Dynein and Cilia Structures

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.

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.