Zaidi Protein Sorting 1

Question 1

Protein trafficking between the nucleus and cytosol is considered to be ____ transport. Transport from the cytosol across an organelle membrane is considered ____ transport. Membrane-enclosed transport between various compartments is considered ____ transport.

Answer 1

Gated.
Transmembrane.
Vesicular.

Question 2

Describe functions of Plasma Membrane (PM), Nucleus, Cytoplasm, ER, Golgi, Mitochondria, Lysosomes, Peroxisomes.

Answer 2

PM: protective barrier, transporting/signaling.

Nucleus: principal site for DNA and RNA synthesis.

Cytoplasm: contains cytosol/organelles. intermediary metabolism.

ER: rough (protein synthesis), smooth (lipid synthesis), protein folding, quality control, calcium storage, signaling.

Golgi: post-translational changes on proteins and lipids, trafficking.

Mitochondria: energy metabolism, signaling, cell differentiation, cell death (leaky mitochondria = cell death).

Lysosomes: contain digestive enzymes degrading organelles and biomolecules.

Peroxisomes: have enzymes used in oxidation reactions.

Question 3

What are the 3 topological divisions of a cell?

Answer 3

1. Nucleus and cytosol
2. Organelles (secretory and endocytic vesicular communication)
3. Mitochondria

Question 4

What is characteristic of a cell with more extensive RER membrane?

What is characteristic of a cell with more SER membrane?

What is characteristic of a cell that has more extensive Inner Mitochondrial membrane?

Answer 4

It produces a lot of enzymes.

It produces more lipids.

It is more metabolically active.

Question 5

What is the purpose of membrane budding and fusion?

Answer 5

Allows the lumen of each compartment to communicate with each other and the cell exterior.

Question 6

Where does gated transport occur?

How does transmembrane transport differ?

How does vesicular transport differ?

Answer 6

Between the nucleus and cytosol through nuclear pore complexes (utilizing active transport and free diffusion)

Membrane protein trans locators directly transport specific proteins from cytosol across an organelle membrane.

Proteins are moved by membrane enclosed transport intermediates between various compartments via vesicles.

Question 7

The inner nuclear membrane and the outer nuclear membrane make up the nuclear ____?

Answer 7

Envelope.

Question 8

What is the function of a Lys-Arg rich signal sequence?

A combination of positively charged and hydrophobic AA serves to…

A signal sequence of hydrophobic AA…

The KDEL signal sequence …

Answer 8

Imports protein into the nucleus.

Import a protein into the mitochondia.

Imports proteins into the ER.

Returns a protein to the ER.

Question 9

Transport between the nucleus and the cytosol is ___, ___, and ___.

Proteins needed in the nucleus and imported from the ___ where they are ___.

___ and ___ are synthesized in the nucleus and exported to ___.

Answer 9

Gated, Bidirectional, and selective.

Cytoplasm, synthesized.

The cytosol.

Question 10

These proteins, ___, compose the nuclear pore complexes and are arranged in a(n) ___ symmetry.

Small molecules move through NPCs via ___ and larger molecules move through NPCs via ___.

Transport is facilitated by binding of particles to ___.

Answer 10

nucleoporins.

octagonal.

small = passive diffusion.
larger = facilitated transport.

fibrils.

Question 11

The nuclear membrane is continuous with the __.

Answer 11

ER membrane

Question 12

Gated transport occurs via two different mechanisms based on the size of the molecule. What are they?

Answer 12

Smaller molecules enter the nucleus via diffusion.

Larger molecules enter the nucleus via active transport.

Question 13

Define Nuclear Localization Signals (NLS).

Answer 13

Sorting signals that direct molecules to the nucleus. They have short sequences rich in positively charged amino acids Lys-Ag. Result is selective import of proteins into the nucleus.

Question 14

Nuclear export

Answer 14

• opposite of nuclear import
• relies on nuclear export signals (NES) and complementary nuclear export receptors (NER).
• NER binds to cargo in nucleus and NPC proteins

1. Binding, 2. Dissociation, 3. Re-binding facilitates transport (like nuclear import)
   - cargo is released into cytoplasm

Question 15

Nuclear localization signals, located on the cargo, are recognized by ____.

Answer 15

Nuclear import receptors (NIRs)

Question 16

Describe the stepwise mechanism of nuclear import

Answer 16

1. Gradient of Ran conformational states drive nuclear transport in appropriate direction
2. Import receptor and cargo enter nucleus by interacting with FG repeats on NPC proteins
3. Ran-GTP binds to complex of impor receptor and cargo
4. Binding causes release of cargo
5. Ran GTP and import receptor leave nucleus
6. In cytoplasm Ran-GTP hydrolysis by Ran-GAP
7. Receptor in released from Ran-GDP in cytoplasm and is ready for another cycle

Question 17

Gene regulatory proteins are kept __ of __ until needed.

Transport is stringently controlled by turning ___ and __ on and off.

The 3 mechanisms included in this process are:

Answer 17

out, nucleus

NLS, NES

Phosphorylation, proteolysis, binding to inhibitory proteins

Question 18

What is the opposite of nuclear export signals (NES)?

Proteins that have both import and export signals, which are ____, function to…?

Steady state localization is dependent upon ____.

Answer 18

Nuclear localization signals (NLS) are used for import.

Shuttling proteins. Proteins that have both NLS and NES shuttle back and forth between nucleus and cytosol.

Localization is dependent on relative rate of transport. If import is greater than export, it is considered nuclear and vice versa.

Question 19

During T cell activation, increased Ca levels in the ____ causes ____. This allows the blocking of the nuclear export signal via binding of ____, which exposes the ____.

Once imported, ____ is activated. Low calcium in the nucleus removes ____, causing this step, ____, to expose the nuclear export signal.

Answer 19

cytosol.
cleavage of phosphates.
calcineurin.
nuclear import signal.

gene transcription.
calcineurin.
phosphorylation by ATP and protein kinase.

Question 20

During conditions of low cholesterol, you want to upregulate ezymes that do what?

SREBP, which stands for ___, undergoes what type of change during conditions of low cholesterol?

Answer 20

synthesize cholesterol.

Sterol response element binding protein (SREBP). During low cholesterol conditions, SREBP is cut into small pieces, with one piece acting as a transcription factor that helps promote cholesterol biosynthesis.

Question 21

Differentiate Direct Binding and Indirect Binding of the Nuclear Import Receptors.

Answer 21

Direct Binding: Cargo protein has nuclear localization signal, which attaches directly to the nuclear import receptor (NIR).

Indirect Binding: Cargo protein first attaches to a nuclear import adaptor protein. The adaptor protein then attaches to the NIR.

Question 22

Where are each of these molecules usually found?

GAP (GTPase activating protein), Ran-GDP, Ran-GTP, GEF (Guanine exchange factor).

What is the function of GAP and GEF?

Answer 22

GAP = cytosol. converts Ran-GTP to Ran-GDP.

Ran-GDP = cytosol.
Ran-GTP = nucleus.

GEF = nucleus. exchanges guanine, converting Ran-GDP to Ran-GTP.