Protein Sorting and Transport Through Golgi complex

Protein Sorting and Transport Through Golgi complex

The Golgi complex was discovered by an Italian physician and Noble Laureate Camillo Golgi in 1898 during an investigation of the
nervous system.

Its electron microscopic structure was described by Dalton and Felix in 1954.

The Golgi apparatus is noticeable with both light and electron microscope. It is also called the Golgi Apparatus.

Morphologically the Golgi is composed of flattened membrane-enclosed sacs (cisternae) and associated vesicles.

1. Protein Sorting and Transport Through Golgi complex Kaushal Sharma M.Sc. Plant & Microbial Biotech. Dayalbagh Educational Institute

2. Discovery of G olgi complex 2 Kaushal Sharma The Golgi complex was discovered by an Italian physician and Noble Laureate Camillo Golgi in 1898 during an investigation of the nervous system . Its electron microscopic structure was described by Dalton and Felix in 1954 . The Golgi apparatus is noticeable with both light and electron microscope. It is also called Golgi Apparatus . Camillo Golgi   (1843 – 1926)

3. Occurrence of Golgi Apparatus: 3 Kaushal Sharma The Golgi apparatus is present in all Eukaryotic cells and absent in Prokaryotes. The Golgi apparatus is specially extensive in the secretory cells . It is absent in few cell types, such as the mammalian RBCs , sperm cells of Bryophytes and Pteridophytes and sieve tubes of plants . A cell may have one large Golgi complex or several very small ones. It occupies different positions in different kind of cells. In secretory and absorptive cells, it usually lies between the nucleus. The invertebrate and plant cells usually have several small Golgi complexes, called Dictyosomes , scattered throughout the cytoplasm.

4. Organization of the Golgi Apparatus 4 Kaushal Sharma Morphologically the Golgi is composed of flattened membrane-enclosed sacs (cisternae) and associated vesicles . Proteins from the ER enter at its cis face (entry face), which is convex and usually oriented toward the nucleus. They are then transported through the Golgi and exit from its concave trans face (exit face). As they pass through the Golgi , proteins are modified and sorted for transport to their eventual destinations within the cell .

5. Regions of the Golgi apparatus 5 Kaushal Sharma Cisternae : The cisternae vary in number from 3-7 in most animal cells and from 10-20 in plant cells. Usually equally spaced in the stack, separated from each other by thin layers of intercisternal cytoplasm . Cisternae may be flat but are often curved. Golgi complex has a distinct polarity, the two poles are called cis face and trans face, which act respectively as the receiving and shipping departments. Convex side of stack forming (cis) face. Concave side of stack maturing (trans) face. Secretory materials reach the Golgi complex from Smooth Endoplasmic Reticulum (SER) by way of transport vesicles which bud off from SER and fuse with golgi cisternae on the cis face.

6. 6 Kaushal Sharma Cisternae : The membranes of the saccules or cisternae are smooth but of variable thickness they enclose a lumen of 60-90 A. Lumen contains a fluid substance or matrix. In a stack, the adjacent cisternae are separated by a distance of 100-300 A. The intercisternal space contains thin layer of cytoplasm having parallel fibrils . From the trans face Secretory vesicles arises that carry the processed material to their destination . Their contents pass through various cisternae with the help of coated vesicles and intercisternal connectives.

7. 7 Kaushal Sharma Tubules: Small , round tubules arise from the periphery of the cisternae. Some of these enlarge at their ends to form vesicles . They form a complicated network towards the periphery and maturing face of the apparatus. Tubules arise due to fenestrations of the cisternae. They have a diameter of 30-50 nm. The tubules interconnect the different cisternae.

8. 8 Kaushal Sharma Vesicles: The vesicles (60 nm in diameter) are of three types: ( i ) Transitional vesicles  are small membrane limited vesicles which are thought to form as blebs from the transitional ER to migrate and converge to cis face of Golgi, where they coalesce to form new cisternae. ( ii) Secretory vesicles  are varied-sized membrane-limited vesicles that discharge from margins of cisternae of Golgi. They, often, occur between the maturing face of Golgi and the plasma membrane. ( iii) Clathrin -coated vesicles  are spherical protuberances, about 50 μm in diameter and with a rough surface. They are found at the periphery of the organelle, usually at the ends of single tubules, and are morphologically quite distinct from the secretory vesicles. The clathrin -coated vesicles are known to play a role in intracellular traffic of membranes and of secretory products, i.e., between ER and Golgi, as well as, between the GELR region and the endosomal and lysosomal compartments.

9. 9 Kaushal Sharma Golgian Vacuoles They are expanded parts of the cisternae which have become modified to form vacuoles . The vacuoles develop from the concave or maturing face. Golgian vacuoles contain amorphous or granular substance. Some of the golgian vacuoles function as lysosomes .

10. Functions of Golgi Complex 10 Kaushal Sharma Transformation of Membranes: Golgi complex brings about membrane transformation , that is, converting one type of membrane (e.g., that of ER) into other types (e.g ., selectively permeable plasma membrane , differentiated membrane of lysosome ). The complex also takes part in the recycling of plasma membrane . Glycoproteins and Glycolipids: Proteins synthesized by the rough endoplasmic reticulum and lipids synthesized by smooth endoplasmic reticulum reach the cisternae of the Golgi apparatus. Here , they combine with carbohydrates to form glycoproteins and glycolipids.

11. 11 Kaushal Sharma Transformation of Membranes: Golgi complex brings about membrane transformation , that is, converting one type of membrane (e.g., that of ER) into other types (e.g ., selectively permeable plasma membrane , differentiated membrane of lysosome ). The complex also takes part in the recycling of plasma membrane . Glycoproteins and Glycolipids: Proteins synthesized by the rough endoplasmic reticulum and lipids synthesized by smooth endoplasmic reticulum reach the cisternae of the Golgi apparatus. Here , they combine with carbohydrates to form glycoproteins and glycolipids . S ynthesis Complex Carbohydrates: Most of the complex carbohydrates, other than glycogen and starch, are synthesized inside the Golgi complex, e.g., pectic compounds, mucopolysaccharides , hyaluronic acid, chondroitin sulphate , hemicelluloses, etc.

12. 12 Kaushal Sharma Fat Transport: Fatty acids and glycerol absorbed by intestinal epithelium are transferred as fat to lacteal through Golgi complex. Synthesis of Pigments: In Chick embryo the retinal pigment has been observed to be synthesized by Golgi complex. Formation of Acrosome: Acrosome is an important constituent of the tip of animal sperms which helps in digesting away the covering sheath of the egg or ovum during fertilization. It is synthesized by Golgi complex with the help of its vesicles. Formation of Lysosomes: Some of the vesicles or vacuoles of the Golgi apparatus store digestive enzymes obtained through ER in the inactive state. They act as primary lysosomes.

13. 13 Kaushal Sharma Formation of Plasma-lemma: Membranes of the vesicles produced by Golgi apparatus join in the region of cytokinesis to produce new plasma-lemma. Formation of New Cell Wall: Pectic compounds of middle lamella and various polysaccharides of the cell wall are secreted by Golgi complex. They are brought to the area of new wall synthesis by secretion vesicles . S ulfation Another task of the Golgi involves the sulfation of certain molecules passing through its lumen via sulfotranferases that gain their sulfur molecule from a donor called PAPS. This process occurs on the GAGs of proteoglycans as well as on the core protein. Sulfation is generally performed in the trans-Golgi network. The level of sulfation is very important to the proteoglycans‘ signalling abilities as well as giving the proteoglycan its overall negative charge.

14. 14 Kaushal Sharma APOPTOSIS The Golgi has a putative role in apoptosis, with several Bcl-2 family members localized there, as well as to the mitochondria. A newly characterized protein, GAAP (Golgi anti-apoptotic protein ) , almost exclusively resides in the Golgi and protects cells from apoptosis by an as-yet undefined mechanism . PHOSPHORYLATION The phosphorylation of molecules requires energy in the form of ATP That ATP is imported into the lumen of the Golgi utilised by resident kinases such as casein kinase 1and casein kinase 2. One molecule that is phosphorylated in the Golgi is Apolipoprotein , which forms a molecule known as VLDL that is a constituent of blood serum.

15. 15 Kaushal Sharma VESICULAR TRANSPORT Vesicles leaving RER transported to the cis face of GA, fuse with the membrane and empty the contents into the lumen. Molecules inside the lumen are modified and sorted for transport to the next destination. Proteins destined for places other than ER and GA, moves to trans face. Gets placed on either of the 3 vesicles, i.e. Exocytotic , Secretory and Lysosomal vesicles .

16. Protein Sorting and Export from the Golgi Apparatus 16 Kaushal Sharma Proteins, as well as lipids and polysaccharides, are transported from the Golgi apparatus to their final destinations through the secretory pathway. This involves the sorting of proteins into different kinds of transport vesicles, which bud from the trans Golgi network and deliver their contents to the appropriate cellular locations.

17. Vesicular transport from ER to Golgi bodies 17 Kaushal Sharma Some proteins are retained in the ER instead of traveling from ER to golgi. Proteins destined to remain in the lumen of the ER are marked by the sequence Lys-Asp- Glu - Leu (KDEL ) at their carboxy terminus . These proteins are exported from the ER to the Golgi, but they are recognized by a receptor in the ERGIC or the Golgi apparatus and selectively returned to the ER. Many proteins are retained in the ER lumen as a result of the presence of the targeting sequence (KDEL) at their carboxy terminus. If this sequence is deleted from the protein , the mutated protein is instead transported to the Golgi and secreted from the cell

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19. Transport from the Golgi Apparatus 19 Kaushal Sharma Transport from golgi apparatus takes place by two pathways. Constitutive Secretory Pathway The constitutive secretory pathway, which operates in all cells , leads to continual unregulated protein secretion . In the absence of specific targeting signals , proteins are carried to the plasma membrane by constitutive secretion .

20. Transport from the Golgi Apparatus 20 Kaushal Sharma Regulated S ecretory Pathway A distinct regulated secretory pathway in which specific proteins are secreted in response to environmental signals. Proteins are sorted into the regulated secretory pathway in the trans Golgi network , where they are packaged into specialized secretory vesicles. These secretory vesicles, which are larger than other transport vesicles, store their contents until specific signals direct their fusion with the plasma membrane. Examples of regulated secretion include:- The release of hormones from endocrine cells. The release of neurotransmitters from neurons. The release of digestive enzymes from the pancreatic acinar cells.

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22. Transport to the plasma membrane of polarized cells 22 Kaushal Sharma The plasma membranes of polarized epithelial cells are divided into apical and basolateral domains that contain specific proteins related to their particular functions . In this example (intestinal epithelium), the apical surface of the cell faces the lumen of the intestine, the lateral surfaces are in contact with neighboring cells, and the basal surface rests on a sheet of extracellular matrix (the basal lamina). The apical membrane is characterized by the presence of microvilli, which facilitate the absorption of nutrients by increasing surface area. Specific proteins are targeted to either the apical or basolateral membranes in the trans Golgi network. Tight junctions between neighboring cells maintain the identity of the apical and basolateral membranes by preventing the diffusion of proteins between these domains.

23. Selective transport of proteins to lysosomes 23 Kaushal Sharma The best-characterized pathway of protein sorting in the Golgi is the selective transport of proteins to lysosomes. Protein destined for incorporation into lysosomes are modified by mannose phosphorylation . This occurs while the protein is still in the cis Golgi network. These phosphorylated mannose residues are specifically recognized by a mannose- 6-phosphate receptor in the trans Goligi network

24. The M-6-P pathway 24 Kaushal Sharma In the trans-Golgi network, the phosphorylated enzymes bind to M-6-P receptors . Which direct the enzymes into vesicles coated with the fibrous protein clathrin . The clathrin lattices is rapidly depolymerized to its subunits, and the uncoated transport vesicles fuse with late endosomes. Within this low pH compartment, the phosphorylated enzymes dissociate from the M6P receptors and then are de-phosphorylated.

25. Transport of proteins to the cells that lack the lysosomes 25 Kaushal Sharma In yeasts and plant cells, which lack lysosomes , proteins are transported from the Golgi apparatus to an additional destination: the vacuole. Vacuoles assume the functions of lysosomes in these cells as well as performing a variety of other tasks, such as the storage of nutrients and the maintenance of turgor pressure and osmotic balance. In contrast to lysosomal targeting , proteins are directed to vacuoles by short peptide sequences instead of carbohydrate markers.

26. Kaushal Sharma M.Sc. Plant and Microbial Biotechnology Thank You