Lecture 10- Intermediate filaments and microtubules

Question 1

General structure of intermediate filaments

Answer 1

10nm diameter filaments are assembled from homo or heterodimers of intermediate filament proteins via tetrameric intermediates

No cofactors
No apparent polarity–not good for motility

Elongated, fibrous proteins
Alpha helical coils and globular heads

Four classes

Question 2

Four classes of intermediate filaments

Answer 2

Keratins
Vimentin and vimentin-related
Neurofilaments
Nuclear lamins

Question 3

Keratins

Answer 3

• Epithelial cells (hair, nails, etc)
• Function: provide strength
• Essential for body surface barrier function of skin
• Typing can be useful in the diagnosis and treatment of epithelial cancers
• Mutation in keratins can cause epidermolysis bullosa–blister disease

Question 4

Vimentin, Desmin, and Glial Fibrillary Acidic Protein (GFA)

Answer 4

• Vimentin widespread in cells of mesodermal origin
• Desmin primarily in muscle cells where it holds together adjacent myofibrils (contractile units)
• GFA in astrocytes and glial cells (supporting cells of CNS)

Question 5

Neurofilaments

Answer 5

• Three subunits co-assemble into filaments in neurons that extend along the length of axons
• Provides long cellular extensions with tensile strength

Question 6

Nuclear lamins

Answer 6

• Meshwork on the inner surface of the nuclear membrane of all cells
• Most dynamic IF
• Disassemble at the beginning of mitosis and reassemble at the end of mitosis
• Mutation in lamins can cause progeria syndrome

Question 7

Microtubules

Answer 7

• 25 nm hollow tubes

- Assembled from dimers of alpha and beta tubulin (both have GTP binding sites but only B can hydrolyze)

Question 8

Assembly of microtubules

Answer 8

• Requires GTP, Mg, and a critical subunit concentration
• Beta tubulin in a microtubule acts as a slow GTPase and GDP must be exchanged for GTP again before a subunit is re-used for assembling another microtubule
• End to end binding in a head to tail orientation results in structural polarity of microtubules which have a plus (fast growing) and a minus end.
• Microtubule associated proteins and drugs can regulate assembly in vitro and in vivo

Question 9

Colchicine

Answer 9

Binds to free tubulin and blocks its assembly into microtubules
Used to treat gout

Question 10

Vinblastine/vincristine

Answer 10

Block MT assembly, also anti mitotic and preferentially kill dividing cells

Used to treat many cancers

Question 11

Taxol

Answer 11

Binds to and stabilizes microtubules and arrests dividing cells in mitosis, powerful anticancer drug especially for some ovarian cancers

Question 12

Dynamic instability model

Answer 12

When all of the GTP is hydrolyzed, plus end disassemble.
Continual rapid disassembly and reassembly of microtubules allows the cell to change microtubule distribution by selective stabilization of plus ends

Question 13

Microtubule organizing centers

Answer 13

Sites for stabilization of minus ends

Paired centrioles

Question 14

Gamma tubulin

Answer 14

Found in centrosomes and functions to nucleate microtubule assembly

Question 15

Microtubule maturation

Answer 15

Post translational modification of alpha tubulin by acetylation and detyrosination contributes to stability of these stable MT arrays

Question 16

Two types of microtubule motors

Answer 16

Dyneins and kinesins

Question 17

Dyneins

Answer 17

one, two or three headed ATPases that produce minus end directed motility

Question 18

Kinesins

Answer 18

two headed atpases that produce plus end directed motility

Question 19

Microtubule skeleton of cilia and flagella

Answer 19

• Cylinders of nine doublet microtubules that project from cell surface
• Axoneme=doublet microtubules and associated proteins
• Basal body=triplet microtubules

Question 20

Kartagener’s syndrome (primary ciliary diskinesia PCD)

Answer 20

-Lack of motility in what should normally be motile cilia, leading to respiratory disease, infertility in males and situs inversus

Question 21

Location and function of non-motile (sensory cilia)

Answer 21

• Also called primary cilia
• Cell surface specializations for localized receptor distribution
• Eyes, nose, ear

Question 22

Polycystic Kidney Disease

Answer 22

Mutation in a gene encoding a protein needed for assembly of primary (non-motile) cilia, normally present in cells lining the kidney tubules

Results in abnormal growth regulation of these epithelial cells and formation of cysts

Question 23

Microtubule associated structures in axonemes

Answer 23

Ciliary double microtubules are interconnected by dynein arms, radial spokes, nexin links, and central sheath projections

Two rows of dynein motors along each doublet microtubule

Question 24

Microtubule sliding produces movement of cilia and flagella

Answer 24

• Dynein ATPase activity is coupled to microtubule sliding
• Regulated sliding results in localized bend formation
• Waves of dynein activity become waves of bend propagation
• Interactions between radial spokes and central pair projections regulate bend formation control beat frequency and waveform