ER and post-ER quality control of integral membrane proteins

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Approximately one-third of all newly synthesized proteins in eukaryotes enter the endoplasmic reticulum (ER). Once associated with this compartment, these nascent polypeptides are post- translationally processed, acquire their native confirmations, oligomerize, and are sorted for extracellular secretion or delivery to other organelles. However, many disease-causing mutations compromise protein folding and maturation, which in turn can generate aggregation- prone species. To off-set the catastrophic effects that accompany the accumulation of protein aggregates, misfolded protein substrates are: (i) selected by molecular chaperones associated with the ER, (ii) modified with ubiquitin, (iii) delivered to the cytoplasm via a process known as retrotranslocation, and (iv) degraded by the 26S proteasome. Brodsky and colleagues named this pathway ER associated degradation (ERAD), and over the past 21 years many of the molecular mechanisms underlying this sequence of events were defined in the Brodsky lab. To date, ~80 human diseases are linked to the ERAD pathway and >1,200 publications have been authored on various aspects of this pathway. Ongoing efforts are defining the pathophysiological foundation of several ERAD-related disorders. In parallel, members of the Brodsky lab have revealed how key components orchestrate each step during ERAD. In the past 5 years, the lab has published 64 papers, and tools and technologies were developed that provide an unprecedented view of the mechanisms that lead to the selection, ubiquitination, retrotranslocation, and degradation of diverse substrates. Nevertheless, recent discoveries dictate that more challenging research directions are pursued: By necessity, these next efforts will require additional method development and a pursuit of longer-term goals. Specific questions that the research program will address include: What biochemical features define an ERAD substrate? Which factors are sufficient to drive the retrotranslocation of ERAD substrates, some of which are aggregation-prone? Do ER-associated proteases function in tandem with the 26S proteasome to destroy substrates that are stably integrated into the ER membrane, and thus might be retrotranslocation resistant? And, how are retrotranslocated membrane proteins— which can reside in the cytosol after being liberated from the ER—retained in a soluble state? Answers to these questions, which lie at the core of research in the field, will significantly advance an understanding of how cellular health is maintained in the face of proteotoxic stress as well as how ERAD-associated diseases arise and might be rectified.