ABSTRACT
Metazoa are equipped with extensively developed intramembranous structures that create a microenvironment, enabling various fundamental reactions for living activities. Each organelle, which is a structure unit delineated by the membranes, has a distinct dynamic shape to fulfill the highly interlinked reactions. Among them, the endoplasmic reticulum (ER) is the compartment having the largest surface that creates a single-large lumen by interconnecting tubules and sheets. The ER is continuous to the nuclear envelop and contains several microdomains required for the export of secretory cargo or physical coupling to mitochondria, endosomes, or plasma membranes. The sheet structure provides a suitable surface for attachment of polysomes-synthesizing secretory proteins. The nascent proteins are sequestrated into the lumen of the ER and extensively edited to obtain the stable conformation in the oxidative environment, which are then transported to the Golgi apparatus through the COPII-coated vesicles budded from one of the ER microdomain, ER exit sites. Extensive studies have identified the essential ER factors for the folding, maturation, and transport of secretory cargo [1,2]. Recent studies also elucidated the physiological role of the attachment sites to the other organelles [3,4]. Still, it remains elusive why such chains of reactions can be efficiently carried out in the microenvironment of nanometer scale.
