Distinct N-terminal regions of the exomer secretory vesicle cargo Chs3 regulate its trafficking itinerary.

Amanda M Weiskoff,J Christopher Fromme

doi:10.3389/fcell.2014.00047

Abstract

Cells transport integral membrane proteins between organelles by sorting them into vesicles. Cargo adaptors act to recognize sorting signals in transmembrane cargos and to interact with coat complexes that aid in vesicle biogenesis. No coat proteins have yet been identified that generate secretory vesicles from the trans-Golgi network (TGN) to the plasma membrane, but the exomer complex has been identified as a cargo adaptor complex that mediates transport of several proteins in this pathway. Chs3, the most well-studied exomer cargo, cycles between the TGN and the plasma membrane in synchrony with the cell cycle, providing an opportunity to study regulation of proteins that cycle in response to signaling. Here we show that different segments of the Chs3 N-terminus mediate distinct trafficking steps. Residues 10–27, known to mediate retention, also appear to play a role in internalization. Residues 28–52 are involved in transport to the plasma membrane and recycling out of endosomes to prevent degradation in the vacuole. We also present the crystal structure of residues 10–27 bound to the exomer complex, suggesting different cargo adaptors could compete for binding to this segment, providing a potential mechanism for regulation.

Highlights

Eukaryotic cells must transport transmembrane proteins between different subcellular compartments, often in response to specific signals or conditions
No coat complexes are known to mediate transport directly from the trans-Golgi network (TGN) to the apical plasma membrane (PM) of polarized cells, which corresponds to secretory vesicles in the model organism Saccharomyces cerevisiae
We have shown the importance of the N-terminal 52 residues of Chs3 for its transport, as deletion of these residues decreases the amount of Chs3 at the bud neck as well as TGN punctae, and the www.frontiersin.org

Summary

Introduction

Eukaryotic cells must transport transmembrane proteins between different subcellular compartments, often in response to specific signals or conditions This transport is mediated by coat complexes, which help to form the shape of transport vesicles (Bonifacino and Glick, 2004). No coat complexes are known to mediate transport directly from the trans-Golgi network (TGN) to the apical plasma membrane (PM) of polarized cells, which corresponds to secretory vesicles in the model organism Saccharomyces cerevisiae (budding yeast). It remains poorly understood how cells regulate this trafficking step. Chs is localized to the bud neck (junction of mother and daughter cells) through its interaction with its activator Chs, which binds the septin-interacting protein Bni (DeMarini, 1997; Reyes et al, 2007)

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