Golgi Complex & Protein Sorting 2024-2025 PDF

GOLGI COMPLEX AND PROTEIN SORTING Asst.Prof. Burcu TÜRKGENÇ Dept. of Medical Biology  GOLGI COMPLEX A Golgi body, also known as a Golgi apparatus, is a cell organelle that helps process and package proteins and lipid molecules, especially proteins destined to be exported...

GOLGI COMPLEX AND PROTEIN SORTING Asst.Prof. Burcu TÜRKGENÇ Dept. of Medical Biology  GOLGI COMPLEX A Golgi body, also known as a Golgi apparatus, is a cell organelle that helps process and package proteins and lipid molecules, especially proteins destined to be exported from the cell. it is located at one side of the nucleus and close to the centrosome In secretory cells Golgi complex are dispersed throughout the cell 2  GOLGI COMPLEX The Golgi complex is prominent (well developed) in secretory cells Examples; In intestine Loading… Goblet cells which secrete large amounts of mucus (mucopolysaccarides) In pancreas acinar cells which secrete various digestive enzymes 3 The number of Golgi Complex varies according to cell function. Animal cells generally contain 10 to 20 Golgi complex per cell. 4  Golgi Complex Structure Golgi Complex consists of flattened, membrane enclosed, disc shaped compartments (sacs) called cisternae Loading… Each golgi compartments consist of four to six cisternae Tubular connections between cisternae links the cisternae forming a single complex. 5 The Golgi complex has two distinct poles (faces) cis-face (entery face or forming face) trans-face (exit face or maturing face) 6 The Golgi complex has two distinct poles (faces) The cis-face is found near the endoplasmic reticulum, The proteins from ER enter the Golgi complex at its cis face for processing (convex face) They exit from concave trans face of the Golgi 7  The endoplasmic reticulum (ER) sends proteins and lipids to the Golgi apparatus. The Golgi apparatus receives lipids and proteins from ER And modify them. 8  Golgi complex functions as factory In which proteins transported from ER are processed, segragated and distributed to ; The plasma, nucleus, and proxisome membranes Secretory vesicles Lysosomes 9 10  A transport vesicle from ER, fuses to the cis face of the Golgi and empties its soluble contents into the Golgi’s cisternal space The proteins pass into the Golgi and through the separate layers of the organelle Proteins are transported from the Golgi apparatus to their final destinations through the secretory pathway Loading… 11  Cis and trans faces are connected to special regions called cis Golgi network (CGN) and trans Golgi network (TGN) (a network of interconnected tubular and cisternal structures) 12  The Golgi complex has five functional regions 1-the cis-Golgi network, CGN 2-cis-Golgi 3-medial-Golgi Golgi stack 4-trans-Golgi 5-trans-Golgi network, TGN Each region contains different enzymes which modify the proteins 13 14  Golgi functions as a central delivery system The primary function of the Golgi apparatus is processing of proteins targeted to; The plasma, nucleus, and proxisome membranes Lysosomes or endosomes, Secretory granules and sorting and packaging them into specific transport vesicles 15  The Golgi complex is compartmentalized Proteins are modified step by step the Golgi complex forms a multi stage processing unit. enzymes catalyzing early processing steps are found in the cisternae near the cis face enzymes catalyzing later processing steps are located in the cisternae toward the trans face. 16  The primary function of the Golgi complex is to modify, process, and sort newly produced proteins that arrive from the ER. Functions of the Golgi complex Glycosylation of the proteins Modification of carbohydrates Processing of proteins Glycolipid and Sphingomyelin biosynthesis Sorting of cytosolic / secreted proteins 17  Modification of proteins in the Golgi complex: Protein Glycosylation within the Golgi One of the major aspects of this processing is the modification of the N-linked oligosaccharides that were added to proteins in the ER. (initial glycosylation in the GER) Following transport to the Golgi apparatus, the N-linked oligosaccharides of these glycoproteins are subject to extensive further modifications. 18  Modification of proteins in the Golgi complex: The first modification of proteins destined for secretion or for the plasma membrane is the removal of four mannose residues. This is followed by the sequential addition of an N-acetylglucosamine, the removal of two more mannoses, and the addition of a fucose and two more N-acetylglucosamine. Finally, three galactose and three sialic acid residues are added. 19  Modification of proteins in the Golgi complex: Consequently, proteins can emerge from the Golgi with a variety of different N-linked oligosaccharides. The enzymes that carry out the addition of sugar residues, glycosyltransferases, and those that remove them, glycosidases. 20 21  Two classes of N-linked oligosaccharides in Golgi Two types of N-linked oligosaccharides are attached to mammalian glycoproteins 1-The complex oligosaccharides: N-linked oligosaccharide is added in the ER It is trimmed and further sugars are added in Golgi 2-The high-mannose oligosaccharides N-linked oligosaccharide is added in the ER It is trimmed but have no new sugars added in the Golgi 22 Specific modifications occur in specific subcompartments Specific sugar molecules are added or deleted to modify the branched sugar structures found on newly formed proteins. eg, some of the mannose sugars are cut from the oligosaccharide branch in the cis Golgi. The protein travels to the medial Golgi where other sugars like N-acetylglucosamine are added to the oligosaccharide chains on the protein. Further modifications to the carbohydrates are completed in the trans Golgi. 23 24  In ER, Oligosaccharide intermediates created by trimming reaction Serve to help protein folding and To help transport of misfolded proteins to the cytosol for degradation. Thus they play an important role In controlling the quality of proteins exiting from the ER. In the Golgi apparatus; Oligosaccharides are modified for new functions. This produces the different oligosaccharide structures seen in the mature proteins. 25  What is the purpose of glycosylation? N-linked oligosac charide 26  What is the purpose of glycosylation? The presence of oligosaccharides makes a glycoprotein resistant to protease digestion e.g. glycocalix which covers the plasma membrane of the absorbtive cells of the small intestine and protects to the intestinal epithelial cells from the digestive enzymes Oligosaccharides on cell-surface glycoproteins also play a role in cell-cell adhesion 27  Lipid metabolism Golgi apparatus functions in lipid metabolism Synthesis of glycolipids and Sphingomyelin occurs in Golgi The glycerol, phospholipids, cholesterol and ceramide are synthesized in the ER Sphingomyelin and glycolipids are then synthesized from ceramide in the Golgi apparatus. 28 Sphingomyelin (SM) is the most abundant sphingolipid found in eukaryotes. SM are found in high quantity in the central nervous system (CNS) especially in the myelin sheath surrounding neuronal axons. Loading… 29  Protein Sorting and Export from the Golgi Apparatus 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. 30  Function modifying, sorting, packaging Cells synthesize a large number of different macromolecules. The Golgi apparatus is involved in modifying, sorting, and packaging these macromolecules for cell secretion (exocytosis) or use within the cell. 31  Function modifying, sorting, packaging Functions: – primarily modified proteins delivered from the RER Enzymes within the cisternae are able to modify substances by the addition of carbohydrates (glycosylation) and phosphates (phosphorylation). – is involved in the transport of lipids around the cell, Phospholipids of plasma membrane – the creation of lysosomes (Lysosomes are formed from the fusion of vesicles from the Golgi complex with endosomes) – plays an important role in the synthesis of proteoglycans, which are molecules present in the extracellular matrix of animals. 32  Function modifying, sorting, packaging In this respect it can be thought of as similar to a post office; – it packages and labels items, then sends to different parts of the cell – Sorting of cytosolic and secreted proteins 33  Cargo Selection, Coat Proteins, and Vesicle Budding Most transport vesicles that carry secretory proteins from the ER to the Golgi and from the Golgi to other targets are coated with cytosolic coat proteins and thus are called coated vesicles. Initially, the secretory proteins are sorted from proteins targeted for other destinations and from proteins that need to remain behind. 34  Cargo Selection, Coat Proteins, and Vesicle Budding The coats assemble as the secretory protein - containing vesicle buds of the donor membrane and are generally removed from the vesicle in the cytosol before it reaches its target. 35 36 Cargo Selection, Coat Proteins, and Vesicle Budding Three kinds of coated vesicles have been characterized: two types of COP-coated vesicles, COPI and COPII (COP indicates coat protein), and clathrin-coated vesicles. 37  Cargo Selection, Coat Proteins, and Vesicle Budding COPI - coated vesicles bud from the ER-Golgi intermediate compartment, or the Golgi apparatus. COPI is the coat protein on vesicles moving from one Golgi cisternae to another during secretion and on the retrieval vesicles that return resident ER proteins marked by the KDEL or KKXX retrieval signals back to the ER from the ER-Golgi intermediate compartment or the cis Golgi network. 38  KDEL signal KDEL is a target signal sequence (Lys-Asp-Glu-Leu) located on the C- terminal end of the amino acid structure on ER-resident proteins. The KDEL sequence prevents a protein from being secreted from the endoplasmic reticulum (ER) and facilitates its return if it is accidentally exported. A protein with a functional KDEL motif will be retrieved from the Golgi apparatus by retrograde transport to the ER lumen by COPI-coated vesicles. It also targets proteins from other locations (such as the cytoplasm) to the ER. Proteins can only leave the ER after this sequence has been cleaved off. 39  Cargo Selection, Coat Proteins, and Vesicle Budding COPII - coated vesicles carry secretory proteins from the ER to the ER-Golgi intermediate compartment or Golgi apparatus, budding from the transitional ER and carrying their cargo forward along the secretory pathway. 40 41  Cargo Selection, Coat Proteins, and Vesicle Budding Clathrin - coated vesicles are the best understood and are responsible for the uptake of extracellular molecules from the plasma membrane by endocytosis as well as the transport of molecules from the trans Golgi network to endosomes, lysosomes, or the plasma membrane. The formation of clathrin - coated vesicles on the trans Golgi network requires clathrin, the GTP-binding protein, ARF1, and at least two types of adaptor proteins 42  Cargo Selection, Coat Proteins, and Vesicle Budding While COPI- and COPII-coated vesicles have limited targets, clathrin-coated vesicles exit the trans Golgi for different destinations: endosomes, lysosomes, or different plasma membrane domains. Since these targets require specific cargoes, different adaptor proteins play a role in the assembly of vesicles for different destinations. 43  Vesicle Fusion The fusion of a transport vesicle with its target involves two types of events. First, the transport vesicle must recognize the correct target membrane; for example, a vesicle carrying lysosomal enzymes deliver its cargo only to lysosomes. Second, the vesicle and target membranes fuse, delivering the contents of the vesicle to the target organelle. 44 45  Vesicular transport Members of the Rab family of small GTP- binding proteins play key roles in this docking of transport vesicles. Rab proteins, like the ARF family, participate in many of the vesicle budding and fusion reactions during vesicular transport. 46  Griscelli Syndrome Griscelli syndrome is a rare disease caused by mutations in the gene encoding Rab27a. Rab27a appears to play a key role in the transport of vesicle trafficking, such as pigment-containing vesicles (melanosomes) to the skin and hair and in the exocytosis of vesicles in T lymphocytes. Patients with Griscelli syndrome exhibit partial albinism (lack of pigment) and are immunodeficient. 47 VIDEOS: Golgi Apparatus https://www.jove.com/science-education/11957/golgi- apparatus?playlist=74b3c7fd 48