upon internalization, many surface membrane proteins are recycled back to the plasma membrane. Although these endosomal trafficking pathways control surface protein activity, the precise regulatory features and division of labor between interconnected pathways are poorly defined. In yeast, we show recycling back to the surface occurs through distinct pathways. In addition to retrograde recycling pathways via the late Golgi, used by synaptobrevins and driven by cargo ubiquitination, we find nutrient transporter recycling bypasses the Golgi in a pathway driven by cargo deubiquitination. Nutrient transporters rapidly internalize to, and recycle from, endosomes marked by the ESCRT-III associated factor Ist1. This compartment serves as both “early” and “recycling” endosome. We show Ist1 is ubiquitinated and that this is required for proper endosomal recruitment and cargo recycling to the surface. Additionally, the essential ATPase Cdc48 and its adaptor Npl4 are required for recycling, potentially through regulation of ubiquitinated Ist1. This collectively suggests mechanistic features of recycling from endosomes to the plasma membrane are conserved.

A second chance at yeast early endosomes

“…They fuse a deubiquitinating enzyme (DUb) to these cargos, which helps define the impact of ubiquitination on trafficking by removing this signal. As expected for the synaptobrevins, ubiquitination is required for Golgi-to-PM recycling and Snc-DUbs are TGN retained, as defined by colocalization with the Arf guanine nucleotide exchange factor Sec7 ( 5 ). In contrast, Mup1- and Fur4-DUbs are PM retained, even in the presence of a substrate that typically triggers endocytosis.…”

“…Time-lapse imaging of cells in a microfluidics chamber allows tracking of Mup1 in response to environmental change ( 5 ). A methionine pulse induces Mup1 endocytosis, and within 2 min Mup1 forms intracellular foci.…”

“…SEs, marked by the inverse-Bin-Amphiphysin-Rvs protein, Ivy1, contain the pool of TORC1 that regulates autophagy ( 3 , 4 ). In this issue, elegant studies by Laidlaw and coauthors ( 5 ) provide evidence for yeast EEs—distinct from LEs or the Golgi—required for post-endocytic recycling of nutrient transporters ( Fig. 1 ).…”

“…Laidlaw and colleagues ( 5 ) first establish clear differences in the trafficking itineraries of the constitutively endocytosed synaptobrevins, Snc1 and Snc2, and the nutrient permeases for methionine and uracil (Mup1 and Fur4, respectively), whose endocytosis is substrate induced ( Fig. 1 ).…”

“…In contrast, Mup1- and Fur4-DUbs are PM retained, even in the presence of a substrate that typically triggers endocytosis. In contrast, a short pulse of substrate induces wild-type nutrient permease endocytosis, resulting in Mup1 and Fur4 colocalization with Vps4, an AAA-ATPase and endosomal marker, but not the Sec7-marked TGN ( 5 ). These findings highlight trafficking differences between synaptobrevins and nutrient permeases.…”