Question

21. Which of the following mechanisms is used by cells to insure that proteins are correctly localized to specific organelles, integrated into the plasma membrane or secreted?
a. Proteins become glycosylated in the rough ER and Golgi and only proteins with sugar modifications are transported to their correct sub cellular location.
b. Proteins encode a sequence that acts as a signal, such as a nuclear localization signal (NLS), to target them appropriately.
c. Any protein being synthesized is immediately inserted into a vesicle yhat is coated with proteins that will target the vesicle to the correct compartment within the cell.
d. After translation, most proteins are transported by binding to chaperones in the cytosol, which actively transports them.
e. Proteins are usually synthesized by ribosomes that are pre-localized to the place the protein will be needed.

Answer 1

The correct option is b.

The mechanism of protein targeting is different for each subcellular organelle. Basically protein targeting depends on signal sequence.

The first branching in transport of proteins happens by presence or absence of N terminal signal peptide.

If N terminal signal peptide is present- Synthesis happens on RER - proteins can be directed to plasma membrane, endoplasmic reticulum or golgi apparatus, lysosomal or proteins for exocytosis

If N terminal signal is absent- synthesis happens on cytoplasmic ribosome - proteins are targeted to nucleus, peroxisome, mitochondria etc.

Some other signals for transport to a particular cell organelles are-

Nuclear localisation signal (NLS) - transport to nucleus

Peroxisomal target sequence containing 3 amino acids- Serine, lycine and leucine- transport to peroxisome

mannose 6 phosphate- transport to lysosome.

Therefore, every cell organelle has it's own signal sequence in order to avoid confusion in protein targeting. There is no one size fits all method for protein targeting.

Option A is incorrect. as glycosylation is not the only way for transportation. It is only needed in few cases like mannose 6 phosphate is added to the proteins which are destined for transport to lysosomes. But, there are others like mitochondrial proteins are not glycosylated before transport. Hence, this option is incorrect

Option C is incorrect- not all the proteins but mainly those proteins which are targeted (after synthesis on Rough endoplasmic reticulum) to plasma membrane or golgi apparatus are transported as vesilcles (transport). Each vesicle has it's own set of coat proteins for correct targeting like clathrin coat protein is found in the vesicles which are programmed for exocytosis. Hence, this option is incorrect

Option D is incorrect- Again, not all the proteins are transported by binding with chaperones. Such transport happens in case where proteins are transported in un folded form. Example. Mitochondrial proteins. Hence, this option is incorrect

Option E is incorrect as there is no such system that ribosome attaches itself to every cellular organelle for protein delivery. Ribosome exists only in two states- either in cytoplasm or bound to endoplasmic reticulum. Hence, this option is also incorrect.