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What are the 3 main types of filaments in the cytoskeleton?

microtubules, actin filaments, and intermediate filaments

Functions of the cytoskeleton:

-maintaining the general shapes and internal structure of cells

-providing mechanical strength

-enabling cell movement (migration, motility, contraction)

-guiding intracellular traffic of organelles and molecules

-facilitating cell division (chromosome separation, breakdown of nuclear membrane)

What is actin filament responsible for?

cell shape/movement/junction/division/ and intracellular transport

What is microtubules responsible for?

intracellular transport, cell division, cilia, flagella

What are intermediate filaments responsible for?

mechanical strength

What type of actin is found in muscle? What type of actin is found in all other cells?

-actin filaments are found in muscles

- g-actin is found in all other cells

Where does actin most commonly nucleate? (Be sure to understand what nucleate means.)

actin commonly nucleate near plasma membrane

What is "nucleate"?

initial formation of multimer from monomers

What is the difference between g-actin and f-actin?

g-actin is globular actin whereas f-actin is filamentous actin

What role does ATP play during filament formation?

ATP binds g-actin, hydrolyzed to ADP post-polymerization into helix shaped filament

What does the structure of f-actin look like?

- f-actin are linear polymers of g-actin subunits and occur as microfilaments in the cytoskeleton and as thin filaments.

- two long pitch helical strands

What does it mean for a filament to have structural polarity?

refers to differences in the filament structures as the opposite poles (i.e. the different ends of the filaments)

Difference between plus and minus end?

- Plus end is where addition and loss happen faster.

- Minus end is where addition and loss happen slower.

What are MFAPs and how do they help determine the structure and functions of actin filaments?

- MFAPs: Microfilament Associated Proteins

- MFAPs aid in actin polymerization and depolymerization

What does it mean when actin is dynamic? Why is it important?

- being able to constantly change/adaptable

- its important because it can continuously adapt and respond to local environment

Where do MTs (microtubules) usually nucleate? Where do they emanate from their nucleation site?

MT nucleate at MTOC (microtubule organizing center) near cell nucleus and emanate to cell periphery.

Describe the structure of MTs. How is GTP involved?

- MTs are stiff, hollow tube formed from 13 protofilaments.

- beta tubulin binds GTP then is hydrolyzed to GDP post-polymerization into a protofilament

MTs have dynamic instability? What does that mean?

MTs can be constructed or deconstructed very quickly. Can also undergo treadmilling.

What is the MTOC/centrosome? What is it made of and where is it located in the cell and what is its function?

The MTOC is made of gamma tubulin, nucleates microtubules near nucleus. MTOC organizes extensions of microtubules.

Do MTs have structural polarity? If so, which end is associated with the MTOC and which end emanates toward the cell membrane?

MTs have structural polarity. Most of the time, the (-) end is towards the nucleus (MTOC), and the (+) end is toward the cell membrane.

Are IFs in all eukaryotic cells? (Are actin and MTs found in all eukaryotic cells?)

Intermediate filaments are less common within the Eukarya domain than actin filaments and microtubules; they are only found in some animals, including vertebrates, nematodes, and molluscs. Actin and MTs are found in all eukaryotic cells.

What are lamins?

Lamins are a type of IFs found in nuclei.

What is the function of molecular motors? How do they accomplish this function?

Molecular motors are proteins that move along microtubules or actin filaments. They are ATP dependent and are important for intracellular transport and cell movement! Molecular motors are essentially ATPases that couple hydrolysis with movement; their globular heads have ATPase activity that dictate filament track, direction, and speed by binding to actin filaments or microtubules. Their fibrous tails have "cargo" binding that comes along for the ride.

Name each molecular motor and its associated cytoskeletal component (if this was not already included as part of your chart in question 1).

1. Myosins: all of the myosins except one move along actin filaments towards the plus end (there is one that moves to the minus end).

2. Kinesins: move along microtubules towards the plus end (away from the MTOC).

3. Dyneins: move along microtubules towards the minus end (toward the MTOC).

What is nucleation? Why is it so important for actin filament polymerization?

Nucleation is the initial formation of multimer form monomers. This is important for actin filament polymerization because it is the rate-limiting step.

What proteins nucleate actin filament polymerization? How are these proteins different?

Formin and ARP 2/3 complexes nucleate f-actin.

Formin nucleates actin polymerization by capturing two actin monomers; once nucleated, the actin polymer rapidly extends. During this process, formin stays at the (+) end. This forms straight, unbranched filaments.

ARP 2/3 complexes nucleates growth and stays attach to (-) ends; binds existing actin fibers at an angle to form a tree-like web of actin. To make a dense actin meshwork (gel), the cell must express and activate ARP complexes.

Describe the interaction between ARP and activating factor. What kind of interaction is this?

Activating factor is required to activate the ARP complex! So, cells can manipulate actin polymerization by manipulating the expression of activating factor.

An inactive ARP complex is activated when an activating factor that binds allosterically to begin nucleating the actin filament. This is an example of positive allostery.

How can a cell prevent actin polymerization by preventing nucleation by ARP without directly affecting ARP expression?

A cell can prevent actin polymerization by preventing nucleation by ARP without directly affecting ARP expression by limiting/inhibiting the expression of activating factor!

What proteins are important for the kinetics of actin filament polymerization? How are these proteins different?

Kinetics: Thymosin, profilin, and capping protein determine the speed of actin subunit addition.

Thymosin and Profilin regulate the speed of actin polymerization. Thymosin inhibits polymerization by keeping g-actin in a "locked state".

Profilin promotes polymerization. It facilitates ADP-> ATP nucleotide exchange.

Thymosin and Profilin compete for g-actin binding!

NOTE: g-actin must be in an ATP-bound state to add to a growing filament!Capping protein helps form long-term stable structures (by preventing polymerization and depolymerization at the (+) end). Caps mean slow growth and stabilized fibers. Capped actin filaments can only have growth at the (-) end.

What class of proteins determines actin filament shape? Describe how each protein influences actin shape.

Filament binding proteins determine the final structural shape of actin (actinin, fimbrin, filamin).Actinin and Fimbrin are bundling proteins that cause the lateral association of filaments.Actinin has wider spacers, causing loose packing which allows myosin-II to enter bundle.Fimbrin has shorter spacers, causing tight packing that prevents myosin-II from entering the bundle.Actin cross-linking proteins (filamin) have a bent connection between their two binding domains which stabilizes actin webs or gels.

Which filament binding proteins is used for actin bundles in the core of microvilli versus the contractile ring? What is the reason for the difference?

Fimbrin is used for actin bundles in the core of microvilli which causes a tightly packed assembly for the microvilli. Actinin is used for filament bundling in the contractile ring because it allows for a lot of space for myosin to enter and perform the powerstroke.

What proteins are required for rapid disassembly of actin filaments? How do they function?

cofilin and profilin

How would loss of gelsolin impact actin disassembly?

reduce severing and capping activity

How does profilin promote actin polymerization?

Profilin promotes actin polymerization because it facilitates ADP-> ATP nucleotide exchange. This enables the filament to elongate because g-actin must be in an ATP-bound state to add to a growing filament!

Give two examples of cellular processes that require actin polymerization.

1. Actin filament assembly of the contractile ring:-Nucleation by formin-Polymerization by profilin-Stabilized by capping protein-Filament bundling with actinin-Myosin mediated contraction of ring-Disassembly with gelsolin and cofilin2. Actin filament formation in Lamellipodia (cell migration)-nucleation by the ARP complex-Filaments engloate via polymerization by profilin-Filaments at the back are depolymerized by cofilinboth profilin and cofilin are required for treadmilling to occur

What are MAPs? How are the MAPs tau and MAP2 similar to actinin, fimbrin, and filamin?

MAPs are microtubule associated proteins.Tau and MAP2 are similar to actinin, fimbrin, and filamin in that they bind the sides of microtubules and the length of their side-arms control how closely MTs can pack together.

How is the MAP +TIPs similar to actin capping protein?

+TIPS are (+) end tracking proteins that bind active (+) ends to help form long-term stable structures. They have a similar function to actin capping protein. +TIPS inhibit dynamic instability!

What are catastrophe factors? How are they similar to cofilin?

Catastrophe factors destabilize MT ends; pry protofilaments apart; aid transition from growth to depolymerization. This is similar to cofilin in that they break apart the polymer.

How is stathmin similar to thymosin?

Stathmin protein binds free tubulin subunits and inhibits MT assembly and elongation. Stathmin activity is binding tubulin. The phosphorylation of stathmin inactivates it because it prevents it binding tubulin.Unphosphorylated stathmin: slow MT growthPhosphorylated stathmin: MT growthStathmin is similar to thymosin in that it inhibits the binding of monomers to the growing polymer.

Are katanin and spastin more similar to gelsolin or cofilin? Why?

Katanin and spastin are MT severing proteins. They are more similar to gelsolin because they are sever the polymer.Stathmin can also depolymerize MTs and it acts more like cofilin because it depolymerizes the microtubules.

What happens to lamins during cell division?

During cell division, lamins (provide the structural integrity of the nuclear envelope) are phosphorylated, which leads to their disassembly and consequent disassembly of the nucleus.

What kind of cellular activity can be controlled by cell to cell communication?

signal transduction - which aids in activities like: migration, growth, proliferation, survival, apoptosis, differentiation, adhesion

Describe how cellular activities are controlled by signaling pathways?

initiated by a signaling ligand that is produced by a signal producing cell.ligand binds to receptor on responding cell.biochemical reaction cascade.* there are a bunch of different ligand producing cells* there are also multiple pathways - convergent and divergent*there are also multiple extracellular signaling molecules that cells are dependent on to illicit a target mechanism.

Why are signaling pathways so complex? What advantage does the complexity provide?

Complexity provides control for regulation and the coordination and integration of multiple different signals (most commonly seen in convergent and divergent cross-talk)

Describe negative and positive feedback loops. What do these feedback loops allow pathways to do?

Positive: when a product of a pathway stimulates its own production or activationNegative: when a product of a pathway inhibits an earlier step in the same pathway.- These can be used for regulation and control of a particular pathway. For example, if like enough of a particular target molecule is made, negative feedback might be initiated to stop that pathway.

Describe what crosstalk is and define the two different types of crosstalk.

Cross-talk: when one or more components of a signal transduction pathway affect another pathway.

Juxtacrine signaling:

contact dependent signalling. This can be via cell-cell, ECM-cell, or gap junctions

Paracrine signaling:

Signaling cell secrets ligand that enters target cell receptors.

Endocrine signaling:

Endocrine cells release hormones which enter the bloodstream and bind to the receptors of target cells.

Synaptic signaling:

At the synapse, NT are released which bind to the receptor at the target cell.

Describe the ligands involved in each type of signaling:

Juxtacrine

- If cell-cell signalling: membrane bound ligands

- if ECM to cell signalling: ECM molecules

- if gap junctions: small cytoplasmic molecules like ions

Describe the ligands involved in each type of signaling:

paracrine

- growth factors

- cytokines

- gases

Describe the ligands involved in each type of signaling:

endocrine

-hormones

Describe the ligands involved in each type of signaling:

synaptic

-NT

What is autocrine signaling? How is it different from paracrine signaling?

autocrine: secreted molecule acts on the same cell

paracrine: does not act on the same cell. only on neighboring target cells.

How are endocrine and synaptic signaling similar? How are they different?

Similar: systemic actiondifferent: synaptic is short distance diffusion all the time and endocrine is long distance diffusion in the circulatory system- synaptic is also faster and a lot more precise

What type of molecules are endorphins? What type of signaling do they participate in? What cell type releases them? What happens when endorphins are released by these cells?

- Endorphins are NT- they participate in synaptic signaling- they are produced by different cells in the CNS in response to various triggers like stress, pain, acupuncture, chili peppers, etc.- Signal to the regions of the brain that respond to pain and deal with emotion. They then inhibit any transmission of pain and affect feelings of pleasure. They then bind to opioid receptors.

What is the cause and consequence of receptor desensitization? Name and describe three mechanisms of receptor desensitization.

It occurs through the uncoupling of the receptor from its signalling cascade which typically occurs after prolonged exposure to the ligand.

1) Inactivation: receptor is altered in a way that disconnects it from its cascade.

2) Sequestration: receptor is temporarily internalized

3) Down-regulation: destroyed in lysosome after internalization

What type of molecule is ghrelin?

hormone

What type of signaling does ghrelin participate in?

endocrine signaling

What cell type releases ghrelin?

produced by endocrine in the stomach

Where are the cells that respond to ghrelin located?

this hormone then travels through the circulatory system to the hypothalamus of the brain

What is the function of ghrelin?

indicating hunger

What type of molecule is leptin?

hormone

What type of signaling does leptin participate in?

endocrine signaling

What cell type releases leptin?

produced by adipose cells

What is the function of leptin?

induces satiety (fullness)

How is leptin similar and different from ghrelin?

ghrelin and leptin work in apposition to one another in the regulation of body weight homeostasis. BUT, they do act on a common population of neurons

What are GPCRs?

G-protein coupled receptors