Misfolded Proteins Induce Aggregation of the Lectin Yos9 [Protein Synthesis and Degradation]

August 1st, 2014 by Smith, M. H., Rodriguez, E. H., Weissman, J. S.

A substantial fraction of nascent proteins delivered into the endoplasmic reticulum (ER) never reach their native conformations. Eukaryotes use a series of complementary pathways to efficiently recognize and dispose of these terminally misfolded proteins. In this process, collectively termed ER-associated degradation (ERAD), misfolded proteins are retrotranslocated to the cytosol, polyubiquitinated and degraded by the proteasome. While there has been great progress in identifying ERAD components, how these factors accurately identify substrates remains poorly understood. The targeting of misfolded glycoproteins in the ER lumen for ERAD requires the lectin Yos9, which recognizes the glycan species found on terminally misfolded proteins. In a role that remains poorly characterized, Yos9 also binds the protein component of ERAD substrates. Here, we identified a 45kDa domain of Yos9 comprising residues 22-421 that is proteolytically stable, highly structured and able to fully support ERAD in vivo. In vitro binding studies show that Yos922-421 exhibits sequence specific recognition of linear peptides from the ERAD substrate, CPY*, and binds a model unfolded peptide, ΔEspP, and protein, Δ131Δ, in solution. Binding of Yos9 to these substrates results in their cooperative aggregation. While the physiological consequences of this substrate-induced aggregation remain to be seen, it has the potential to play a role in the regulation of ERAD.