Functional Glycomics homepage

Article navigation

Featured Articles

ERAD trafficking: trimming the glycan tree

Functional Glycomics (09 December 2010) | doi:10.1038/fg.2010.40

Inhibition of mannose trimming defines EDEM1 and Derlin-1 interactions as occurring at an early stage in glycoprotein degradation.

Image courtesy of Gerardo Lederkremer

Protein synthesis and trafficking often rely on the attachment of N-linked glycans that are remodeled following proper refolding (for transit to the Golgi) or misfolding (to send the protein for endoplasmic reticulum (ER)–associated degradation (ERAD)). The number of mannose residues that are removed from the glycans controls protein fate, with trimming of more mannose residues resulting in ERAD targeting. However, identifying specific binding partners or functional consequences of these sugar residues has been challenging. A new report in the Journal of Biological Chemistry by Lederkremer and colleagues now identifies an early step in ERAD processing that does not require mannose trimming, improving our understanding of this complex process.

The uncleaved precursor of asialoglycoprotein receptor, H2a, when expressed in hepatocytes is cleaved and secreted. In other cell types, however, this processing does not occur efficiently, and H2a is eventually degraded, making it a useful ERAD model substrate for which mannose trimming is required.

In vitro studies have suggested that the homologous lectins OS9 and XTP3-B may be important for binding demannosylated substrates. However, ER degradation–enhancing mannosidase-like protein 1 (EDEM1) has also been implicated in recognizing the trimmed substrates, a function that is further complicated by the presence of a mannosidase-like domain in this protein. Thus, the activity of these proteins within the larger ER quality control compartment (ERQC) — a staging ground for ERAD substrates — is not known.

To explore this system in HEK 293 and NIH 3T3 cells, the authors relied on mannosidase and proteasome inhibitors as well as enzyme knockdowns and fusion proteins to inform a series of co-immunoprecipitation and localization studies. They first demonstrated that, as expected, mannose trimming is required for successful trafficking of H2a to the proteasome and degradation. This process is dependent on EDEM1, but interaction of H2a with EDEM1 did not require prior trimming. Importantly, the colocalization behavior of EDEM1 with H2a (observed in several experimental conditions) was different from that of ER mannosidase I (ERManI) with H2a (observed in few conditions), suggesting that EDEM1 actively participates in trafficking, whereas ERManI is instead a critical stop along the ERAD pathway.

The authors further confirmed the role of XTP3-B, but not OS9, in receiving the trimmed H2a substrate. Additional studies of other ERQC proteins defined HRD1 and SCF-Fbs2, two ubiquitin ligases, and SEL1L, a ligase adaptor, as important for downstream processing. Surprisingly, Derlin-1 — a protein that has been implicated in late-stage retrotranslocation — also associated with the substrate before mannose trimming, defining an early role for this protein.

Taken together, these results suggest a model in which EDEM1 and Derlin-1 are important for the early stages of ERAD processing, including delivery of the substrate to the ubiquitination and degradation machinery. The work also raises several new questions, and it should therefore spur additional research into how these proteins and those later in the ERQC work together to control this important pathway.

Catherine Goodman

Original research paper

  1. Groisman, B. et al. Mannose trimming is required for delivery of a glycoprotein from EDEM1 to XTP3-B and to late ER-associated degradation steps. J. Biol. Chem. (9 November 2010) doi: 10.1074/jbc.M110.154849 | Article