![]() ![]() Bulk flow is the default secretory pathway in which proteins in the ER lumen can freely and passively leave the ER ( 20, 21). Finally, SEC13 and SEC31 are recruited as an outer coat to complete the coat assembly ( 19) ( Figure 1).īulk flow and concentrative transport. SEC23 stimulates SAR1 GTPase hydrolysis, thereby functioning as the GTPase-activating protein (GAP) for SAR1, while SEC24 mediates cargo recruitment and concentration in the ER lumen ( 18). GTP-bound SAR1 inserts its hydrophobic N-terminus into the ER membrane and recruits SEC23-SEC24 heterodimers to the ERES by directly interacting with SEC23 ( 16, 17). SEC12 is a type II ER transmembrane protein that functions as a guanine nucleotide exchange factor (GEF) for SAR1 while also recruiting SAR1 to the ER membrane ( 14, 15). COPII coat assembly begins in the membrane of the smooth ER (lacking ribosomes) where the localization of SEC12 defines the ER exit site (ERES). ![]() Finally, SEC13-SEC31 heterotetramers are recruited as the outer coat to complete the coat assembly process.Ĭoat assembly. SEC23 also functions as the GTPase-activating protein for SAR1 and stimulates SAR1 GTP hydrolysis. SEC24 mediates cargo recruitment via direct physical interaction with transmembrane proteins through their cytoplasmic tails or with soluble cargoes via cargo receptors. GTP-bound SAR1 inserts its hydrophobic N-terminus into the ER membrane and recruits SEC23-SEC24 heterodimers to the ER exit site via direct interaction with SEC23. SEC12 recruits GDP-bound SAR1 to ER exit sites (ERESs) and acts as a guanine nucleotide exchange factor for SAR1. ER to Golgi transport by COPIIĬOPII coat assembly on the ER membrane. Genetic disorders associated with deficiency in protein components of COPII will also be described. This Review will focus on the early phase of conventional protein secretion, in which proteins are transported from the ER to the Golgi in a process mediated by coat protein complex II (COPII). A few proteins have been shown to exit the cell via an ER/Golgi–independent process referred to as unconventional protein secretion ( 8– 11). ![]() Properly folded proteins are then transported from the ER to the Golgi apparatus for further processing and sorting before reaching their final extracellular or intracellular destinations ( 6, 7). Proteins enter the intracellular secretory pathway at the endoplasmic reticulum (ER), where they are cotranslationally embedded into the ER membrane or deposited into the ER lumen ( 4, 5). This pathway is also shared by many proteins destined for the cell surface and various intracellular organelles, such as endosomes and lysosomes ( 2, 3). In eukaryotes, the majority of secreted proteins must transit through a series of membrane-bound organelles before arriving at the plasma membrane for release into the extracellular environment ( 1– 3). Cells secrete proteins into the extracellular environment for multiple purposes, including intercellular communication, defense, and modulation of the external environment. ![]()
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