Cell and Molecular Bio Exam #2

Where does synthesizing proteins begin?

Ribosome

What are sorting signals?

Signals defined by a synthesized protein’s amino acid sequence that tell the cell where to sort a protein to

True or False: All proteins have sorting signals

False

What are the four pathways of a protein? (regardless of if it has or does not have sorting signals)

1. It goes into another organelle

2. It remains free floating in the cytoplasm

3. It leaves the cell altogether

4. It embeds itself into the membrane

What are the three types of protein transport?

1. Gated Transport

2. Protein Translocation

3. Vesicular Transport

What are some features of gated transport?

Selective, requires energy (active transport)

What is an example of gated transport?

Protein and RNA moving between the nucleus and cytoplasm via nuclear pore complexes

What do molecules that travel between the nucleus and cytoplasm typically travel through?

Nuclear pore complexes

Describe protein translocation

Transmembrane (across a membrane) movement of a protein assisted by protein translocators

What is a protein translocator?

A channel embedded in a membrane that helps move polypeptides across it

What must usually be true in order for protein translocation to occur?

The protein must unfold in order to “snake through”

What is partial translocation and what does it achieve?

It is the partial crossing of a protein across a membrane, embedding a transmembrane protein.

What is vesicular transport?

Transport via vesicles, membrane enclosed

Where does vesicular transport often begin from?

Golgi or Endoplasmic Reticulum

What do vesicles do in vesicular transport?

They pinch or bud off of a membrane, enclosing molecules in the process that can then be transported elsewhere

The nuclear envelope/outer nuclear membrane is continuous with _________

endoplasmic reticulum

What does the inner nuclear membrane have attached to it that act as binding sites?

Nuclear lamina (structural meshwork)

What is the nuclear lamina and where is it located?

On the inner nuclear membrane, they are proteins/filaments that act as binding sites

What molecules/structures enter the nucleus via nuclear pore transport?

Histones, DNA polymerase, RNA polymerase

What is the nuclear pore complex composed of?

~30 nucleoporins

What is the sorting signal to the nucleus?

Nuclear localization signal (NLS)

What must recognize the nuclear localization signal?

Nuclear import receptors (importans)

What are features of the nuclear localization signal?

It is responsible for selectivity, contains 1-2 short sequences of positively charged amino acids (especially arginine and lysine), and can be present anywhere on the amino acid chain

How does import to the nucleus occur?

Nuclear import receptors bind the nuclear localization signal on the cargo protein —> transport to the nucleus through the nuclear pore complex

What is RanGAP and what is its full form

It is an enzyme known as Ran GTPase activating protein

What does RanGAP do?

It hydrolyzes GTP, converting RanGTP to RanGDP by removing a phosphate

Where is RanGAP localized to?

It is an enzyme localized to the cytoplasm

What is RanGEF and what is its full form

It is an enzyme known as Ran Guanine Exchange Factor

What does RanGEF do?

It exchanges GDP for GTP on the Ran protein

Where is RanGEF localized to?

It is an enzyme localized to the nucleus

Describe the cycle of movement between the cytoplasm and nucleus of the Ran protein

Ran-GDP in the cytoplasm binds to nuclear import receptors and travels to the nucleus, where there are high concentration of Ran-GEF. In the nucleus, the Ran-GEF replaces the GDP on Ran to a GTP. Now, Ran-GTP binds to nuclear export receptors and travels back to the cytoplasm, where there is a high concentration of Ran-GAP. In the cytoplasm, the Ran-GAP hydrolyzes Ran-GTP, removing a phosphate and generating Ran-GDP.

Describe the process of protein import into the nucleus

1. Nuclear import receptor binds the nuclear localization signal

2. Fibrils on the nuclear pore complex help with docking

3. The NLS associated Import Receptor enters the nucleus

4. Ran-GTP binds to import receptor when protein reaches nucleus because of high concentration of Ran-GEF

5. Ran-GTP bound import receptor releases NLS/cargo which can now execute its functions

6. Ran-GTP bound import receptor returns to cytoplasm

7. Ran-GTP bound import receptor is hydrolyzed by Ran-GAP, releasing Ran-GDP

8. Nuclear import receptor is recycled for another round of import

Describe the process of protein export from the nucleus

1. Ran-GTP in the nucleus promotes the binding of protein to the export receptor (export receptor is bound to Ran-GTP) - complex forms

2. Complex is transported through nuclear pore to cytoplasmic side

3. In the cytoplasmic side, the complex encounters Ran-GAP, causing the GTP hydrolysis of Ran-GTP to Ran-GDP

4. Everything in complex is released: Ran-GDP, export receptor, and the protein

5. Protein can execute its functions in the cytoplasm

6. Export receptor is recycled and can be used for another round