Protein sorting and protein traffic Flashcards
Why is it important for proteins to be sorted to the correct compartments?
Different organelles need different proteins in order to function correctly. Some proteins (enzymes) could be harmful to other parts of the cell.
Where does protein synthesis occur?
In the cytoplasm.
What happens to proteins once they are synthesised?
They can remain in the cytoplasm, they can be targeted into the nucleus, they can go to the ER for trafficking or be distributed to the mitochondrion.
What are sorting signals?
Provide information regarding where the proteins should be sorted to. They are amino acid sequences.
What are the three types of sorting signals?
Those responsible for sorting to the nucleus, those to the mitochondria and those to the ER.
What amino acids are important in nuclear targetting?
Lysine and Arginine.
How does nuclear targeting work in Nucleoplasmin?
It has two stretches of positively charged amino acids that are close to each other when the protein is folded. A positive patch forms that can be recognised by sorting machinery.
How has the importance of the positively charged amino acids in nuclear targeting been shown?
T-antigen with a single amino acid change were expressed in cells in culture. The two types were sorted into different places, showing that the change in the stretch of positively charged amino acids means the protein no longer contains the correct import signal.
What does it mean that the double membrane is contiguous with the endoplasmic reticulum?
The space between the inner and outer membranes is directly connected with the lumen of the endoplasmic reticulum.
Where does movement of molecules into and out of the nucleus occur?
Through nuclear pores.
What is each nuclear pore composed of?
A large number of protein subunits. Fibrils protrude from both sides of the complex.
What is the word for when two subunits are the same?
Heterodimeric.
What are the words for when two subunits are different?
Homodimeric.
How are proteins imported through the nuclear pore complex?
Nuclear import receptors recognise NLS’ (nuclear localisation sequences). The complex of the receptor and cargo binds to the fibrils on the cytoplasmic side. Repeats on the fibrils guide the complexes to the nuclear pore. The binding of nuclear protein to the pore opens the pore and there is active transport into the nucleus.
What is the conformation of proteins that are imported into the nucleus?
They are fully folded.
