Intra-cellular Traffic and Sorting of Protein

Q-1. Agranular endoplasmic reticulum takes part in synthesis of
a) Lipids
b) Proteins
c) Carbohydrates
d) Vitamins

Answer: Lipids
Agranular reticulum:
It is characterized by the absence of ribosomes due to which its walls are smooth.
This Agranular reticulum is most common in those cells which are concerned with steroid or lipid synthesis.

Q-2. Secretory proteins are synthesized in:
a) Cytoplasm
b) Endoplasmic reticulum
c) First in cytoplasm and then in Endoplasmic Reticulum
d) First in Endoplasmic Reticulum and then in cytoplasm

Answer: Endoplasmic reticulum
Secretory proteins are synthesized in rough endoplasmic reticulum
Transport route:
Rough Endoplasmic Reticulum -> Golgi body->Secretory vesicles

Q-3. The collagen triple helix structure is not found in:
a) Cytoplasm
b) Golgi apparatus
c) Lumen of endoplasmic reticulum
d) Intracellular vesicles

Answer: Cytoplasm.
Transport route:
Synthesis of collagen: RER-> GB-> Intra-cellular secretory vesicles-> Extra-cellular space
Important points:
Collagen is the major insoluble fibrous protein in the extracellular matrix and in connective tissue.

Q-4. Proteins are sorted by
a) Golgi bodies
b) Mitochondria
c) Ribosomes
d) Nuclear membrane

Answer: Golgi bodies
The Golgi apparatus is involved in glycosylation and sorting proteins.

Q-5. Intracellular sorting and packing done by
a) ER
b) Golgi apparatus
c) Ribosome
d) Cytoplasm
e) Nucleus

Answer: Golgi apparatus
The Golgi apparatus, or Golgi complex, functions as a factory in which proteins received from the ER are further processed and sorted for transport to their eventual destinations.
Important point:
In addition, glycolipids and sphingomyelin are synthesized within the Golgi.

Q-6. Following organelles are involved in the formation N-glycosylated product
a) Golgi apparatus
b) Nucleolus
c) SER
d) RER
e) Peroxisome

Answer: a and d
Organelles involved in the formation N-glycosylated product:
RER (Synthesis)
Golgi apparatus (Final processing)
Important points:
N-glycan chains if present are added as these proteins transverse the inner part of the ER membrane- a process called co-translational glycosylation.
Subsequently the proteins are found in the lumen of Golgi Apparatus where the further changes in glycan chains occur.

Q-7. Refsum’s disease is due to deficiency of which of the following enzyme?
a) Malonate dehydrogenase
b) Thiophorase
c) Succinate thiokinase
d) Phytanic alfa oxidase

Answer: Phytanic alfa oxidase
Refsum disease (RD) is a neuro-cutaneous syndrome that is characterized biochemically by the accumulation of phytanic acid in plasma and tissues.
Patients with Refsum disease are unable to degrade phytanic acid because of a deficient activity of phytanoyl-CoA hydroxylase, a peroxisomal enzyme catalyzing the first step of phytanic acid alpha-oxidation.

Q-8. Proteins targeted for destruction in eukaryotes are covalently linked to
a) Clathrin
b) Pepsin
c) Laminin
d) Ubiquitin

Answer: Ubiquitin
Proteins that are defective or destined for rapid turnover are often marked for destruction by ubiquitination- the attachment of small, highly conserved protein, called Ubiquitin.
Proteins marked in this way are rapidly degraded by a cellular component known as the proteasome, which is complex, ATP-dependent, proteolytic system located in cytosol.
Important points:
Chaperone associate with nascent protein and ensures final proper conformation of the nascent protein
Signal peptide guides the trans-membrane proteins into endoplasmic reticulum.
Ubiquitin marks aged and abnormal proteins for degradation
Lysosomal protease doesn’t require ATP.

  1. Which of the following groups of proteins assist in the folding of other proteins?
    a) Proteases
    b) Proteosomes
    c) Templates
    d) Chaperones

Answer: Chaperones
Chaperons are proteins that prevent faulty folding and un-productive interactions of other proteins.
Many chaperones are heat shock proteins, that is, proteins expressed in response to elevated temperatures or other cellular stresses.
Human chaperone proteins:
General chaperones: GRP78/BiP
Lectin chaperones: Calnexin and Calreticulin
Non-classical molecular chaperones: HSP47 and ERp29
Folding chaperones:
Protein disulfide isomerase (PDI)
Peptidyl prolyl cis-trans-isomerase (PPI)

Q-10. The ligand-receptor complex dissociates in the endosome because
a) Of its large size
b) The vesicle loses its clathrin coat
c) Of the acidic pH of the vesicle
d) Of the basic pH of the vesicle

Answer: Of the acidic pH of the vesicle
Endocytosed cell-surface receptors dissociate from their ligands within late endosomes.
The acidic pH of late endosomes causes most receptors and ligands to dissociate.