Endosome Maturation and SNARE Proteins

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Questions and Answers

What occurs during the maturation from an early endosome to a late endosome?

The endosome begins to store nutrients for the cell.

Membrane proteins scheduled for degradation are internalized into intralumenal vesicles. (correct)

The endosome starts to increase in size due to vesicle fusion.

The endosome generates more tubular projections.

What is the pH change in the endosome during maturation?

The pH remains constant throughout maturation.

The pH inside the endosome becomes more acidic. (correct)

The pH becomes more alkaline.

The pH fluctuates but trends to become more basic.

What characterizes fully matured late endosomes?

They remain isolated from other organelles.

They generate new tubular projections.

They fuse with one another and with endolysosomes and lysosomes. (correct)

They actively send vesicles to the plasma membrane.

What is formed when a lysosome fuses with a late endosome?

An endolysosome. Signup and view all the answers

How are the stages of endosome maturation connected to the trans-Golgi network (TGN)?

Via transport vesicles that deliver lysosomal proteins. Signup and view all the answers

Which proteins are specifically responsible for facilitating the fusion of transport vesicles with target membranes?

SNARE proteins Signup and view all the answers

What roles do v-SNAREs and t-SNAREs play in membrane fusion?

They interact to form a trans-SNARE complex Signup and view all the answers

How does the specificity of vesicle fusion with target compartments occur?

By the types of Rab proteins and the presence of specific tethering proteins Signup and view all the answers

What happens during the bilayer fusion process mediated by SNARE proteins?

Lipid molecules flow between membranes to form a connection Signup and view all the answers

What is the primary mechanism through which cholesterol is transported into cells?

Receptor-mediated endocytosis Signup and view all the answers

How many types of SNARE proteins exist, and what is their importance?

35; each type is associated with a specific organelle Signup and view all the answers

Which component is essential for the binding of LDL particles to cell-surface receptors?

Apolipoprotein B Signup and view all the answers

What surrounds the core of cholesterol esters in LDL particles?

A lipid monolayer of phospholipids and cholesterol Signup and view all the answers

What is found in the core of Low Density Lipoproteins (LDL)?

Cholesterol esters Signup and view all the answers

Which protein complex is involved in the formation of intralumenal vesicles during endocytosis?

ESCRT protein complexes Signup and view all the answers

What is the primary role of the endoplasmic reticulum in cellular processes?

Biosynthesis of lipids and proteins Signup and view all the answers

What is the nature of proteins that are directed to specific intracellular locations?

They contain sorting signals recognized by sorting receptors. Signup and view all the answers

Which of the following accurately describes the arrangement of the endoplasmic reticulum?

It is organized as an interconnected network of tubules and flattened sacs. Signup and view all the answers

What types of proteins are produced by the endoplasmic reticulum?

All transmembrane proteins and resident proteins of organelles Signup and view all the answers

What role does the sarcoplasmic reticulum play in cellular function?

It stores and releases calcium ions. Signup and view all the answers

What role do chaperone proteins play in the endoplasmic reticulum?

They assist in the proper folding of proteins. Signup and view all the answers

What happens to proteins that fail to fold correctly in the endoplasmic reticulum?

They are exported to the cytosol for degradation. Signup and view all the answers

What is necessary besides chaperon proteins for proper protein folding?

The correct environmental conditions in the ER. Signup and view all the answers

How do oligosaccharides contribute to protein folding in the ER?

They mark the state of protein folding. Signup and view all the answers

What activates the unfolded protein response in the ER?

The accumulation of misfolded proteins. Signup and view all the answers

What is the function of Sec13 and Sec31 in vesicle formation?

Create the outer shell of the COPII coat Signup and view all the answers

How does the presence of different phosphoinositides (PIPs) influence protein recruitment?

They determine the specificity of protein domains for membrane regions Signup and view all the answers

Which of the following statements best describes phosphoinositides?

They can be phosphorylated in various positions Signup and view all the answers

What role do v-SNARES and t-SNARES play in vesicle and compartment identity?

They facilitate the docking and fusion of vesicles Signup and view all the answers

Why might different types of PIPs be associated with specific vesicle transport events?

As they attract different adaptor proteins to the membrane Signup and view all the answers

Which of these proteins is an example of an adaptor protein that mediates vesicle assembly?

AP2 Signup and view all the answers

What is a key reason why clathrin-coated vesicles do not form on the ER membrane?

The ER lacks sufficient phosphoinositides Signup and view all the answers

Which component is crucial for the identification of different organelles?

The specific Rab proteins associated Signup and view all the answers

Study Notes

Intracellular Organization and Trafficking

Eukaryotic cells have the same basic set of membrane-enclosed organelles
Organelles have different internal environments compared to the surrounding cytosol due to membranes
Major intracellular compartments in an animal cell include: mitochondrion, Golgi apparatus, smooth endoplasmic reticulum, nucleolus, rough endoplasmic reticulum, endosome, lysosome, cytosol, peroxisome, and free ribosomes
Proteins travel between organelles in various ways: protein translocation, gated transport, vesicular transport, and engulfment
Sorting signals within the amino acid sequence of a protein direct its movement. These signals are recognized by sorting receptors present in different organelles
The secretory pathway’s environment is similar to the outside of a cell
The synthesis of phospholipids takes place in the ER membrane. The lipids are only formed in one of the two layers of the bilayer that faces the cytosol. The ER assembles most lipid bilayers

Biomolecule Condensates

Cells also have biomolecule condensates, places with high concentrations of certain nucleic acids and proteins serving as biochemical factories
Specific biomolecule condensates include: nucleolus, pyrenoid, stress granules, P-granules, Balbiani body, Cajal body, paraspeckles, RNA transport granule, PML body, and postsynaptic density
Biomolecule condensates can include a series of macromolecules (nucleic acids and/or proteins) that serve as a scaffold for the whole condensate
These scaffolds attract and recruit other proteins and/or nucleic acids, which perform specific tasks
The nucleolus is an example of a biomolecule condensate that is involved in rRNA transcription, ribosome assembly, and pre-rRNA processing
The nucleolus is composed of dense and granular components.

The Endoplasmic Reticulum

The ER plays a significant role in the biosynthesis of lipids and proteins
It stores Ca²⁺ ions, which are used in various cellular processes
The ER is an interconnected network of tubules and flattened sacs, continuous with the outer nuclear membrane
The ER extends throughout the entire cytosol, with all regions close to some portion of the ER membrane
The ER is structurally and functionally diverse, with rough ER (with ribosomes) for the secretory pathway and smooth ER (without ribosomes) for lipid biosynthesis and metabolism
The proportion of rough ER to smooth ER varies among cell types

Proteins Present in the ER

Proteins that need to be secreted, resident proteins of the ER and Golgi, and transmembrane proteins are produced in the ER
The process of protein translocation across the ER membrane may or may not require ongoing polypeptide chain elongation.
- Co-translational translocation
- Post-translational translocation
Proteins that need to be embedded in the cell membrane commonly contain hydrophobic segments which are recognized like signal sequences
Some proteins are integrated into the ER membrane by a post-translational mechanism using chaperones like Get3
Some membrane proteins acquire a covalently attached glycosylphosphatidylinositol (GPI) anchor.
After the completion of protein synthesis, the precursor protein remains anchored to the ER membrane by a hydrophobic C-terminal sequence, while the rest of the protein is in the ER lumen. This process is managed through the action of a transamidase enzyme.
Resident ER proteins include translocons and receptors, signal peptidase, chaperones that aid proper folding, proteins that catalyze disulfide bonds, proteins that add polysaccharides to proteins (glycosylation), and proteases
Specialized proteins that fold other proteins in the ER

The Endocytic Pathway

Cells take up molecules from the outside via endocytosis, a process through which endocytic vesicles containing the materials from the outside fuse into the cell.
Recycling endosomes serve as intracellular protein storage sites. The protein storage is mobilized as needed
Endocytic vesicles form primarily from clathrin-coated regions of the plasma membrane, though there are exceptions
Early endosomes mature into late endosomes
ESCRT protein complexes mediate the formation of intralumenal vesicles in multivesicular bodies, where intralumenal vesicles (small membrane-bound vesicles inside a bigger compartment) are used to transport materials among different parts of the cell.

The Golgi Apparatus

The Golgi apparatus is an ordered series of flattened, membrane-enclosed compartments (cisternae) connected by tubular connections.
Materials pass between the cisternae in either vesicular transport or cisternal maturation mechanisms
The Golgi apparatus also processes proteins, including modifying/ removing/ substituting original sugars added at the ER and adding extra sugars elsewhere. This is carried out by Golgi resident proteins at different parts of the Golgi, a complex process that has been coined as glycosylation
The final step in the Golgi process is sorting of each vesicle into the correct compartment.

Autophagy

Autophagy is a cellular process where a portion of the cell's cytoplasm (including organelles and proteins) is engulfed in a double-membrane structure called the autophagosome, and then sent to lysosomes to degrade the contents
Autophagy can occur for these reasons: During starvation to supply with materials to survive, damaged organelles are repaired or replaced, specific structures are removed during development or in changing conditions, presence of bacteria or viruses inside cells, and presence of macroaggregates.
Many more reasons also trigger autophagy, as well

Lysosomes

Lysosomes are the terminal destination for degradation of proteins, microorganisms, dead cells, and other ingested materials.
The lysosome lumen contains a low pH that is maintained by a H⁺ pump.

Mechanisms of Membrane Transport and Compartment Identity

Vesicles and compartments have different types of coat proteins, phosphoinositides (e.g., PIPs), Rab proteins, v-SNARES and t-SNARES, therefore, establishing identity. Different coat-proteins and their properties facilitate many different kinds of transport pathways, including the ER to Golgi and vesicle to compartment.
The assembly and disassembly of clathrin-coated vesicles involve the interaction between adaptor proteins and various membrane-bending and fission proteins. The assembly of the coat introduces curvature into the membrane
Dynamin is in charge of pinching off clathrin-coated vesicles
Polymerization of actin filaments occurs near the vesicle neck, and helps transport the budding vesicle away from the plasma membrane

Cholesterol Transport

LDL contains a core of cholesterol esters
LDL is recognized by LDL receptors in the plasma membrane to form a clathrin-coated vesicle
The vesicle fuses to other vesicles, forming the early endosome; the clathrin coat is lost.
As the acidity increases, the LDL particle dissociates from the LDL receptor, and the LDL is retained in the lumen
The endosome matures into a endolysosome and lysosomes to be used by the cell for membrane production.

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Description

This quiz explores the intricate process of endosome maturation, focusing on the transition from early to late endosomes. It covers pH changes, the role of SNARE proteins in membrane fusion, and the connection to the trans-Golgi network. Test your knowledge on the specifics of cholesterol transport and the characteristics of late endosomes.