9.3 Cell Junctions

Question 1

What are cell junctions?

Answer 1

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.

Question 2

What are the three general types of cell junctions?

Answer 2

1) Occludens (tight) junctions
2) Anchoring junctions: 3 types of anchoring junctions (adherence junctions, desmosomes, and hemidesmosomes)
3) Communication junctions

Question 3

What are occludens (tight) junctions?

Answer 3

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.

Question 4

What are anchoring junctions?

Answer 4

anchoring junctions are physical structures that attach cells to each other or to their surroundings to the extracellular matrix

Question 5

What are the three type of anchoring junctions and their role?

Answer 5

1) adherence junctions: which are supported by actin filaments
2) desmosomes (supported by intermediate filaments)
3) hemidesmosomes (supported by intermediate filaments)

Question 6

What are communication junctions?

Answer 6

allow movement of small ions and other signals between cells. They are not physically strong and they lack attachment to the cytoskeleton.

Question 7

Which junction is found closest to the top of a layer of cells?

Answer 7

occludens (tight) junctions

Question 8

What is the role of the occludens junctions?

Answer 8

to separate the top membrane from the membrane found in the lateral and basal regions of the cell

Question 9

How do occludens junctions help epithelial cells lining tubular compartments (glands and cells of the digestive and circulatory system)?

Answer 9

they help to regulate the passive movement of fluid across the cell layer.

Question 10

What is a hallmark of the occludens junctions?

Answer 10

there’s no elaboration of the surrounding cytoplasm aka they lack attachment to the cytoskeletal systems.

Question 11

What are occludens (tight) junctions called ininvertebrates?

Answer 11

septate junctions; some tight junctions can be pretty leaky in some cell types

Question 12

What is the structure of a tight junction? how are they formed?

Answer 12

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.

Question 13

What are septate junctions?

Answer 13

junctions that function like tight junctions but are found in invertebrates and are more regularly arranged.

Question 14

What proteins form septate junctions?

Answer 14

a protein called “discs-large” and is related to occludins

Question 15

What is the structure of an anchoring junction?

Answer 15

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

Question 16

How can anchoring proteins be organized/classified?

Answer 16

based on their function and the supporting cytoskeletal system

1) cell to cell
2) cell to matrix

Question 17

Adherens junctions are supported by what? Which proteins do they use?

Answer 17

Actin filaments

Transmembrane proteins: cadherins (e-cadherin)

Intracellular anchoring proteins: (Vinculin and alpha actinin)

Question 18

Where are cadherin super family of proteins found? what kind of proteins are they?

Answer 18

They are transmembrane proteins that are found in adherens junctions.

Question 19

How did cadherins get their name?

Answer 19

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

Question 20

What are catenin proteins?

Answer 20

coupling proteins that link the transmembrane adherens to the actin cytoskeleton.

Question 21

What is the name of proteins that attach most epithelial cells?

Answer 21

e cadherins, where e stands for epithelial

Question 22

What are focal adhesions and what transmembrane and intracellular anchoring proteins are used?

Answer 22

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

Question 23

What is the structure of an integrin? what is significant about this?

Answer 23

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.

Question 24

Why do integrins require calcium?

Answer 24

in order to maintain the correct shape necessary to allow them to bind to their target proteins.

Question 25

Desmosomes communicate how and what are the transmembrane proteins and intracellular anchoring proteins?

Answer 25

cell to cell.

Transmembrane: desmoglein and desmocollin

Intracellular: desmoplakin and plakoglobin

Question 26

What is the structure of desmosomes?

Answer 26

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)

Question 27

Why are desmosomes and hemidesmosomes stronger than actin supported junctions (adherens junctions)?

Answer 27

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.

Question 28

What forms stable attachments between epithelial cells?

Answer 28

desmosomes

these attachments are necessary since the function of an epithelial cell monolayer is to act as a barrier between fluid filled compartments.

Question 29

What are the the transmembrane integral and intracellular anchoring proteins of hemidesmosomes? how does it communicate?

Answer 29

cell to matrix

Transmembrane: integrins

Intracellular: plectins

Question 30

How are hemidesmosomes formed?

Answer 30

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.

Question 31

Where are hemidesmosomes typically found? where are they usually not found?

Answer 31

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.

Question 32

What is the structure of communication junctions?

Answer 32

lack supporting cytoskeletal elements and can be thought of as a specialized transmembrane pore that are formed by the cooperation of two neighboring cells.

Question 33

What are communication junctions called in animals?

Answer 33

gap junctions.

Question 34

Connexons are unique to which junction? Why?

Answer 34

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.

Question 35

What is the proteins that make up connexons?

Answer 35

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

Question 36

Gap junctions have small pores or channels, what can move through these channels?

Answer 36

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.

Question 37

Where are gap junctions commonly found?

Answer 37

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.

Question 38

How can the permeability of gap junctions be altered?

Answer 38

by changing the size of the central pore.

Question 39

What is the structure of communication junctions in plants?

Answer 39

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.

Question 40

What is a desmotubule?

Answer 40

the extension of the endoplasmic reticulum in plant communication junction. found in the center of plasmodesmata.

Question 41

What are plasmodesmata?

Answer 41

channels of cytoplasm that join neighboring plant cells through small holes in the cell wall.