Lecture 10 - Intermediate filaments and microtubules

Question 1

What is the general structure of intermediate filaments?

Answer 1

Relatively stable homo- or heterodimers made of tetrameric repeats. Assembly requires no cofactors and these filaments are relatively stable (except lamins). No apparent functional polarity, not used in motility. High tensile strength.

Question 2

What are the four classes of intermediate filament proteins?

Answer 2

Keratins, vimentins (and desmins, glial fibriallary acid protein), neurofilaments, nuclear lamins

Question 3

Describe keratins

Answer 3

found in epithelial cells, hair, nails, etc. Main fxn to provide strength, essential for the body surface barrier fxn of skin… Desmosomes mediate cell-cell attachment (can be helpful in determining and treatment of epithelial cancer)

Question 4

Describe vimentin, desmin, and glial fibrillary acidic protein

Answer 4

Vimentin widespread cells of mesodermal origin, desmin primarily in muscle cells where it holds together adjacent myofibrils, GFA is in astrocytes and glial cells

Question 5

Describe neurofilaments

Answer 5

three subunits co-assemble into filaments in neurons that extend along the length of axons; providing these long extensions with tensile strength

Question 6

Describe nuclear lamins

Answer 6

meshwork on the inner surface of the nuclear membrane of all cells - most dynamic of the IFs - disassemble at the beginning of mitosis and reassemble at the end of mitosis. Phosphorylation regulates this cycle. Mutations can lead to progeria

Question 7

Describe the structure of microtubules

Answer 7

25 nm diameter hollow tubes assembled from dimers of alpha and beta tubulin (contain GTP binding sites), only beta tubulin can hydrolyze GTP

Question 8

Describe how microtubules are assembled

Answer 8

Requires Mg++, GTP, and enough subunits - beta tubulin acts as a slow GTPase - if GTP is present, microtubule can’t be degraded. Structural polarity (+ end = fast growing, - end = slow growing). When subunits contain GDP, tubule is unstable and peels away.

Question 9

Give examples of anti-microtubule drugs.

Answer 9

Colchine - blocks assembly of microtubles by binding to free tubulin, treats gout. Taxol (Paclitaxel) - binds and stabilizes microtubules, arrests cells in mitosis.

Question 10

Describe the organization and function of microtubules

Answer 10

Dynamic instability model - when all GTP is hydrolyzed, plus ends disassemble - allows cell to selectively stabilize MTs. Microtubule organizing centers (MTOCs) determine microtubule distribution by providing sites for nucleation (gamma tubulin). MTOC + centriole = centrosome.

Question 11

Are all microtubules dynamic?

Answer 11

No, maturation is the process when MTs fail to show dynamic instability. Capping proteins, and acetylation of subunits make MTs stable.

Question 12

What is the function of microtubule associated proteins?

Answer 12

create specialized MT arrays in different places within cells and tissues, and change the surface for interaction with other proteins.

Question 13

Describe the two microtubule motors

Answer 13

1. Dyneins - minus-end directed motility - cytoplasmic dyneins are two-headed (cilia and flagella, 1 or 3 heads) - carry large MW items, like the nucleus
2. Kinesins - two headed ATPases that produce plus-end directed motility - can carty neurotransmitters

These motor proteins central to organelle/vesicle transport and segregation of chromosomes at mitosis.

Question 14

What are special microtubule “organelles”?

Answer 14

1. Mitotic apparatus - will be covered later
2. Cilia and flagella - ( 9 doublet + 2 configuration), may be motile, sensory, or both. Axoneme is the name of the structure and all of the associated proteins. Basal body is 9 triplet microtubule, serves as the nucleation site

Question 15

Describe the location and function of motile cilia and flagella - example of disease

Answer 15

Sperm, cilia in airways, fallopian tubes, and brain ventricles (to provide flow for cerebral spinal fluid).

Kartagener’s syndrome (Primary ciliary diskinesia) - genesis is lack of motility in what should normally be motile cilia - leads to resp diseases, infertility in males, and left-right asymmetry

Question 16

Location and function of non-motile (sensory) cilia, and example of disease.

Answer 16

Non-motile cilia lacking dynein arms and central microtubules are found on most cell types and called primary cilia. Found in the ear (balance), eye (rods and cones), and the kidney.

Polycystic kidney disease( PKD) - mutations in gene that encodes protein needed for assembly of primary cilia in lining of kidney tubules. Abnormal growth regulation and formatino of cysts.

Question 17

Describe structures in axonemes

Answer 17

ciliary doublet microtubules interconnected by dyenin arms, radial spokes, etc. Two rows of dyenin moters along each doublet microtubule. Two MTs in the middle do not have dyneins, but have associated proteins that regulate beating activity of flagellum.

Question 18

Describe the process that produces movement in cilia and flagella…

Answer 18

Dyenin ATPase activity is coupled to microtubule sliding. Regulated sliding results in localized bend formation.

Interconnections between radial spokes regulat bend formation to control beat frequency.