![]() ![]() Proteins destined for secretion or incorporation into the ER, Golgi apparatus, lysosomes, or plasma membrane are initially targeted to the ER. The entrance of proteins into the ER thus represents a major branch point for the traffic of proteins within eukaryotic cells. Still other proteins travel through the initial steps of the secretory pathway but are then retained and function within either the ER or the Golgi apparatus. Plasma membrane and lysosomal proteins also travel from the rough ER to the Golgi and then to their final destinations. Further studies extended these results and demonstrated that this pathway is not restricted to proteins destined for secretion from the cell. These experiments defined a pathway taken by secreted proteins, the secretory pathway: rough ER → Golgi → secretory vesicles → cell exterior. ![]() Pancreatic acinar cells, which secrete most of their newly synthesized proteins into the digestive tract, were labeled with radioactive amino acids to study the intracellular pathway taken by secreted proteins. Following longer chase periods, the radiolabeled proteins traveled from the Golgi apparatus to the cell surface in secretory vesicles, which then fused with the plasma membrane to release their contents outside of the cell. If the cells were then incubated for a short time in media containing nonradioactive amino acids (a process known as a chase), the radiolabeled proteins were detected in the Golgi apparatus. After a brief exposure of pancreatic acinar cells to radioactive amino acids, newly synthesized proteins were detected in the rough ER, which was therefore identified as the site of synthesis of proteins destined for secretion. The location of the radiolabeled proteins within the cell was then determined by autoradiography, revealing the cellular sites involved in the events leading to protein secretion. Because most proteins synthesized by these cells are secreted, Palade and coworkers were able to study the pathway taken by secreted proteins simply by labeling newly synthesized proteins with radioactive amino acids. These investigators studied the fate of newly synthesized proteins in specialized cells of the pancreas (pancreatic acinar cells) that secrete digestive enzymes into the small intestine. The role of the endoplasmic reticulum in protein processing and sorting was first demonstrated by George Palade and his colleagues in the 1960s ( Figure 9.2). GLUT SGLT endomembrane glucose transporter.The Endoplasmic Reticulum and Protein Secretion However, recent studies revealed the intracellular localization and functional features of some glucose transporters the aim of the present paper was to summarize the collected knowledge. Despite the obvious necessity, the mechanism of glucose transport and the molecular nature of mediating proteins in the endomembranes have been hardly elucidated for the last few years. Autophagic degradation of polysaccharides, glycoproteins, and glycolipids leads to glucose accumulation in lysosomes. Posttranslational processing of the oligosaccharide moiety of glycoproteins also results in intraluminal glucose formation in the endoplasmic reticulum (ER) and Golgi. The glucose-6-phosphatase system catalyzing the last common step of gluconeogenesis and glycogenolysis generates glucose within the lumen of the endoplasmic reticulum. Glucose transport through intracellular membranes has not been elucidated yet however, glucose is formed in the lumen of various organelles. GLUT (glucose transporter) or SLC2A (Solute carrier 2A) families represent the main glucose transporters in mammalian cells, originally described as plasma membrane transporters. This role is fulfilled by glucose transporters, mediating the transport of glucose by facilitated diffusion or by secondary active transport. ![]() Glucose is a basic nutrient in most of the creatures its transport through biological membranes is an absolute requirement of life. ![]()
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