La célula. 5.Tráfico vesicular
From the RETICULUM to the GOLGI
|« Endoplasmic reticulum||Golgi complex »|
Most of the molecules synthesized in endoplasmic reticulum are delivered to the Golgi complex.
Endoplasmic reticulum and Golgi complex communicate between each other through vesicles. ERGIC is an intermediate compartment between these two organelles.
COPI and COPII are vesicle coating proteins involved in the trafficking between endoplasmic reticulum and Golgi complex. COPII vesicles travel from the endoplasmic reticulum to the Golgi complex, whereas COPI vesicles do the opposite journey.
Most of the proteins and lipids which are going to leave the endoplasmic reticulum are packaged in vesicles. The Golgi complex is the target compartment for these vesicles. There are areas free of ribosomes distributed along the endoplasmic reticulum membrane, referred as transition zones, where these vesicles with their cargoes are assembled.
Vesicles and tubulo-vesicular structures are originated by the participation of proteins known as COPII. COPII is assembled on the cytosolic face of the endoplasmic reticulum membrane, at the transition areas, and participates in the evagination of the membrane and vesicular budding, as well as in the anchoring of adaptor proteins, which select and bind the cargoes. Transmembrane cargoes and receptors that recognize soluble cargoes have cytosolic domains that are bound selectively by the adaptor proteins. Thus, COPII, through adaptor proteins, can select and concentrate cargoes in the nascent vesicles, which will be transported to the ERGIC (endoplasmic reticulum Golgi intermediate compartment). Beside that, any protein to be transported must be properly folded. Otherwise, it will be removed. A quality control of proteins is always working in the endoplasmic reticulum.
COPII vesicles are released from transition zones of the endoplasmic reticulum, are moved away and fused with the ERGIC (endoplasmic reticulum-Golgi intermediate compartment), which moves toward the cis face of the Golgi complex. Vesicles and ERGIC movements are mediated by microtubules and motor proteins. In the cis side of the Golgi complex, all the ERGIC bodies coming from different parts of the cell are fused to form the cis cisternae of the Golgi complex. At the same time, from the ERGIC, COPI coated vesicles are released and fused with the endoplasmic reticulum as part of a recycling process.
COPII coated vesicles are released from the transition zones of the endoplasmic reticulum, and are fused together to form the ERGIC (endoplasmic reticulum-Golgi intermediate compartment), which is moved by microtubules and motor proteins to the cis face of the Golgi apparatus. There all the ERGIC bodies are fused to form the cis cisternae of the Golgi apparatus.
ERGIC body matures during the way toward the Golgi apparatus. This maturing process brings a progressively change of the molecular composition of its membrane, that in turn allows the attachment of some cytosolic proteins to its membrane. Thus, COPI proteins will be recruited to the ERGIC membrane and will form vesicles with selected cargoes. This is similar process to that of the generation of COPII vesicles in the endoplasmic reticulum, but COPI vesicles will be sent to the endoplasmic reticulum. It is a recycling pathway that will send back resident proteins belonging to the endoplasmic reticulum. Endoplasmic reticulum resident transmembrane proteins will be directly selected by COPI proteins, but soluble resident proteins should be picked up by transmembrane receptors, known as KDEL receptors, which in turn are catched by COPI proteins. All these interactions are mediated by specific amino acid sequences or signal peptides. So, COPI vesicles fuse with the endoplasmic reticulum and release transmembrane proteins in the endoplasmic reticulum membrane by lateral diffusion, whereas soluble proteins are uncoupled from KDEL and set free in the lumen. KDEL, now without any bound ligand, will be included again into COPII vesicles for a new journey, so it is permanently traveling between the endoplasmic reticulum and ERGIC-Golgi compartments. There is an increase in the pH value as we move from the endoplasmic reticulum to the Golgi apparatus, so that ERGIC is more basic that the endoplasmic reticulum. KDEL binds ligands in more basic enviroments, but the affinity for them decreases in more acidic environments, so that ligands are released, which happens in the endoplasmic reticulum. In other words, the differences in pH values between ERGIC-Golgi and endoplasmic reticulum allow fetching ligands by KDEL in the ERGI-Golgi and releasing them in endoplasmic the reticulum.
Antonny B, Schekman R . ER export: public transportation by the COPII coach. 2001. Current opinion in cell biology 13:438-443.
|« Endoplasmic reticulum||Golgi complex»|
Updated: 2017-09-19. 11:51
Atlas of Plant and Animal Histology
Dep. of Functional Biology and Health Sciences.
Faculty of Biology.
University of Vigo