Sorting Things Out Through Endoplasmic Reticulum Quality Control

Molecular Membrane Biology, November 2010; 27(8): 412–427

REVIEW

Sorting things out through endoplasmic reticulum quality control

TAKU TAMURA1, JOHAN C. SUNRYD1,2, & DANIEL N. HEBERT1,2
1

Department of Biochemistry and Molecular Biology, and 2Program in Molecular and Cellular Biology, University of Massachusetts, Amherst, MA, USA

(Received 29 April 2010; and in revised form 13 May2010)

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Abstract The endoplasmic reticulum (ER) is a highly organized and specialized organelle optimized for the production of proteins. It is comprised of a highly interconnected network of tubules that contain a large set of resident proteins dedicated to the maturation andprocessing of proteins that traverse the eukaryotic secretory pathway. As protein maturation is an imperfect process, frequently resulting in misfolding and/or the formation of aggregates, proteins are subjected to a series of evaluation processes within the ER. Proteins deemed native are sorted for anterograde trafficking, while immature or non-native proteins are initially retained in the ER in anattempt to rescue the aberrant products. Terminally misfolded substrates are eventually targeted for turnover through the ER-associated degradation or ERAD pathway to protect the cell from the release of a defective product. A clearer picture of the identity of the machinery involved in these quality control evaluation processes and their mechanisms of actions has emerged over the past decade.Keywords: Protein folding, carbohydrates, molecular chaperones, ERAD

Introduction Ribosome-attached secretory pathway cargo are cotranslationally targeted to the endoplasmic reticulum (ER) translocon Sec61 by the signal recognition particle or SRP. Nascent chains co-translationally emerge into the ER lumen through this ER membrane pore in a vectorial manner (N- to C-terminus). As protein maturationbegins co-translational and translocationally, the vectorial nature of these processes helps to taper the ensemble of possible intermediates by starting maturation with shorter nascent chains. These early events, which are assisted by a group of factors localized in close proximity to the Sec61 entry site, include protein folding, processing and modification. The majority of the secretory pathwaycustomers are modified by N-linked glycans (Apweiler et al. 1999), and these modifications are used extensively as maturation and quality control tags that assist with the sorting process. This review will focus on the machinery and mechanics for how these sorting decisions are mediated with the help of N-linked glycans and their related factors.

Getting started: Translocation and early maturationevents Proteins containing the consensus N-linked glycosylation site Asn-X-Ser/Thr are co-translationally modified with a pre-assembled 14-member carbohydrate comprised of three glucoses, nine mannoses and two N-acetylglucosamines (Glc3Man9GlcNAc2; Figure 1A) through the action of the oligosaccharyl transferase (OST). The addition of these large bulky hydrophilic modifications greatly alters thefundamental properties of the protein. The enzymatic transfer of the glycan requires that the Ser or Thr, situated two residues C-terminal to the Asn attachment site, position their hydroxyl side chain to render the Asn amide group more nucleophilic to support the progression of the transfer reaction. Therefore, this reaction mechanism favors the transfer of glycans that are situated on flexibleregions. These regions are frequently located on the aqueous exposed surface of the protein. This optimally positions glycans for their role as maturation and quality control tags to support the recruitment of protein folding and degradation factors.

Correspondence: Prof. Daniel N. Hebert, Department of Biochemistry and Molecular Biology, University of Massachusetts, 710 N. Pleasant St., Amherst,...