Question

Explain the transport of extracellular membrane proteins following their post translational modification in the ER.

Answer 1

Once post-translational modification of proteins is achieved inside ER they are then transported to Golgi Membrane (GM) and finally transported to their targeted locations on the cell surface.

Transport of proteins from ER to cis Golgi :

1. The export proteins are exported from ER to cis Golgi network, the first compartment of Golgi apparatus. This transport is carried out by the formation of transport vesicles followed by the targeting and fusionn of these vesicles.

2. Transport vesicles arise from specialized coated regions of membranes which are surrounded by a coat of proteins coverin gthe cytosolic face so that these membranes eventually slough off as coted transport vesicles.

3. Transport vesicless are typically of three types depending on the composition of the protein coat and various adaptor proteins and small GTP binding proteins required for their formation. The three types are as follows :

a. Clathrin coated - Clathrin associated with AP2 forms vesicles from the plasma membrane during endocytosis that transport to the early endosomes.

b. COP I coated - It forms vesicles for both intra- Golgi transport and retrograde transport from Golgi to ER.

c. COP II coated - It forms vesicles for the anterograde transport from ER to Golgi.

4. COP II coated vesicles then bud off from ER and carry the proteins to cis Golgi.

Oligosaccharide processing in Golgi compartments :

As the protein enters the Golgi Network, it undergoes a series of sorting within the cisternae of Golgi and is then transported to respective target sites.

1. Golgi membrane is subdivided into cis- cisternae, medial cisternae and trans- cisternae.

2. In the cis-cisternae, the lysosomal proteins undergo phosphorylation of oligosaccharides and other proteins may undergo removal of Mannose subunits..

3. In the medial cisternae, again removal of Mannose and addition of N-acetylglucosamine (N- linked glycosylation) can take place.

4.  In trans- Golgi, Galactose subunits and N-acetylmuraminic acid (NANA) can be added depending on the protein which is being transported.Sulfation of tyrosines and carbohydrates also takes place in trans Golgi.

5. The intramembrane transport within Golgi cisternae is thought to occur through cisternal maturational model where each Golgi cisterna matures as it migrates outward through a stack and at each stage, the Golgi resident proteins that are carried forward in cisterna are moved backward to an earlier compartment. In this way each cisterna progresses into the next one.

6. From here the proteins are then enclosed within respective transport vesicles and targeted either to lysosome or Plasma membrane.

7. The vesicles then fuse with the plasma membrane with the help of SNARE proteins.

PS -For your reference I have added certain images of the complete process from Alberts - Molecular Biology of the Cell 6th Edition.

ER 7 cis Golgi network cis SORTING phosphorylation of oligosaccharides on lysosomal proteins • removal of Man . removal of Man addition of GINAC • addition of Gal . addition of NANA • sulfation of tyrosines and carbohydrates SORTING Golgi apparatus Figure 13-29 Oligosaccharide processing in Golgi compartments. The localization of each processing step shown was determined by a combination of techniques, including biochemical subfractionation of the Golgi apparatus membranes and electron microscopy after staining with antibodies specific for some of the processing enzymes. Processing enzymes are not restricted to a particular cisterna; instead, their distribution is graded across the stack, such that early-acting enzymes are present mostly in the cis Golgi cisternae and later-acting enzymes are mostly in the trans Golgi cisternae. Man, mannose; GlcNAc, N-acetylglucosamine; Gal, galactose; NANA, N-acetylneuraminic acid (sialic acid). cisterna medial cisterna trans cisterna Golgi stack trans Golgi network lysosome plasma membrane nelabrane secretory vesicle secesito ry

glucosidase! 2 UDP UDP 3 CMP Golgi mannosidase II glucosidase 11 Golgi mannosidase N-acetylglucosamine transferase ER mannosidase - UDP UDP 5 UDP +3 CMP - Asn Asn - An next added here high-mannose oligosaccharide Endo H- sensitive Endo H- resistant complex oligosaccharide GOLGI LUMEN ER LUMEN KEY: = N-acetylglucosamine (GICNAC) = mannose (Man) = glucose (Gl) = galactose (Gal) = N-acetylneuraminic acid (sialic acid, or NANA) Figure 13-31 Oligosaccharide processing in the ER and the Golgi apparatus. The processing pathway is highly ordered, so that each step shown depends on the previous one. Step 1: Processing begins in the ER with the removal of the glucoses from the oligosaccharide initially transferred to the protein. Then a mannosidase in the ER membrane removes a specific mannose. The remaining steps occur in the Golgi stack. Step 2: Golgi mannosidase i removes three more mannoses. Step 3: N-acetylglucosamine transferase I then adds an N-acetylglucosamine. Step 4: Mannosidase ll then removes two additional mannoses. This yields the final core of three mannoses that is present in a complex oligosaccharide. At this stage, the bond between the two N-acetylglucosamines in the core becomes resistant to attack by a highly specific endoglycosidase (Endo H). Since all later structures in the pathway are also Endo H-resistant, treatment with this enzyme is widely used to distinguish complex from high-mannose oligosaccharides. Step 5: Finally, as shown in Figure 13-30, additional N-acetylglucosamines, galactoses, and sialic acids are added. These final steps in the synthesis of a complex oligosaccharide occur in the cisternal compartments of the Golgi apparatus: three types of glycosyl transferase enzymes act sequentially, using sugar sub that have been activated by linkage to the indicated nucleotide; the membranes of the Golgi cisternae contain specific carrier proteins that allow each sugar nucleotide to enter in exchange for the nucleoside phosphates that are released after the sugar is attached to the protein on the lumenal face.

vesicular tubular cluster or Golgi apparatus secretory protein vesicular tubular cluster COPIL coat a soluble ER resident protein FORWARD PATHWAY KDEL KDEL receptor C protein empty KDEL receptor RETRIEVAL PATHWAY COPI coat COPI coat soluble ER resident protein ER (B) cis Golgi network cis, medial, and trans Golgi cisternae trans Golgi network