Chapter 15: Intracellular compartments and Protein Transport

What is the function of the ER?

Its function is to synthesize proteins and most lipids, and distribute them to many organelles and to the plasma membrane

What is the function of the golgi apparatus?

Its function is to modify, sort, and package proteins and lipids for either secretion or distribution to other organelles

What is the function of lysosomes?

Its function is to cause intracellular degradation

What is the function of Endosomes?

its function is to sort endocytosed materila

What is the function of the mitochondria?

Its function is to synthesize ATP through oxidative phosphorylation

What is the function of peroxisomes?

Its function is to oxidate the breakdown of toxic materials

What do the following organelles each have?

• ER

• Nucleus

• Golgi apparatus

• Lysosomes

• Endosomes

• Mitochondria

• Peroxisomes

They each have distinct compartments that are separated from the cytosol by at least one selectively permeable membrane

What is the function of the chloroplast?

Its function is to synthesize ATP and carbon fixation through photosynthesis

What does the cytosol contain?

It contains many metabolic pathways, such as where protein synthesis occurs, and cytoskeletons

What are the 3 mechanisms that organelles can use to transport proteins?

Proteins can be transported..

• Through nuclear pores

• Across the membrane

• By vesicles

With each of the 3 mechanisms requiring an input of energy to occur

Which transport mechanisms transport folded proteins? And which mechanism transports unfolded proteins?

Nuclear pore transport and vesicle transport are able to transport folded proteins, while Membrane transport is able to transport unfolded proteins

What are signal sequneces?

They are stretches of amino acids that direct proteins to their correct destination

• Additionally, the signal sequence of one protein can be removed and added to another protein, changing both proteins destination

What do nuclear pores form?

They form gates, made out of many proteins, with a nuclear basket structure on the nuclear side and cytosolic fibrils on the cytosol side

What do the nuclear pore gates allow for?

They allow for selected macromolecules and larger complexes to enter or exit the nucleus

What are nuclear import receptors?

They are receptors in the nucleus that dictates whether a protein is imported into the nucleus depending on whether they have nuclear localization signals

What has to be present for proteins to be imported into the nucleus through the nuclear pore?

A supply of energy from GTP hydrolysis is required for the import of proteins into the nucleus

How do nuclear pores transport proteins into the nucleus?

1. A nuclear import receptor binds to a protein with a nuclear localization signal in the cytosol

2. The receptor then guides the nuclear protein into the nuclear pore by interacting with its cytosolic fibrils

3. Inside the nucleus, Ran-GEF converts Ran-GDP into Ran-GTP by replacing GDP with GTP

4. Afterwards, Ran-GTP binds to the receptor, causing it to release the imported protein inside the nucleus and remain there

5. The receptor-Ran-GTP complex then returns to the cytosol, where Ran-GAP hydrolyzes Ran-GTP into Ran-GDP, releasing the receptor and allowing for new nuclear proteins to bind

How are proteins imported into the mitochondria?

1. A mitochondrial protein recognizes a mitochondrial import receptor on the outer mitochondrial membrane and binds to it

2. The translocator in the outer membrane (TOM) associated with the receptor then allows for the protein to enter into the inner membrane

3. Once in the inner membrane, the protein continuously diffuses laterally in the outer membrane until the signal sequence is recognized by the translocator in the inner membrane (TIM) and enters into the matrix where the signal sequence is cleaved off from the protein

What occurs to a protein as it is transported into the mitochondrial matrix?

It becomes unfolded in the process

How are proteins transported into the ER?

1. Ribosomes translating proteins with an ER signal sequence are directed to the ER membrane and SRP binds to them, slowing down protein synthesis

2. The newly formed SRP-ribosome complex then binds to the SRP receptor on the ER membrane and SRP is released as a result

3. The ribosome then moves from the SRP receptor to a protein translocator where protein synthesis continues

4. As protein synthesis continues, the signal sequence on the protein is cleaved off by a signal peptidase, allowing for the soluble ER protein to cross the ER membrane and enter into the lumen

What happens when a protein with an N-terminal ER signal sequence and a stop-transfer sequence enters the ER translocator?

The stop-transfer sequence halts translocation and anchors the protein to the membrane. With the N-terminal signal sequence being cleaved off, producing a single-pass transmembrane protein.

What happens when a protein has both a start-transfer and stop-transfer sequence as it enters the ER translocator?

The start-transfer sequence initiates membrane anchoring, and the stop-transfer sequence halts translocation, resulting in a double-pass transmembrane protein.

How are lipids transferred from the ER to a mitochondrion?

Lipid transfer proteins anchored on both organelles form junctional complexes that allow for the organelles to interact with each other and allow lipids to transfer between the two

What happens in the inward endocytic pathway?

Extracellular molecules are taken in by endocytosis and delivered to the early endosome, then to the late endosome, and finally to the lysosome for degradation 

What happens in the outward secretory pathway?

Water-soluble proteins move from the ER to the G.A, entering from the cis face, and exiting from the trans face into the P.M for exocytosis or to lysosomes for degradation

What is the retrieval pathway?

A pathway where material is returned from the G.A back to the ER, including proteins that accidentally left the ER and were meant to remain in the ER

How do proteins move through the G.A?

By Vesicles shuttling the proteins between individual golgi cisternae from the cis to the trans face

How do Clathrin-coated vesicles transport selected cargo molecules?

1. Cargo receptors bind to specific cargo molecules and allow for adaptins to bind to the receptor

2. Clathrin coat proteins then bind to the adaptins, forming a clathrin-coated budding vesicle

3. Afterwards, Dynamin wraps around the neck of the budding vesicle and hydrolyzes GTP to pinch off the vesicle from the membrane

4. Once released, the vesicle removes its clathrin coat proteins and becomes a naked transport vesicle, that is able to fuse with its target membrane

How are vesicles directed to their target membrane?

1. A Rab protein on the vesicle surface binds to a tethering protein on the target membrane, and triggers the vesicle to begin docking onto the target membrane

2. The v-SNARE on the vesicle then binds to the complementary t-SNARE on the target membrane to ensure the vesicle properly docks

3. After being docked, the SNAREs wrap around each other and pull the two membranes close together, driving membrane fusion with the help of SNARE proteins

4. Once fusion is complete, the SNAREs are pried apart and v-SNARE is recycled for future use

What do unfolded proteins cause in the ER?

They cause stress in the ER as misfolded proteins buildup and if too much stress is built up, then cell death is triggered

How do Misfolded proteins cause the production of chaperone proteins?

1. When a misfolded proteins binds to an ER sensor protein, it triggers the coding genes that code for chaperone proteins to produce the protein

2. The chaperone proteins then bind to unfolded proteins and help guide them to their proper folded formation, reducing the # of misfolded proteins and reducing stress in the ER

What happens in the constitutive secretion pathway?

Many soluble proteins are continuously secreted from the cell due to unregulated exocytosis, with the pathway also continuously supplying the P.M with new proteins and lipids

What happens in the regulated secretion pathway?

Selected proteins in the trans Golgi network divert into secretory vesicles and remain stored until an extracellular signal stimulates their secretion

How do LDLs release cholesterol into the cell?

1. An LDL binds to an LDL receptor on a cells surface which allows it to enter the cell, becoming internalized in a clathrin coated vesicle once inside

2. The vesicle then loses their coat and fuses with the endosome, allowing for LDL to dissociate from the receptor

3. From there, LDL is delivered into a lysosome and broken down, releasing cholesterol while LDL receptors return to the P.M

What are the 3 different fates a receptor protein can have?

They can either be:

• recycled back into the apical plasma membrane

• Moved from the endosomal compartment into lysosome to be degraded

• Returned to a different domain of the plasma membrane through transcytosis