Sorting of secretory proteins at the trans-Golgi network by human TGN46

Abstract

Secretory proteins are sorted at the trans-Golgi network (TGN) for export into specific transport carriers. However, the molecular players involved in this fundamental process remain largely elusive. Here, we identified the transmembrane protein TGN46 as a cargo receptor for the export of secretory proteins in CARTS – a class of protein kinase D-dependent TGN-to-plasma membrane carriers. We show that TGN46 is necessary for cargo sorting and loading into nascent carriers at the TGN. By combining quantitative fluorescence microscopy and mutagenesis approaches, we further discovered that the TGN46 lumenal domain encodes for its cargo sorting function. Interestingly, this domain forms liquid droplets in vitro by means of liquid-liquid phase separation, a physical mechanism that may assist in TGN46-mediated cargo sorting. In summary, our results define a cellular function of TGN46 in sorting secretory proteins for export from the TGN. These findings could open new therapeutic avenues for diseases that parallel secretory malfunction.

SARS-CoV-2 remodels the Golgi apparatus to facilitate viral assembly and secretion

“…TGN46 is a glycoprotein that recycles between the TGN and cell surface (Ponnambalam et al, 1994; Reaves et al, 1993). TGN46 was previously reported to directly interact with integrin β1 to modulate its trafficking (Wang and Howell, 2000), while recently TGN46 was reported to serve as a sorting receptor at the TGN for CARTS, a class of protein kinase D-dependent TGN-to-plasma membrane carriers (Lujan et al, 2022). Therefore, we postulate that the increased TGN46 level indicates a need for TGN46 to accommodate the high flux of virion trafficking through the late secretory pathway, and our results support this hypothesis.…”

Protein sorting from endosomes to the TGN

“…The biological relevance of this cycling between the TGN and the plasma membrane remains unclear because the function of TGN46 and its isoforms has not been revealed yet. Recently, also the TGN-derived carriers involved in anterograde transport of TGN46 and isoforms have been described ( Wakana et al, 2012 ; Wakana et al, 2013 ; Wakana et al, 2015 ; Lujan et al, 2022 ), and the sorting machinery mediating their retrograde traffic from endosomes ( Saint-Pol et al, 2004 ; Lieu et al, 2007 ; Lieu and Gleeson, 2010 ). Also, the ‘Golgi-resident proteins’ such as galactosyltransferase or sialyltransferase can escape the Golgi compartment.…”

Partitioning to ordered membrane domains regulates the kinetics of secretory traffic

“…Non-resident proteins of the Golgi flux through distinct sub-compartments from cis-to-trans Golgi via several possible non-exclusive paths, including vesicles that exchange between static cisternae and/or dynamic cisternae that themselves change their composition and function over time ( Lujan and Campelo, 2021 ; Pantazopoulou and Glick, 2019 ). Post-Golgi sorting generally occurs at the trans-Golgi network (TGN), where exiting proteins are packaged into transport carriers targeted to other organelles ( Luini et al, 2008 ; Lujan et al, 2022 ; Nishimura et al, 2002 ; Tan and Gleeson, 2019 ). These sorting steps are at least partially mediated by clathrin ( Ford et al, 2021 ) and its cargo-binding adapter proteins ( Farías et al, 2012 ; Nishimura et al, 2002 ; Park and Guo, 2014 ).…”