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Intracellular transport (Luirink)

33 important questions on Intracellular transport (Luirink)

One side of the translocation gate is always closed, what closes this gate?

It can be closed by a ribosome or a plug, called Sec61.

What is the end stage of translocation?

End-stage of translocation: signal sequence is cleaved and degraded. The mature protein folds and can be further sorted.

What are the four different types of membrane(-associated) proteins?

Transmembrane
Membrane-associated
Lipid-linked
Protein-attached

What are the three different types of transmembrane proteins?

Single-pass
Double-pass
Multi-pass

How are single-pass membrane proteins inserted?

There is one signal for targeting (on the N-terminus) and one signal for anchoring (in the middle). The targeting signal gets bound into the translocation channel. The rest of the protein goes into the lumen, until the anchoring signal goes into the translocation channel. Translocation stops and the targeting signal is cleaved. The N-terminus is now inside the lumen and the C-terminus is in the cytosol.

A single-pass membrane can also be formed with only one signal, what is this process?

There is an internal start-transfer sequence. This signal gets bound to the translocation channel. The N-terminus stays outside in the cytosol. The C-terminus goes inside the lumen.

What are the characteristics of vesicular transport?

Transport of both soluble and membrane proteins (“cargo”)
Different types of vesicles to ensure correct sorting (right cargo to right organel)
Transport must be specific and well-regulated
Continuous budding and fusion of vesicles maintains identity and diversity of vesicles

What are the functions of the protein coat of a vesicle?

Shape
Selection of cargo
Specificity of transport

What is the function of dynamin in vesicle budding?

Dynamin forms a ring structure around the neck of each deeply invaginated coated pit.

Does the coat always remain around the vesicle?

The coat is removed when the vesicle is close to its destination.

What is the connection between Rab proteins and tethering proteins?

Rab protein are found on the surface of a vesicle, they are recognized by the corresponding tethering proteins on the cytosolic surface of the target membrane.

What are SNARE proteins?

SNARE proteins — "SNAP" (Soluble NSF Attachment Protein) REceptor" — are large protein complexes. It mediates vesicle fusion.

How is vesicle fusion mediated by SNARE proteins?

The v-SNARE in the vesicle docks at t-SNARE in target membrane; many different SNAREs for specific fusion. V-SNARE + t-SNARE leads to a trans SNARE complex, which results in membrane fusion. When the v-SNARE and the t-SNARE are connected all of the surrounding water is pushed away.

How do SNARE proteins catalyze membrane fusion?

Once appropriately triggered, the tight pairing of v-SNAREs and t-SNAREs draws the two lipid bilayers into close apposition. The force of the SNAREs winding together squeezes out any water molecules that remain trapped between the two membranes, allowing their lipids to flow together to form a continuous bilayer. In a cell, other proteins recruited to the fusion site help to complete the fusion process. After fusion, the SNAREs are pried apart so that they can be used again.

What is the direction of the secretory pathway?

From endoplasmic reticulum to Golgi apparatus, to the plasma membrane to the external milieu.

What happens during transport of the secretory pathway?

Modification
Quality control
Sorting

What happens during modification in the secretory pathway?

While synthesizing the protein (going inside the lumen) asparagines gets recognized and glycosylated. A branched oligosaccharide side chain is attached.

What implies quality control in the secretory pathway?

Only proteins that were folded correctly can proceed to the Golgi apparatus. Unfolded proteins or proteins that were not folded correctly are bound to a chaperone protein. The chaperones hold the proteins in the ER until proper folding or assembly occurs. When folding doesn't occur properly the protein is exported to the cytosol where it is degraded.

What is a retention signal?

A retention signal is a combination of amino acids that ER resident proteins contain. When such a protein 'escapes' from the ER it will go back to the ER because of this signal.

What is the structure of the Golgi apparatus?

The Golgi apparatus is a stack of cisternae and tubules. It is polarized, the cis-side faces the ER and the trans-side faces the plasma membrane.

What is the function of the Golgi apparatus?

Modification of proteins, primarily trimming of sugars (glycosylation).

What are the two pathways of exocytosis?

Constitutive secretion
Regulated secretion

What is constitutive secretion?

It is a default, continuous process. It is present in all cells. It consists of a permanent flow of material to the membrane (i.e. extracellular matrix proteins).

What is regulated secretion?

Regulated secretion depends on stimuli (i.e. hormones or neurotransmitters, like insulin) and is only present in specialized cells. The proteins are stored in the vesicles near PM. Only released when there is a signal, so it is regulated.

What are the influences of signals on the sorting of proteins from the trans Golgi network?

No signal --> constitutive secretion
Unknown signal --> selective aggregation --> storage in vesicles for regulated secretion
Mannose-6-P --> targeting to lysosomes

What is the process of phagocytosis?

Uptake of bacteria --> large endocytic vesicles (phagosomes) --> fusion with lysosomes --> degradation.

What is the process of pynocytosis?

Uptake via clathrin coated pit--> clathrin coated vesicle --> uncoating --> fusion with early endosome --> late endosome --> lysosome

What are the two types of pynocytosis?

Ordinary pinocytosis
Receptor-mediated endocytosis (RME)

Describe the example of RME: LDL.

Cholesterol in blood in LDL --> binds to LDL receptor --> concentration in coated pit --> uptake --> endosome --> dissociation of LDL-receptor complex by low pH --> transfer to lysosome --> LDL is degraded/receptor returns to PM

What is the process of an early endosome to a lysosome?

Early endosomes contain the extracellular material that is taken up by pinocytosis. More early endosomes (or an early endosome plus a late endosome) fuse to form larger (late) endosomes. The late endosomes develop into or fuse with lysosomes.

What are the characteristics of lysosomes?

Degrades macromolecules by acid hydrolases
Low pH
One membrane that protects cytosol, contains many transporters and ATPase

What are the four pathways to enter lysosomes?

Pinocytosis/endocytosis
Phagocytosis
Autophagy
Vesicular traffic from GA

What is the process of vesicular traffic from the Golgi apparatus to lysosomes?

Patch signal sequence in hydrolase is recognized and M6P is attached in the Cis Golgi Network. M6P binds to M6P receptor in Trans Golgi Network. It is packed in clathrin coated vesicles. Delivery of cargo (hydrolase) at lysosome. M6P is dissociated from receptor by low pH, the receptor is recycled. Phosphate from hydrolase is removed and it is now an active mature enzyme.

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Cell biology - Histology

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