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Related Concept Videos

Eukaryotic Compartmentalizations01:46

Eukaryotic Compartmentalizations

One of the distinguishing features of eukaryotic cells is that they contain membrane-bound organelles, such as the nucleus and mitochondria, that carry out specialized functions. Since biological membranes are only selectively permeable to solutes, they help create a compartment with controlled conditions inside an organelle. These microenvironments are tailored to the organelle's specific functions and help isolate them from the surrounding cytosol.
For example, lysosomes in the animal cells...
Eukaryotic Compartmentalization01:46

Eukaryotic Compartmentalization

One of the distinguishing features of eukaryotic cells is that they contain membrane-bound organelles, such as the nucleus and mitochondria, that carry out specialized functions. Since biological membranes are only selectively permeable to solutes, they help create a compartment with controlled conditions inside an organelle. These microenvironments are tailored to the organelle's specific functions and help isolate them from the surrounding cytosol.
For example, lysosomes in the animal cells...
Intralumenal Vesicles and Multivesicular Bodies01:38

Intralumenal Vesicles and Multivesicular Bodies

Intraluminal vesicles (ILVs) are small vesicles 50-80 nm in diameter formed during the maturation of early endosomes. A specialized endosome containing numerous ILVs is called a multivesicular body (MVB). ILVs contain internalized molecules such as antigens, nucleic acids, proteins, and metabolites. Some of these molecules are released from the MVBs inside exosomes and are transported to other cells. Other MVBs contain molecules that are retained in the ILVs and are later degraded within the...
SNAREs and Membrane Fusion01:43

SNAREs and Membrane Fusion

Once a transport vesicle has recognized its target organelle, the vesicular membrane needs to fuse with the target membrane to unload the cargo. Transmembrane proteins called SNAREs present on organelle membranes and their vesicles, mediate vesicle fusion.
SNAREs exist in pairs that symmetrically interact and catalyze the fusion of the lipid bilayers in vesicle and target organelle. v-SNARE in the vesicle membrane are single polypeptide chains that bind to a complementary t-SNARE, composed of 2...
Eukaryotic Compartmentalization01:37

Eukaryotic Compartmentalization

One of the distinguishing features of eukaryotic cells is that they contain membrane-bound organelles, such as the nucleus and mitochondria, that carry out specialized functions. Since biological membranes are only selectively permeable to solutes, they help create a compartment with controlled conditions inside an organelle. These microenvironments are tailored to the organelle's specific functions and help isolate them from the surrounding cytosol.
For example, lysosomes in the animal cells...
Mechanisms of Membrane Domain Formation00:59

Mechanisms of Membrane Domain Formation

Different physical properties of lipids and proteins allow them to localize and form distinct islands or domains in the membrane. Some membrane domains are formed due to protein-protein interactions, whereas others are formed due to the presence of specific lipids such as sphingolipids and sterols—for example, large proteins, such as bacteriorhodopsin, aggregate and create distinct domains.
Another mechanism for membrane domain formation involves membrane proteins interacting with cytoskeletal...

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Related Experiment Video

Updated: Jun 29, 2026

SNARE-mediated Fusion of Single Proteoliposomes with Tethered Supported Bilayers in a Microfluidic Flow Cell Monitored by Polarized TIRF Microscopy
10:58

SNARE-mediated Fusion of Single Proteoliposomes with Tethered Supported Bilayers in a Microfluidic Flow Cell Monitored by Polarized TIRF Microscopy

Published on: August 24, 2016

Endosomal compartmentalization in three dimensions: implications for membrane fusion.

J L A N Murk1, B M Humbel, U Ziese

  • 1Department of Cell Biology, University Medical Center Utrecht, Heidelberglaan 100, 3584 CX, Utrecht, The Netherlands.

Proceedings of the National Academy of Sciences of the United States of America
|November 5, 2003
PubMed
Summary

Endosomes sort cellular components. New research shows inner membranes are vesicles, requiring fusion for retrograde transport and T cell activation, revealing a novel fusion mechanism.

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SNARE-mediated Fusion of Single Proteoliposomes with Tethered Supported Bilayers in a Microfluidic Flow Cell Monitored by Polarized TIRF Microscopy
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Area of Science:

  • Cell Biology
  • Immunology
  • Membrane Trafficking

Background:

  • Endosomes act as central sorting hubs in the endocytic pathway, directing proteins and lipids to various cellular destinations.
  • Multivesicular endosomes contain internal vesicles, with proteins destined for degradation sorted internally and recycling proteins remaining on the outer membrane.
  • A retrograde pathway allows molecules like MHC class II to return from internal structures to the outer membrane for T cell activation.

Purpose of the Study:

  • To investigate the structural basis of retrograde transport from endosomal internal vesicles to the outer membrane.
  • To determine whether retrograde transport occurs via membrane fusion or direct continuity between membrane domains.
  • To elucidate the mechanism of protein transport from inner to outer endosomal membranes.

Main Methods:

  • Cryo-electron tomography was used to visualize the intricate architecture of endosomes in cryo-immobilized B-lymphocytes and dendritic cells.
  • 3D reconstructions were generated to analyze the structural relationship between inner and outer endosomal membranes.
  • Clathrin-coated areas on the outer membrane were examined in relation to inward budding vesicles.

Main Results:

  • Multivesicular endosomes possess inner membranes that are distinct, free vesicles, not continuous with the outer membrane.
  • Protein transport from inner to outer membranes necessitates fusion between these membrane domains.
  • A novel fusion mechanism is implied, involving the exoplasmic leaflets of the membranes, distinct from typical intracellular fusion.
  • Clathrin-coated regions were observed on the outer membrane, associated with inward budding, suggesting roles in cargo sorting.

Conclusions:

  • Retrograde transport from endosomal internal vesicles to the outer membrane requires a specific fusion event.
  • This finding challenges previous assumptions about membrane continuity and reveals a unique fusion mechanism.
  • The study highlights the complex sorting and trafficking processes within endosomes, crucial for immune responses.