SFP: cytoskeleton

Question 1

Describe the molecular structure and functions of the 3 main types of cytoskeletal components

Answer 1

Microfilaments: F-actin. The smallest filament. They’re dynamic polymers

Intermediate filaments: stable polymers of heterogeneous proteins

Microtubules: the largest filament; dynamic polymers of alpha and beta tubulin subunits.

Question 2

Explain the dynamics of microfilament (F-actin) assembly and disassembly.

Answer 2

The plus end has fast growing f actin, while the minus end has slower growth. As such, growth typically occurs at the plus end while shrinkage occurs at the minus end as G-actin-ADP dissociates more rapidly from this end. The actin removes Pi to form ADP as the g actin subunits move along the protein. It almost looks like a treadmill movement from plus to minus end!

Question 3

Describe how actin binding proteins regulate the structure and function of microfilaments.

Answer 3

Profilin: removed ADP and puts ATP back on so G-actin can get back to the plus end
Cofilin: will severe actin and depolymerize

Question 4

Describe the molecular structure and corresponding functions of myosin classes I, II, V, and VI.

Answer 4

I: all cell types; link f actin to the plasma membrane, intracellular membrane trafficking, membrane tension, struts in the plasma membrane
II: conventional; sarcomeric, smooth muscle, non-muscle
V: all cell types; two MHCs, MLCs, and calmodulin. Can “walk” along the microfilament and move cargo!
VI: all cell types; structurally similar to myosin V but walks toward the negative end of the filament! It traffics endocytic vesicles and forms autophagosomes

Question 5

Explain how contraction of non-muscle Myosin II (NM II) is regulated.

Answer 5

Regulatory light chain phosphorylation; calmodulin binds to calcium and activates myosin light chain kinase. MLCK phosphorylates the regulatory light chains on the neck of myosin. This activates myosin ATPase and facilitates assembly of the myosin tail region.

Question 6

Which myosin forms “struts” between bundles of microfilaments in the plasma membrane?

Answer 6

MHC I

Question 7

In myosin I, what does the motor head bind to?

Answer 7

F-actin; moved toward the plus end

Question 8

When actin and myosin interact, what functions can myosin be capable of?

Answer 8

“walking” along microfilaments, generating tension on microfilaments, cell contraction and movement

Question 9

Describe the basic structure of myosin II

Answer 9

Has a motor head that connects to actin and has a myosin ATPase domain, a neck that binds to myosin light chain or calmodulin, and a tail that has an alpha helical domain

Question 10

Specify the basic steps involved in cell motility and describe the role of F-actin microfilaments and myosin.

Answer 10

1. polymerization of F actin at the leading edge at the plus end. This generates force that pushes on the membrane to create lamellipodia and filopodia
2. Microfilaments in lamellipodia form adhesions via integrins
3. NM II (myosin II) interacts with actin to generate tension, allowing filaments to slide and adhesions to detach. This allows for translocation of the cell body

Question 11

Specify the main types of intermediate filament proteins and explain how subunits are
assembled into intermediate filaments.

Answer 11

There is a central helical rod domain, which dimerizes. An antiparallel alignment of IF protein dimers then forms tetramers. These align end to end to form protofilaments. Complete Ifs contain 8 protofilaments wound into a rope structure.

Question 12

Intermediate filaments are a ___ group (hetero vs homogeneous)

Answer 12

Heterogeneous

Question 13

Describe the functions of intermediate filaments and their role in intercellular adhesion.

Answer 13

• IFs anchor into desmosomes to mechanically couple cytoskeletons between cells
• IFs anchor into hemidesmosomes to couple cytoskeleton to ECM

Question 14

Describe epidermolysis bullosa simplex

Answer 14

The basal layer of keratins don’t anchor properly into the hemidesmosomes, causing the basal layer to peel from the underlying dermis, causing blistering and peeling of the skin

Question 15

Describe desminpathies

Answer 15

Desmin is an intermediate filament that tethers sarcomeres to the membrane. If these desmins have a defect this process is disrupted, and pathologies like cardiomyopathy can occur.

Question 16

Describe the use of Ifs as tumor cell markers

Answer 16

Specific antibodies for IF proteins can be used to differentiate types of tumors based on the IFs present (cytokeratin in carcinomas and GFAP in glial cells)

Question 17

Describe the molecular structure of microtubules including the arrangement of subunits.

Answer 17

The largest filament consisting of alpha and beta subunits. They have a growth end and a negative end. Alpha tubulin is always bound to GTP, and the beta will jump on and polymerize with GTP attached, but the GTP hydrolyzes once the bond is formed. This forms the protofilaments that form a larger cylindrical shape.

Question 18

Explain how microtubules are assembled and disassembled.

Answer 18

1. Alpha-Beta tubulin forms the plus end (geta has to be in its GTP form)
2. The beta gets hydrolyzed as it moves toward the negative end
3. The GDP can dissociate . as such, the growing MTs contain a beta tubulin GTP cap on the plus end while they’re growing. When they’re shrinking, they lose their cap, exposing beta tubulin GDP-Pi that can dissociate

Question 19

Explain the concept of dynamic instability of microtubules.

Answer 19

The cycle of growth and shrinkage is dependent of the concentration of alpha-beta-tubulin GTP heterodimers. More of these means rapid growth as the cap is maintained on the positive end. A lack of these heterodimers causes “catastrophe” and the cap is lost. This causes shrinkage.

Question 20

Describe alterations in structure and function of microtubules produced by microtubule
associated proteins (MAPs).

Answer 20

MAPS CAN BIND TO TUBULIN HETERODIMERS OR MTS to assemble, cross-link, stabilize, destabilize, severe, and cross link. An example is the gamma tubulin used in capping.

Question 21

Describe the structure and function microtubule organizing centers (MTOCs).

Answer 21

They are gamma tubulin containing complexes that cap the minus end of MTs. A big example is the centrosome. Centrosomes are composed of centrioles that contain 9 triplets of MTs in a pinwheel. The pericentriolar material contains alpha tubulin rind complexes that cap the minus end and nucleate MTs so growth occurs with assembly at the plus and disassembly at the minus.

Question 22

Describe the molecular structure of kinesins and dyneins and their corresponding roles in intracellular transport.

Answer 22

Kinesins: transport cargo to the plus end of MT
Dyneins: transport cargo to the minus end of MTs

The heads walk, the tails bind to cargo

Question 23

Describe the structure and function of cilia and flagella and explain the molecular basis of movement.

Answer 23

Dynein is affixed to alpha tubules. The motor heads of the dynein move along adjacent beta tubules toward the minus end, causing the MTs to bend. Adjacent doublets are connected by nexes, allowing for coordinated movement of cilia and flagellum

Question 24

Describe an axoneme

Answer 24

Core microtubule structure containing a 9+2 array connected via nexins and sheath proteins. This helps coordinate the movement of cilia and flagella.

Question 25

What are basal bodies?

Answer 25

They’re derived from centrioles; they anchor the minus ends of MT forming the axoneme

Question 26

Explain why drugs targeting microtubules are used as chemotherapeutic agents

Answer 26

They inhibit or promote assembly or disassembly of microtubules. Microtubules are important in forming the mitotic spindle, so the idea is impacting the microtubulin can impact cellular division