Cell Bio Exam 4

What does the cytoskeleton help provide structure for?

The interior of a eukaryotic cell

What is the cytoskeleton?

A complex network of interconnected filaments and tubules that extends through the cytosol

What does the cytoskeleton provide for cellular function?

Provides architectural framework

What are the characteristics of the cytoskeleton?

High internal organization, dense, fibrous, dynamic, changeable

What does the cytoskeleton enable?

Assumption and maintenance of complex shapes that otherwise wouldn’t be possible

What does the cytoskeleton play a role in?

Cell movement and division, movement of organelles in cytosol, cell signaling, cell-cell adhesion

What is a fundamental feature of the cytoskeleton in both eukaryotes and prokaryotes?

Modularity

What is modularity in terms of the cytoskeleton?

A small number of cytoskeletal elements are deployed in different locations and arranged in different ways to meet the needs of a particular cellular structure

What are the three main structural elements of the cytoskeleton in eukaryotes?

Microtubules, microfilaments, and intermediate filaments

What was the technique of immunofluorescence important for?

Localizing specific proteins to the cytoskeleton

What is each structural element of the cytoskeleton formed by?

The polymerization of a different kind of protein subunit

What is the smallest of the structural elements of the cytoskeleton?

Microfilaments

What is the largest of the sturctural elements of the cytoskeleton?

Microtubules

What is the structure of microtubules?

Hollow tube with a wall consisting of typically 13 protofilaments

What is the structure of microfilaments?

Two intertwined chains of F-actin

What is the structure of intermediate filaments?

Eight protofilaments joined end to end with staggered overlaps

What are septins sometimes called?

The fourth cytoskeleton

What are septins closely associated with?

The contractile ring

What is the contractile ring?

A structure involved in the pinching off of daughter cells during cell division

How do the polymer systems of bacteria and archaea function?

Similarly to microfilaments, microtubules, and intermediate filaments

What is the cytoskeleton best known for its roles in?

Cell motility

What are microfilaments essential components of?

Muscle fibrils

What are microtubules the structural elements of?

Cilia and flagella

Where most cells dynamically regulate, what happens to specific cytoskeletal structures?

They are assembled and disassembled

What do modern cell biologists use to study the cytoskeleton?

Fluorescence microscopy, digital video microscopy, and electron microscopy

What two general groups can microtubules be classified into?

Cytosolic and axonemal

What do cytosolic MTs do in animals?

Maintain axons

What are axons?

Specialized extension of nerve cells

What do cytosolic MTs do in plant cells?

Govern the orientation of cellulose microfibrils

What do cytosolic MTs form that are essential for the movement of chromosomes?

Mitotic and meiotic spindles

What are axonemal microtubules?

Microtubules present in highly ordered bundles in the axonemes of eukaryotic cilia and flagella

What is an axoneme?

The central shaft of a cilium or flagellum consisting of a highly ordered bundle of axonemal MTs and associated proteins

What are the protein building blocks of microtubules?

Tubulin heterodimers

What is the structure of MTs?

Straight, hollow cylinder with an outer diameter 25 nm and inner diameter of 15 nm

What does the MT wall consist of?

Longitudinal arrays of protofilaments

What are protofilaments?

Linear polymer of tubulin subunits

What are protofilaments usually arranged side by side around?

The lumen of the MT

How do the structures of alpha and beta tubulins compare?

They are nearly identical 3D structures despite sharing only 40% amino acid sequence identity

What does each a and b tubulin have?

A binding domain at the N-terminus, a domain in the middle to which colchicine can bind, and a domain at the C-terminus that interacts with MAPs

What does the uniform orientation of tubulin dimers mean?

One end of the protofilament differs structurally from the other

What are tubulin isoforms?

Several closely related but nonidentical genes for each of the a- and b-tubulin subunits

What can some axonemal MTs contain?

Doublet or triplet MTs

What do doublets and triplets contain?

One complete 13-protofilament microtubules and one or two incomplete 10 or 11-protofilament tubules

Where are doublets found?

Cilia and flagella

Where are triplets found?

Basal bodies and centrioles

What are doublets and triplets stabilized by?

Microtubule inner proteins

What do microtubules form by?

The reversible polymerization of tubulin dimers

What do oligomers serve as?

Seeds from which new microtubules can grow in a process called nucleation

What is elongation?

Addition of subunits at either end of an MT after it has been nucleated

What is the lag phase of MT assembly?

The initial slow period of microtubule formation

What is the plateau phase of MT assembly?

When MT assembly is balanced by disassembly

What is critical concentration?

The tubulin heterodimer concentration at which MT assembly is exactly balanced with disassembly

Where does addition of tubulin dimers occur more quickly?

The plus ends of microtubules

What are the two ways that actin exists?

G-actin (monomer) or F-actin (polymer)

What end of the microfilament grows faster?

Barbed (+) end

What do actin-binding proteins do?

Regulate capping, severing, cross-linking, and nucleation; control actin filament organization and turnover

What coordinate polymerization sites?

Signaling pathways

What are the functions of the actin cytoskeleton?

Cell motility, shape, division, intracellular transport

What are two kinds of capping proteins?

CapZ and tropomodulins

What does filamin do?

Bind filaments together at their crossing points and causes microfilaments to become large 3D networks

What does gelsolin do?

Severs microfilaments then caps the newly formed ends

What does ⍺-actinin do?

Makes adhesive connections between filaments in focal contacts and focal adhesions

What does the bundling protein fascin do?

Holds actin in the core of filopodia

What are the actin binding proteins in the microvilli?

Fimbrin and villin

What is the terminal web?

Dense mesh of proteins spectrin and myosin

What are anchored to the terminal web?

Actin filaments

What do membrane linking proteins do?

Connect microfilaments to the membrane of a cell for structural support

What is the formation of dendritic filament networks controlled by?

WASP and the Arp2/3 complex

What is the Arp2/3 complex?

Complex of proteins necessary for branching of microfilaments

What is the WASP?

Complex of proteins that activate the Arp2/3 complex

What do phospholipids and GTPases do?

Regulate the breakdown, formation, and stability of microfilaments

When are Rho family GTPases active?

When bound to GTP

What is myosin?

A family of protein-motors located on the actin cytoskeleton

How many known members does the myosin family have?

At least 24

What are the three main sections of the structure of myosin?

Head domain, neck domain, and tail domain

What is the head domain also known as?

The motor domain

What is the head domain made up of?

A globular protein with active sites to bind actin and ATP

What is the function of the head domain?

To bind to actin and ATP

What does ATP being hydrolyzed cause?

A conformational change that causes the myosin to pull on the actin filaments

What stabilize the head structure?

Light chains

What is the function of light chains?

Allowing the head to move more freely to regulate ATP hydrolysis by the myosin head

What can the tail be bound to?

Cargo or other myosin filaments

What dictate the destination of the myosin?

Specific binding partners in the tail

What is the function of myosin as a whole?

Generation of movement or tension using ATP to exert force on actin filaments

How much efficiency do myosin work with when pulling a moderate load?

50%

What is Myosin I utilized for?

Intracellular transport and membrane interactions

What is Myosin II utilized for?

Muscle contraction and cytokinesis

What is Myosin V utilized for?

Intracellular transport of vesicles and mRNA