Bio Lecture 18 Cell Junctions

Question 1

What do adherens junctions do and where are they found?

Answer 1

Also called zonula adherens.
Links actin cytoskeleton of cells to make a continuous framework of proteins that make up the structure of the epithelia.
Found only on the lateral sides of the cells.

Question 2

What are desmosomes?

Answer 2

Spot welds that link the keratin or intermediate filaments of adjacent cells. This is what gives the tissue ability to resist tearing and shearing forces.

Question 3

What two junctions regulate molecular traffic between and through cells?

Answer 3

Tight junctions (zonula occludin): these restrict flow between cells and maintain cell membrane polarity, their permeability can be adjusted by the cell
Gap junctions: 6 connexins form a connexon that is a channel for molecules to pass through, these can open and close

Question 4

What junctions bind a cell to the basal lamina?

Answer 4

Focal adhesions: using actin filaments

Hemidesmosomes: using intermediate fibers

Question 5

What is the transmembrane protein for both focal adhesions and hemidesmosomes?

Answer 5

integrins

Question 6

What are the three major proteins that make up the basal lamina?

Answer 6

Laminin: the cross looking protein, made of 3 interwoven proteins held together by disulfide bonds, the 3 heads can bind to each other or to collagen

Type IV Collagen: 3 protein strands wrapped linearly, can bind to itself or laminin

Fibronectin: v-shaped molecule made of two proteins connected by disulfide bonds, binds to integrins and collagen and proteoglycans

Question 7

Explain the process for collagen formation.

Answer 7

1) Pro alpha chain synthesized into the ER
2) hydroxylation of lysines and prolines
3) glycosylation of hydroxylysines
4) Self assembly of 3 pro alpha chains
5) Procollagen triple-helix is formed
6) Secreted out of cell into ECM
7) Propeptides are cleaved and become tropocollagen
8) Tropocollagen is hydrofobic so it self assembles into fibrils which have the striated look
9) Fibrils can then aggregate to form collagen fibers

Question 8

What amino acids are involved in the stabilization of the tropocollagen triple-helix?

Answer 8

Repeating sequences of glycine, proline, and hydroxyproline cause kinks in the strands that allow them to interact and form a helix.

Question 9

How does vitamin C affect the formation of collagen?

Answer 9

Vit. C is needed for the functioning of Prolyl and lysyl hydroxylases which hydroxylate proline and lysine in the ER.
Hydroxyl proline is needed to form kinks in the tropocollagen and hydroxylysine is needed to form intermolecular bonds in the collagen.

Question 10

What is the role of hydroxylysine in the collagen molecule?

Answer 10

Hydroxylated lysine and normal lysine both form cross-link bonds within and between tropocollagen molecules. Lysyl oxidase carries out this reaction. The hydroxylysines form stronger links than normal lysine links.

Question 11

What is scurvy?

Answer 11

Lack of Vit. C causes decrease hydroxylation of proline and lysine in newly made collagen fibers. This affects tissues that frequently make and break down collagen such as the gums and ligaments around teeth. The collagen can no longer resist mechanical stress.

Question 12

What fiber in ECM allows for strength and flexibility?

Answer 12

Elastin fibers

Question 13

What structures help tissues to resist tension and compression forces?

Answer 13

Collagen fibers resist tension

GAGs are cushions for compressive forces