Emergent properties of proteostasis-COPII coupled systems in human health and disease

In eukaryotic membrane trafficking, emergent protein folding pathways dictated by the proteostasis network (the 'PN') in each cell type are linked to the coat protein complex II (COPII) system that initiates transport through the exocytic pathway. These coupled pathways direct the transit of protein cargo from the endoplasmic reticulum (ER) to diverse subcellular and extracellular destinations. Understanding how the COPII system selectively manages the trafficking of distinct folded states of nascent cargo (comprising one-third of the proteins synthesized by the eukaryotic genome) in close cooperation with the PN remains a formidable challenge to the field. Whereas the PN may contain a thousand component, the minimal COPII coat components that drive all vesicle budding from the ER include Sar1 (a GTPase), Sec12 (a guanine nucleotide exchange factor), Sec23-Sec24 complexes (protein cargo selectors) and the Sec13-Sec31 complex (that functions as a protein cargo collector and as a polymeric lattice generator to promote vesicle budding). A wealth of data suggests a hierarchical role of the PN and COPII components in coupling protein folding with recruitment and assembly of vesicle coats on the ER. In this minireview, we focus on insights recently gained from the study of inherited human disease states of the COPII machinery. We explore the relevance of the COPII system to human biology in the context of its inherent link with the remarkably flexible folding capacity of the PN in each cell type and in response to the environment. The pharmacological manipulation of this coupled system has important therapeutic implications for restoration of function in human disease.