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.
Desmosomes communicate how and what are the transmembrane proteins and intracellular anchoring proteins?
cell to cell.
Transmembrane: desmoglein and desmocollin
Intracellular: desmoplakin and plakoglobin
What is the structure of desmosomes?
there are transmembrane proteins associated with desmosomes which attache the cells to each other. These attachment proteins are also members of the cadherin super fam (desmogelin and desmocollin)
The couplin proteins found in desmosomes are plakoglobin and desmoplakin (found in dense plaque of protein)
Why are desmosomes and hemidesmosomes stronger than actin supported junctions (adherens junctions)?
in general intermediate filaments are stronger and more stable than actin filament and so the junctions that have them are also stronger and more stable than adherens junctions.
b/c of ther strong cell to cell connection, desmosomes are most common in mechanically stressed tissues.
What forms stable attachments between epithelial cells?
desmosomes
these attachments are necessary since the function of an epithelial cell monolayer is to act as a barrier between fluid filled compartments.
What are the the transmembrane integral and intracellular anchoring proteins of hemidesmosomes? how does it communicate?
cell to matrix
Transmembrane: integrins
Intracellular: plectins
How are hemidesmosomes formed?
formed by a single cell and can be sort of considered as half of a normal desmosome. Their function is to stably attach a cell to the surface that it’s growing on.
Where are hemidesmosomes typically found? where are they usually not found?
not generally found in motile cells but they are found in a number of specialized situations where strong and stable attachments are needed.
example: attaching the cells lining the tooth cavity to a tooth.
What is the structure of communication junctions?
lack supporting cytoskeletal elements and can be thought of as a specialized transmembrane pore that are formed by the cooperation of two neighboring cells.
What are communication junctions called in animals?
gap junctions.
Connexons are unique to which junction? Why?
Gap junctions. gap junctions can have small pores or channels. Each channel is formed by a hexagonal group of six proteins created by an individual cell called a connexon. Two connexons (one from each cell) will fuse to form a channel.
What is the proteins that make up connexons?
connexins. there are a number of different connexin proteins and they can be grouped together in different ways to create connexons. This allows an individual connexon to have a unique property
Gap junctions have small pores or channels, what can move through these channels?
Gap junctions allow free movement of molecules smaller than about 5000 daltons from one cell to its neighbor. 5000 daltons is about 1/8th the size of an actin monomer. Gap junctions
therefore allow very rapid movement of small signaling molecules between cells much more rapidly than would be the case if some sort of secretion event was needed.
Where are gap junctions commonly found?
in tissues where the response of large numbers of cells need to be coordinated together rapidly.
Ex: cells in the heart are coupled by gap junctions, which helps them coordinate contraction at the same time.
How can the permeability of gap junctions be altered?
by changing the size of the central pore.
What is the structure of communication junctions in plants?
a channel is formed between the cell walls of adjacent cells and a region of endoplasmic reticulum extends between cells through this tube. The cytoplasm of the two cells is therefore continous.
What is a desmotubule?
the extension of the endoplasmic reticulum in plant communication junction. found in the center of plasmodesmata.
What are plasmodesmata?
channels of cytoplasm that join neighboring plant cells through small holes in the cell wall.
