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cell shows a high degree of dynamics through the uptake and secretion of vesicles. Proteins from the ER are also transported in this way to the Golgi apparatus and from the Golgi apparatus to the lysosomes and endosomes as well as the cytoplasmic membrane (Figure 5.8).
Figure 5.8 Vesicle transport pathways in the cell.
The pinching off of vesicles and their uptake is a complex process that involves a large number of internal and external proteins (many of them not yet known). The budding of vesicles only occurs when a specific protein coat is formed on the vesicle surface:
Vesicles that bud from the ER carry COPII proteins.
Vesicles that migrate between the cis and trans sides of the Golgi apparatus carry COPI proteins.
Vesicles that are sent from the cis‐Golgi to the endosomes or endocytotic vesicles from the plasma membrane are covered with a coat of clathrin molecules (Figure 5.9).
Figure 5.9 Structure of clathrin‐coated vesicles: (a) electron micrograph and (b) three‐dimensional representation of a clathrin coat, derived from an electron microscope photo.
Source: Courtesy of Barbara Pearse, Medical Research Council, Cambridge, UK.
These surface proteins are connected to membrane‐bound cargo receptors via adapter proteins, which recognize cargo proteins that are present within the vesicle.
Vesicles must be able to recognize a target compartment and to bring the content to the correct location. Further receptor molecules termed SNARE proteins serve this purpose. Every vesicle carries specific v‐SNARE proteins on the surface, which can be recognized by the target compartment with specific t‐SNARE receptors. In this context, Rab proteins are important (Table 5.2): Rab proteins are monomeric GTPases that with the help of other proteins (Rab cascade) ensure that the vesicle finds the right partner.
Table 5.2 Occurrence of some Rab proteins.
| Rab | Localization |
|---|---|
| Rab1 | ER and Golgi |
| Rab2 | cis‐Golgi network |
| Rab3A | Synaptic and secretory vesicles |
| Rab4/Rab11 | Recycling endosomes |
| Rab5 | Early endosomes, clathrin‐coated vesicles |
| Rab6 | Medial and trans‐Golgi |
| Rab7 | Late endosome |
| Rab8 | Cilia |
| Rab9 | Late endosomes, trans‐Golgi |
The most researched SNARE proteins are those associated with the neurovesicles in the presynapse. Neurovesicles can only carry out exocytosis when synaptobrevin (v‐SNARE) on the vesicle membrane interacts with syntaxin (t‐SNARE) on the inside of the presynapse. Additionally a further peripheral membrane protein, SNAP25 (t‐SNARE), must enter the complex. The exocytosis is initiated via a calcium signal: when an action potential occurs in the synapse, the voltage‐gated calcium channels open, and Ca2+ flows into the synapse for a short time.
In the different compartments of the Golgi apparatus, the sugar residues of the proteins are altered in different ways. For example, the mannose residues of the lysosomal proteins are phosphorylated and therefore recognized by their mannose‐6‐phosphate residues. In other proteins, the mannose residues are removed and replaced by N‐acetylglucosamine, galactose, or N‐acetylneuraminic acid (NANA).
In the trans‐Golgi, proteins with mannose‐6‐phosphate residues are recognized by a specific transmembrane receptor. The loading of these receptors results in a conformation change in the proteins, which is then recognized by clathrin molecules (Figure 5.9). This leads to the budding of the vesicle, which is loaded with lysosomal enzymes. These vesicles fuse with vesicles of the late endosomes, finally resulting in the formation of the endosomes and lysosome.
Proteins that are sent to the cytoplasmic membrane, where they bud into the extracellular space via exocytosis, are also processed in the Golgi apparatus. The fusion of the Golgi vesicle with the cytoplasmic membrane is termed exocytosis. In this process, water‐soluble proteins, such as peptide hormones or antibodies, are released into the extracellular space (e.g. the blood). Membrane‐associated proteins remain as membrane proteins in the cytoplasmic membrane and are orientated with their sugar residues into the extracellular space. Exocytosis can be both continuous and signal controlled. An example of the latter is the release of insulin or histamine from their respective storage vesicles.
The opposite process, endocytosis, also occurs continuously at the cytoplasmic membrane. In this process, vesicles bud off and migrate from the early endosomes to the late endosomes and finally deliver their contents to the lysosomes or the Golgi apparatus (Figure 5.8).
The processes of endocytosis are subdivided:
Phagocytosis (uptake of microorganisms or dead cells).
Pinocytosis (uptake of liquids and smaller molecules).
Phagocytosis (Figure 5.8) is the function of the phagocytes (macrophages, neutrophils, and dendritic cells) of the cellular immune system. The phagocytosed cells are degraded in the lysosomes.
Pinocytosis is a continuous process: macrophages take up about 25% of their cell volume per hour via pinocytosis; in relation to the cytoplasmic membrane, this corresponds to a budding rate of the membrane in vesicles of 3% per minute. The surface of the membrane, which is taken up via endocytosis, corresponds to the cell surface, which is released via exocytosis (endocytosis–exocytosis cycle). During endocytosis the vesicles are filled with liquids and molecules that are present in the extracellular