Dynein and Cilia Structures

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Questions and Answers

What structure is formed when two monomers of intermediate filaments pair together?

Dimer (correct)

Tetramer

Filopodia

Neurofilament

How many polypeptide chains are present in an antiparallel tetramer of intermediate filaments?

Six

Four (correct)

Two

Eight

What is the diameter of the final intermediate filament structure?

5 nm

10 nm (correct)

15 nm

20 nm

What occurs to cells in the basal layer of the skin due to a mutant keratin gene?

Cell rupture Signup and view all the answers

What happens to the organization of tetramers in the structure of intermediate filaments?

They are packed together in a rope-like array. Signup and view all the answers

What causes the defect in the keratin filament network in mutant epidermis?

Assembly of a truncated keratin protein Signup and view all the answers

What is the result of the interaction between normal and defective keratins in the skin?

Disruption of keratin filament network Signup and view all the answers

Where do the cells rupture in the mutant epidermis as indicated by electron microscopy?

Between the nucleus and hemidesmosomes Signup and view all the answers

What role do axonemal dynein heads play in flagellum function?

They produce a sliding force between adjacent microtubule doublets. Signup and view all the answers

What happens to the axoneme when treated with the proteolytic enzyme trypsin?

It breaks the flexible protein links between microtubule doublets. Signup and view all the answers

What is the primary cilium's function before a cell enters the cell-division cycle?

To be shed or resorbed. Signup and view all the answers

What structural feature connects the A microtubule of one doublet to the B microtubule of another in a flagellum?

Dynein arms. Signup and view all the answers

What is the consequence of the flexible protein links in an intact axoneme?

They enable the flagellum to beat and create waves. Signup and view all the answers

Where is the axoneme of the primary cilium nucleated?

By the mother centriole at the basal body. Signup and view all the answers

What is the primary function of neurofilaments in nerve cell axons?

To give tensile strength Signup and view all the answers

What drives the movement of dynein heads along the microtubules?

The hydrolysis of ATP. Signup and view all the answers

What occurs to centrioles before a cell enters the cell-division cycle?

They duplicate and form centrosomes. Signup and view all the answers

Which protein is responsible for forming cross-bridges in neurofilaments?

NF-H Signup and view all the answers

What distinguishes glial filaments from neurofilaments in terms of structure?

They appear smoother with fewer cross-bridges Signup and view all the answers

What role do septins play in eukaryotic cells?

They compartmentalize membranes into distinct domains Signup and view all the answers

In what manner do septins interact with other cytoskeletal elements?

By establishing cross-links between intermediate filaments and microtubules Signup and view all the answers

What structural characteristic do septins possess?

They assemble into nonpolar filaments Signup and view all the answers

What is the primary activity of the septin filaments at the base of primary cilia?

They act as a diffusion barrier Signup and view all the answers

What visual technique is used to observe the structure of neurofilaments in axons?

Freeze-etch electron microscopy Signup and view all the answers

Study Notes

Dynein

Dynein, a large macromolecular assembly, mediates attachment to cargoes.
Cytoplasmic dynein, itself a large protein complex, needs dynactin and an adaptor protein for organelle translocation.
Dynactin includes an actin-like filament made from Arp1.

Motile Cilia and Flagella

Cilia and flagella are motility structures made of microtubules and dynein.
Flagella are found on sperm and protozoa, enabling swimming.
Cilia beat with a whip-like motion, used for swimming (e.g., Paramecium) or moving fluid over tissues (e.g., respiratory tract).
Cilia in the oviduct sweep eggs toward the uterus.

Microtubule Arrangement

Microtubules are arranged in a 9+2 array in flagella/cilia.
There are nine outer doublet microtubules surrounding a centre of two single microtubules.
Projections from one microtubule connect with another (e.g., radial spokes, dynein arms).
High-resolution images show details of inner protein structures within microtubules.

Axonemal Dynein

Axonemal dynein forms bridges between microtubule doublets.
The motor domain of dynein molecules "walks" along adjacent doublets, causing sliding.
Sliding motion is driven by ATP hydrolysis.
This sliding force generates bending in cilia/flagella causing a beating/wave motion.

Bending of Axoneme

Trypsin breaks flexible protein links between microtubule doublets in cilia/flagella.
ATP binding allows dynein heads to move microtubule doublets against each other.
In intact cilia/flagella, protein links prevent sliding; dynein action creates bending motions.

Primary Cilia

Animal cells contain non-motile primary cilia, specialized compartments.
Primary cilia have similar structures to motile cilia but perform signaling functions.
Basal body anchored structures contain centrioles.
Intraflagellar transport (IFT) is necessary for axoneme machinery creation.

Intermediate Filaments

Three major types of cytoskeletal protein, including intermediate filaments found in metazoans.
Intermediate filaments particularly prominent in cells subjected to mechanical stress.
In humans, various families of intermediate filaments exist. They provide tissue strength and support.
They are more diverse than actins and tubulins.

Intermediate Filament Structure

Intermediate filaments form through lateral bundling/twisting of coiled-coil structures.
Parallel dimers associate in an antiparallel fashion to form tetramers.
Eight parallel protofilaments of tetramers form the complete filament (32 coiled-coils).
These structures have high resistance to breakage and stretch considerably.

Intermediate Filaments in Animals

Keratins are the most diverse filament family; various types are found in hair, nails and other tissues that need strength.
Disulfide bonds in these cross-linked networks give them great stability against cell death.
Keratins can support tissues and structures.

Intermediate Filaments in Nervous Cells

Neurofilaments are a family of intermediate fibers found in high concentrations within the axons of neurons.
These neurofilaments are linked by cross-bridges of protein providing stability for these long nerve cells.

Vimentin-like Filaments

Vimentin and other related proteins are a class of intermediate filaments found in cardiac, smooth and skeletal muscle.
They form a scaffold surrounding the Z disc of the sarcomere in muscles.
These filaments provide a stable structural framework for the muscle cells.

Linker Proteins

Plakins connect the intermediate filament network to the rest of the cytoskeleton.
Plectin is a significant example; linking intermediate filaments to microtubules, actin, myosin and other structures/components.
Plectin and related proteins link intermediate filaments to the cytoplasmic and nuclear cytoskeletons.

Septins

GTP-binding septins are additional filament systems in most eukaryotes apart from terrestrial plants.
Septins form rings and cage-like structures, compartmentalizing membranes or recruiting additional proteins.
In primary cilia, a ring of septins controls membrane protein movement.

Cell Polarity

Cell polarity controls behaviours such as protein secretion, cell division orientation and migration pathways.
Polarity signals often regulate the actin cytoskeleton, and coordinate cell behaviour.

Cell Polarity and Small GTPases

Small GTPases (e.g., Rho family proteins) regulate actin cytoskeleton by reacting to external/internal signals.
These GTPases cycle between active (GTP-bound) and inactive (GDP-bound) states, controlled by GEFs and GAPs.
GTPases are involved in the establishment of cell polarity.

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Explore the role of dynein in cellular motility and the structure of cilia and flagella. Learn about the arrangement of microtubules and the importance of dynactin in organelle transport. This quiz will enhance your understanding of these essential biological components.