Question

Match the item on the left that identifies a protein modification with the best-fitting description on the right GPl-anchor O-glycosylation [Choose] [Choose] Occurs in cis-Golgi by protein kinase activity Added to select ser or the residues in Golgi Catalyzed co-translationally in the oxidizing conditions of ER Occurs on cytosolic surface of ER Binding of glycoprotein by lectin chaperone Added co-translationally to C-terminus Catalyzed by flippase in Golgi Disulfide bridge formation Early states of ER-associated degradation pahthway [Choose) Phosphorylation of glycoproteins destined for lysosome [Choose] Synthesis of new lipid [Choose Enrichment of phosphatidylserine in one leaflet [Choose

Match the description on the left with the best-fitting term on the right

Answer 1

Answer

• GPI anchors : Added co translationally to C- terminus

Explaination: GPI anchors: Glycosylphosphatidylinositol anchors are glycolipids containing phsophatidyl inositiol helps in attachment of a protein to a membrane with the help of these anchors rather than with transmebrane spanning hydrophobic regions of a protein. GPI anchors are assembled in the ER membrane and are added to the protein after completion of protein synthesis at the C terminus. C terminal sequence is cleaved and exchanged for the GPI anchors, so that these proteins remain attached to the membrane by these glycolipids.

• O glycosylation: Added to select ser or thr residues in Golgi

Explaination: Proteins can also be modified by the addition of carbohydrates on the serine (ser) and threonine (thr) amino acid residues of the protein in the Golgi apparatus by the sequential addition of single sugar residues called as O linked glycosylation.

• Disulfide bridge formation: Catalyzed co-translationally in the oxidizing conditions of ER

Explaination : The formation of disulfide bond between the side chain of cysteine residues is important for the correct folding of the protein and thus, for its function.  This disulfide bond firmation takes place in ER, which is promoted by oxidizing environment of ER and catalyzed by enzyme PDI (Protein Disulfide Isomerase).

• Early states of ER associated degradation pathways: Binding of the glycoprotein by lectin chaperon  

Explaination: In ER associated degradation pathway -  any mis-folded proteins are identified, returned form the ER to the cytosol and degraded by the ubiquitin - proteasome system. Protein which can bind to carbohydrates and recognize them are called as lectins. In this pathway, as the glycoprotein exits the translocon channel, 2 glucose residues are removed, allowing lectin chaperone calreticulin to bind this misfolded glycoprotein and assist in correct folding.

• Phosphorylation of glycoproteins destined for lysosome: Occurs in cis-Golgi by protein kinase activity

Explaination: Proteins destined to the lysosomes are modified in the cis- Golgi by phosphorylation of the mannose residues. This phosphorylation is carried out specific protein kinases.

• synthesis of new lipid: Occurs on the cytoplasmic surface of ER

Explaination: Smooth ER is the major site of synthesis of new lipids. Eukaryotic cell membrane are extremely hydrophobic. Therefore, some lipids are synthesized with already existing cellular membranes of the ER. For example, phospholipids.

• enrichment of phosphatidyl serine in one leaflet: Catalyzed by flippase in golgi

Explaination: To maintain a stable membrane, some of these newly synthesized phospholipids like phosphatidyl serine, have to be transferred on one of  the side of the Golgi which is equivalent to the inner leaflet of plasma membrane and enrich it to ensure even growth of both halves of the bilayer. This transfer is catalyzed by enzymes called flippases.