Protein Targeting

What are three primary destinations for proteins synthesized in a cell?

Proteins may exit the cell, become part of the membrane, or enter a subcellular compartment.

What is the primary function of an N-terminal signal sequence?

It directs proteins to the endoplasmic reticulum (ER).

What are the structural characteristics of an ER signal sequence?

It is 13-20 amino acids long, contains 10-15 hydrophobic amino acids, has one or more positively charged amino acids near the N-terminus, and a polar C-terminal sequence.

What molecule binds to the signal sequence as the peptide emerges from the ribosome?

The signal recognition particle (SRP).

Where does a protein go after being synthesized and modified in the ER?

It is transported via vesicles to the Golgi apparatus for sorting.

What is the difference between N-linked and O-linked glycosylation in terms of attachment site?

N-linked glycosylation attaches to the nitrogen of Asn or Arg, while O-linked glycosylation attaches to the oxygen of Ser, Thr, or Tyr.

Where does N-linked glycosylation typically begin?

In the rough endoplasmic reticulum (RER).

Where does O-linked glycosylation typically occur?

In the Golgi apparatus or the cytosol for proteins not entering the ER.

What is the role of dolichol phosphate in glycosylation?

It serves as the scaffold upon which the core oligosaccharide is built before being transferred to an Asn residue on a protein.

Which antibiotic is known to block the first step of N-linked glycosylation?

Tunicamycin.

How are proteins destined for mitochondria or chloroplasts delivered to these organelles?

They bind to chaperone proteins in the cytosol and are delivered to receptors on the exterior of the organelle.

What does NLS stand for in the context of protein targeting?

Nuclear Localization Sequence.

What are the typical characteristics of a Nuclear Localization Sequence (NLS)?

It is usually 4-8 amino acids long and contains several consecutive basic residues, such as Arginine (Arg) or Lysine (Lys).

Is the NLS removed after the protein reaches the nucleus?

No, the NLS is not removed.

Which proteins are responsible for binding a protein with an NLS to facilitate nuclear import?

Importin alpha and beta.

What molecule promotes the dissociation of importin beta inside the nucleus?

Ran-GTP.

What is the function of CAS (cellular apoptosis susceptibility protein) in nuclear transport?

It binds to importin alpha and Ran-GTP to help release the nuclear protein and recycle importin alpha out of the nucleus.

What happens to Ran-GTP after nuclear import is complete?

It is transported back into the nucleus.

What is the primary function of the Golgi apparatus in the secretory pathway?

It sorts proteins and sends them to their final destinations, such as the plasma membrane, lysosomes, or other organelles.

What modifications occur to N-linked oligosaccharides once they reach the Golgi complex?

They are further modified, and O-linked oligosaccharides may be added to some proteins.

Why is subcellular organization important in eukaryotic cells?

It aids in compartmentalizing metabolic pathways, allowing for specific protein targeting and regulation.