9.3 Cell Junctions Flashcards
What are cell junctions?
cell junctions are specialized structures that play roles in physical and chemical interactions between cells and their surroundings. Most cell junctions are complex structures and include a variety of accessory proteins found in the cytoplasm.
What are the three general types of cell junctions?
1) Occludens (tight) junctions
2) Anchoring junctions: 3 types of anchoring junctions (adherence junctions, desmosomes, and hemidesmosomes)
3) Communication junctions
What are occludens (tight) junctions?
usually found at the top of a polarized cell. They restrict the movement of extracellular fluid past the epithelial cell and they also restrict the movement of membrane components from one cell compartment to another.
What are anchoring junctions?
anchoring junctions are physical structures that attach cells to each other or to their surroundings to the extracellular matrix
What are the three type of anchoring junctions and their role?
1) adherence junctions: which are supported by actin filaments
2) desmosomes (supported by intermediate filaments)
3) hemidesmosomes (supported by intermediate filaments)
What are communication junctions?
allow movement of small ions and other signals between cells. They are not physically strong and they lack attachment to the cytoskeleton.
Which junction is found closest to the top of a layer of cells?
occludens (tight) junctions
What is the role of the occludens junctions?
to separate the top membrane from the membrane found in the lateral and basal regions of the cell
How do occludens junctions help epithelial cells lining tubular compartments (glands and cells of the digestive and circulatory system)?
they help to regulate the passive movement of fluid across the cell layer.
What is a hallmark of the occludens junctions?
there’s no elaboration of the surrounding cytoplasm aka they lack attachment to the cytoskeletal systems.
What are occludens (tight) junctions called ininvertebrates?
septate junctions; some tight junctions can be pretty leaky in some cell types
What is the structure of a tight junction? how are they formed?
tight junctions are formed by sealing strands which are two cells that form a branched network of contacts between plasma membranes
STRUCTURE: not a solid band of material but instead it’s made up of a network of tiny ridges.
What are septate junctions?
junctions that function like tight junctions but are found in invertebrates and are more regularly arranged.
What proteins form septate junctions?
a protein called “discs-large” and is related to occludins
What is the structure of an anchoring junction?
There are some transmembrane proteins that interact with partners on the adjacent cell or attach to extracellular matrix components but in addition there are cytoskeletal filaments. There is also some sort of coupling system that links the two coupling proteins..
All anchoring junctions have both intracellular anchor proteins and transmembrane adhesions proteins
How can anchoring proteins be organized/classified?
based on their function and the supporting cytoskeletal system
1) cell to cell
2) cell to matrix
Adherens junctions are supported by what? Which proteins do they use?
Actin filaments
Transmembrane proteins: cadherins (e-cadherin)
Intracellular anchoring proteins: (Vinculin and alpha actinin)
Where are cadherin super family of proteins found? what kind of proteins are they?
They are transmembrane proteins that are found in adherens junctions.
How did cadherins get their name?
from being calcium dependent adhesion proteins
Each cadherin is made up of a number of globular domains. Molecular calcium binds to these domains and causes them to protrude from the cell’s surface. Protruding cadherins from opposite cells join together
The interacting domains at the end of cadherins also require calcium in order to generate attachments
What are catenin proteins?
coupling proteins that link the transmembrane adherens to the actin cytoskeleton.
What is the name of proteins that attach most epithelial cells?
e cadherins, where e stands for epithelial
What are focal adhesions and what transmembrane and intracellular anchoring proteins are used?
focal adhesions are cell to surface junctions instead of cell to cell. They are supported by actin filaments.
Transmembrane proteins: Integrins
Intracellular anchoring proteins: vinculin and alpha actinin
What is the structure of an integrin? what is significant about this?
heterodimers made up of an alpha and a beta subunit.
There are a large number of genes for alpha and beta subunits which can combine in a larger number of combinations. Each combination allows the cell to interact with a specific component of the extracellular matrix
There are at 24 alpha subunit types and 9 beta subunit types. Various combinations show
specificity for different extracellular matrix and cell-surface proteins, and have different binding
strengths.
Why do integrins require calcium?
in order to maintain the correct shape necessary to allow them to bind to their target proteins.