Presented by: M. Hammad LatifPresented to: Dr. Qandeel Fatima

Protein sorting in Golgi Bodies

Protein targeting or protein sorting is the biological mechanism by which proteins are transported to the appropriate destinations in the cell or outside of it. Proteins can be targeted to the inner space of an organelle, different intracellular membranes, plasma membrane, or to exterior of the cell via secretion.

Protein Sorting

An organelle, consisting of layers of flattened sacs, that takes up and processes secretory and synthetic products from the endoplasmic reticulum and then either releases the finished products into various parts of the cell cytoplasm or secretes them to the outside of the cell.

The Golgi Apparatus

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.

Organization of the Golgi Apparatus

a. Vesicles from the ER fuse to form the ER-Golgi intermediate compartment, and proteins from the ER are then transported to the cis Golgi network. 

b. The medial and trans compartments of the Golgi stack correspond to the cisternae in the middle of the Golgi complex and are the sites of most protein modifications. 

c. Proteins are then carried to the trans Golgi network, where they are sorted for transport to the plasma membrane, secretion, or lysosomes.

Regions of the Golgi apparatus

Protein processing within the Golgi involves the modification and synthesis of the carbohydrate portions of glycoproteins. One of the major aspects of this processing is the modification of the N-linked oligosaccharides that were added to proteins in the ER.N-linked oligosaccharides are processed within the Golgi apparatus in an ordered sequence of reactions.a. The first modification of proteins destined

for secretion or for the plasma membrane is the removal of three additional mannose residues.

b.  This is followed by the sequential addition of an N-acetylglucosamine

c. The removal of two more mannoses d. The addition of a fucose and two more N-

acetylglucosamines.e.  Finally, three galactose and three sialic

acid residues are added.

Protein Glycosylation within the Golgi

Ceramide, which is synthesized in the ER, is converted either to sphingomyelin (a phospholipid) or to glycolipids in the Golgi apparatus.In the first reaction, a phosphorylcholine group is transferred from phosphatidylcholine to ceramide. Alternatively, a variety of different glycolipids can be synthesized by the addition of one or more sugar residues (e.g., glucose).

Lipid and Polysaccharide Metabolism in the Golgi

Synthesis of sphingomyelin and glycolipids

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.

Protein Sorting and Export from the Golgi Apparatus

a. Some proteins are retained in the ER instead of traveling from ER to golgi.

b. 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.

c. 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

Vesicular transport from ER to Golgi bodies

Transport from golgi apparatus takes place by two pathways.

Constitutive secretory pathwayThe 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.

Transport from the Golgi apparatus

a. A distinct regulated secretory pathway in which specific proteins are secreted in response to environmental signals.

b.  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 

Regulated secretory pathway

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 ofextracellular 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.

Transport to the plasma membrane of polarized cells

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

Selective transport of proteins to lysosomes

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.

The M-6-P pathway

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.

Transport of proteins to the cells that lack the lysosomes