Biochem Lecture 9

Question 1

What are the major roles of the cytoskeleton?

Answer 1

Cell movement
Cell shape
Moving and positioning cell components
Mechanical strength to tissues/cells

Question 2

What are the three major structural elements of the cytoskeleton?

Answer 2

Microfilaments (actin)
Microtubules
Intermediate filaments

Question 3

What are the general properties of Microtubules?

Answer 3

hollow tube about 25nm in outer diameter
made up of two monomers alpha and beta tubulin
has polarity with + and - ends
is used for chromosome segregation in mitosis, intracellular transport, movement of cilia and flagella

Question 4

What are the general properties of Actin?

Answer 4

8nm diameter
two intertwined chains of F-actin
made up of G-actin monomers
has polarity with + and - ends
used for cell shape, cell movement, muscle contraction

Question 5

What are the general properties of intermediate filaments?

Answer 5

8-12nm diameter
made of 8 protofilaments joined end to end with staggered overlap
protofilaments are made of polypeptide dimers that have wrapped around each other and stacked end to end
no polarity
used for structural support, and provide strength to cells

Question 6

What proteins are in intermediate filaments?

Answer 6

Keratin (epithelial cells)
Vimentin (fibroblasts)
Neurofilaments (neurons)

Question 7

What causes ALS or Lou Gherig’s disease?

Answer 7

Neurofilaments mutate and cause an abnormal accumulation and assembly of neurofilaments which causes a progressive loss of motor neurons which leads to muscle atrophy, paralysis, and death.

Question 8

What are desmosomes?

Answer 8

They link keratin intermediate fibers of the epithelia into a continuous protein network that provides structural stability. Desmosomes interacting with intermediate fibers allows tissues to withstand tearing and shearing forces.

Question 9

What is epidermolysis bullosa simplex?

Answer 9

A mutation in the keratin genes prevent assembly into protofilaments so the skin blisters really easy.

Question 10

How are microfilaments formed?

Answer 10

Microfilaments are actin based. G-actin (actin bound to ATP) is globular. It binds to the + end of the chain and assembles to make a microfilament. Eventually the ATP is hydrolyzed into ADP at the - end of the chain.
The filamentous form of actin is called F-actin.

Question 11

What are three ways that actin filaments arrange into structures?

Answer 11

Parallel bundles: make up microvilli
Contractile bundles: make up stress fibers
Networks: cell cortex which determines cell shape

Question 12

How do Myosin and actin interact and what types of Myosin are there?

Answer 12

Myosin I is a monomer that moves cargo along actin fibers.

Myosin II are in muscles and convert ATP into mechanical energy by muscle contraction.

Question 13

Where are microfilaments found in the cell and what are they anchored to?

Answer 13

Located at the cell periphery at the plasma membrane.

They are anchored to adherens junctions between cells, and at focal adhesions in cell-extracellular matrix.

Question 14

Where are intermediate filaments found and what are they anchored to?

Answer 14

Found throughout the cell.

Anchored to Desmosomes (cell-cell) and hemidesmosomes (cell-extracellular matrix).

Question 15

What are the two groups that microtubules can be classified into?

Answer 15

1. Cytoplasmic microtubules: form mitotic and meiotic spindles, spatial distribution and trafficking of vesicles and organelles
2. Axonemal microtubules: form central shaft or axoneme of cilia and flagella

Question 16

How do microtubules grow and disintegrate?

Answer 16

While GTP bound tubulin is available, the tubule will grow mostly at the plus end until GTP tubulin is no longer available. This allows the GTP at the cap of the tubule to be hydrolyzed into GDP and cause catastrophe which is depolymerization. GTP at the tip stabilizes it, but once hydrolyzed, it becomes unstable.

Question 17

What are microtubule organizing centers?

Answer 17

These are centrosome which are made up of two centrioles. They are located near the nucleus and anchor the minus end of microtubules.

Question 18

What are kinesin and dynein?

Answer 18

Proteins that walk along microtubules to transport vesicles and other things to and from the cell membranes. Kinesin moves towards the plus end of microtubules, dynein moves towards the minus end. Both use ATP for their duties.

Question 19

How are microtubules involved with Cilia and flagella?

Answer 19

Axonemal microtubules form the central shaft of cilia and flagella. They bend and move through the action of dynein that makes the tubes slide past each other.

Question 20

What is Kartagener syndrome?

Answer 20

Dynein is absent in flagella and cilia. This causes male sterility and chronic respiration infection because the flagella of the sperm can’t move, and the cilia in the lungs can’t sweep mucus up and out.

Question 21

What are the two regions of the golgi and what are the two roles the golgi fulfills for the cell?

Answer 21

REGIONS:

1. cis-Golgi network (CGN): closest to the ER and continually receives newly synthesized lipids/proteins
2. trans-Golgi network (TGN); farthest from ER, proteins and lipids are packaged and bud off here

ROLES:

1. Post translational modification
2. Sorting of proteins to their final destination

Question 22

How do proteins destined for the ER get there from the Golgi?

Answer 22

Proteins destined for the ER have a KDEL sequence at their tails that signal the Golgi to send them back to the ER.

Question 23

What are constitutive and regulated secretion?

Answer 23

Constitutive: the default pathway, products are secreted as they are made, done in all cells, membrane proteins are secreted this way

Regulated: in specific cells, always mediated through calcium signaling, secretory vesicles build up and wait for a signal to release, hormones such as insulin and glucagon do this as well as neurotransmitters epinephrine and acetylcholine, also digestive enzymes

Question 24

What is the pH environment inside lysosomes?

Answer 24

There is an H+ pump at the membrane of lysosome that creates a low pH inside the organelle.

Question 25

How do proteins destined for lysosomes get to their destination?

Answer 25

Lysosome enzymes are post-translationally modified by the addition of mannose-6-phosphate which acts as a tag for transport to the lysosome. The mannose sugar is added in the ER and the phosphate is added in the cis-golgi. It then binds to a M6P receptor that transports it to a late endosome that becomes a lysosome once the pH gets low enough.

Question 26

What is I-cell disease?

Answer 26

The enzyme that adds M-6-P to lysosome enzymes doesn’t work, so the body gets an accumulation of undegraded macromolecules because lysosomes can’t do their jobs. This causes developmental delays, short trunk dwarfism, and death by 7 years. This is one example of a lysosomal storage disease.