1.14: Extracellular Matrix II: Matrix Proteins

Question 1

What is a basal lamina?

Answer 1

The basal lamina is a layer of extracellular matrix on which epithelium sits and which is secreted by the epithelial cells

• Fibrous and flexible layer that separates organized groups of cells. It’s secreted by adjacent cells, 40-50mm thick.

Question 2

What are the four major proteins found in the ECM?

Answer 2

Collagen, Fibronection, Laminin, Elastin

Question 3

The major proteins found in the ECM serve two major purposes. What are they?

Answer 3

adhesion and structure

Question 4

Adhesion is achieved by which two proteins?

What do these two proteins do?

Answer 4

Fibronectin and Laminin

These two proteins attach cells to the extracellular matrix.

Question 5

Structure is acheived by which two proteins?

What do these two proteins allow?

Answer 5

Elastin and Collagen?

These two proteins allow body parts to stretch and return to the original shape, such as skin and inhaling air into the lungs

Question 6

What do integrins provide?

Answer 6

Provide the critcal link between cells and their surrounding matrix.

-Cells attach to ECM by means of integrins

Question 7

The extracellular portion of integrains binds to what?

The intracellular portion of integrines binds to what?

Answer 7

1) binds to various types of ECM proteins: fibronectin, laminins, Collagens
2) binds to actin filaments

Question 8

What is the most abundant type of fibrous protein?

How much percentage of total protein mass does it make up?

Answer 8

Collagen

30%

Question 9

What’s the function of collagen?

Answer 9

Provides tensile strength, regulates cell adhesion, supports chemotaxis and migration, and directs tissue development.

Question 10

Collagen is transcribed and secreted by what?

Answer 10

fibroblasts

Question 11

How are fibroblasts able to organize collagen fibrils?

Answer 11

By exerting tension onto matrix, they can organize collagen fibrils into sheets and cables.

This can dramatically influence the alignment of collagen fibers

Question 12

What’s the composition of collagen?

Answer 12

They are glycosylated proteins rich in glycine, proline, hydroxyproline and hydroxylysine

Normally is Gly-Pro-Y and Gly-X-Hyp

where X and Y are any amino acids

Question 13

How many types of collagen are there?

How many major classes?

Answer 13

28 type and 5 major classes

Question 14

What is type I collagen chain composition and representative tissues?

Answer 14

Skin, bone, tendon, blood vessels, cornea

Question 15

How are collagens constructed?

Size and Shape?

Answer 15

Constructed from 3 left handed helical peptides twisted into a right-handed triple helix – homotrimer or heterotrimer

Collagen is a rod-shaped molecule about 3000A long and only 15A thick. Its three helically intertwined alpha chains may have different sequences, but each chain has about 1000 amino acids residues.

Question 16

How are collagen fibrils made up?

Answer 16

made up of collagen molecules aligned in a staggered fashion and cross-linked for strength

Question 17

How is collagen synthesized?

Answer 17

*1) Synthesis of Pro-alpha chain

2) Hydroxylation of selected prolines and lysines (in ER/Golgi compartment)

*3) Glycosylation of lysine

4) self-assembly of three pro-alpha chains and procollagen triple-helix formation
5) Secretion

*6) in cell: Cleaveage of N- and C-terminal propeptides

7) Self-assembly into fibril –> collagen fibrils aggregate with others to form collagen fiber

Question 18

In the synthesis of collagen, what enzyme/vitamin is requried in order for the hydrozylation of Proline and Lysine residues?

Answer 18

Ascorbate (Vitamin C)

-Scurvy: disease due to lack of vitamin C

Question 19

In the synthesis of collagen, what follows after the hydrozylation of lysine and proline?

Answer 19

Covalent crosslinking btwn lysine and residues:

-Following cleavage, collagen fibrils are strengthened by the covalent crosslinking btwn lysine residues of the constituent collagen molecules by lysyl oxidases (LOX)

Question 20

Role of Lysyl oxidase?

Answer 20

an extracellular copper enzyme that catalyzes formation of aldehydes from lysine residues in collagen.

-These aldehydes are highly reactive, which will allow them to undergo spontaneous chemical rxns with other lysyl oxidase-derived aldehyde residues, or with unmodified lysine residues. This resules in the cross-linking of collagen.

Question 21

What is collegen Fibrillogenesis?

What four extracellular chaperones are required for appropriate protein folding during collagen fibrillogenesis?

Answer 21

Process of forming a collagen fibril

Requirements: Fibronectin, Collagen V, Integrin alpha2beta1, integrin alpha5beta1

Question 22

How are non-fibrillar collagens made and how are they different from fibrillar collagens?

Answer 22

• Triple stranded helical structure are interrupted by one or two short nonhelical domains. This allows the molecules to be more flexible.
• They are not cleaved after secretion and therefore retain their propeptides.
• They do not aggregate with one another to form fibrils in the extracellular space. Instead, they bind in a periodic manner to the surface of fibrils formed by the fibrillar collagens.

Question 23

What is type IV collagen?

Answer 23

a sheet forming collagen.

• There’s a small globular domain at the N terminus, a large one at the C terminus.
• Triple helix is interrupted by nonhelical segments that introduce flexible kinks.
• Lateral interactions between triple helical segments, as well as head to head and tail to tail interactions between the globular domains.

Question 24

Where would you see interactions of fibrous and non-fibrous collagens?

Answer 24

1) Tendons
2) Cartilage
3) Different tissues contain combinations of fibrous and non-fibrous collagens

Question 25

What are the three steps in collagen degradation?

Answer 25

a) Enzyme binding b) Unwinding of triple helix c) Cleavage of collagen peptides - initial cleavage yields gelatin: Cleared by denaturation and protease digestion followed by phagocytosis. Degradation occurs in extraceullar matrix

Question 26

What are the only two enzymes that has this collagenous property for collagen degradation?

Answer 26

Matrix metalloproteinases (MMPS) and Cathepsins ( L and K) -Hard to make, hard to break... that's why only a few enzymes can do this collagen degradation

Question 27

Role of Elastin? Elastin stretch is limited by what?

Answer 27

another major ECM fiber. - Provides recoil to tissue that undergo repeated stretches. - Elastin stretch is crucially limited by tight association with collagen fibrils.

Question 28

Synthesis of Elastin is similar to collagen. How?

Answer 28

- Soluble tropoelastin secreted into the extracellular space and assembled into elastic fibers - After secretion, the tropoelastin become highly cross-linked to one another, generating an extensive network of fibers and sheets - The cross-links are formed btwn lysines. - like collagen, crosslinks occurs etracellularly and provides stability

Question 29

Where can elastin be found?

Answer 29

lungs, walls of large blood vessels, elastic ligaments.

Question 30

Structure of elastin?

Answer 30

750 amino acids long with predominantly small nonpolar amino acids such as glycine, alanine, and valine. - also rich in proline and lysine - elastin can be in relaxed and stretched conformations - formed as a 3D network of cross-linked polypeptides that have an irregular conformation. Cross-link involve lysine making a desmosine cross-link btwn elastin chains.

Question 31

What regulates the limit of elastin stretch?

Answer 31

Long, inelastic collagen fibrils are interwoven with the elastic fibers to limit the extent of stretching and prevent the tissue from tearing.

Question 32

Structure of fibronectin

Answer 32

- a multiadhesive matrix protein that binds cells to the ECM - 440 kDa glycoprotein with dimers of two similar polypeptides linked at their C-termini by two disulfide bonds - secreted as a soluble protein dimer and is then assembled into an insoluble matrix. - FN can be stretched several times over its resting length thus exposing integrin-binding sites

Question 33

What does fibronectin bind?

Answer 33

Binds integrin, collagen fibrin and heparan sulfate

Question 34

Through interactions with adhesion receptors, what does fibronectins influence?

Answer 34

Influence shape and movement of cells and the organization of cytoskeleton and conversely cells can regulate the matrix

Question 35

Fibronectin plays role in what else? (6)

Answer 35

- cell adhesion/mediating cell attachment and function - growth - cell migration during development and differentiation - wound healing - embryonic development

Question 36

What are four ECM proteins found in ALL basal laminae?

Answer 36

Laminin Perlecan Type IV collagen Entactic/Nidogen

Question 37

Structure of Laminin? Short arm, long arm?

Answer 37

major protein in basal lamina - trimeric glycoprotein (aby), chains covalently linked by disulfide bonds - Short arms good at binding other laminin molecules in order to form sheets (multiadhesive) - Long arm specialized for binding to cells. - Principal basal lamina ligan of integrin

Question 38

Strucure of Integrins

Answer 38

consists of alpha and beta transmemebrane polypeptides - extracellular domains combining to form binding sites for divalent metal ions and proteins of the extracellular matrix or specific surface proteins of other cells. - receptors for fibronectin, collagen, laminin, fibrinogen, vitronectin, Ig superfamily