Cytoskeleton and Cell Motility Quiz

Cytoskeleton and Cell Motility

• The cytoskeleton is a network of protein filaments that provides structural support and allows for cell motility.

Actin and Intermediate Filaments

• Actin filaments (microfilaments) are flexible structures composed of actin monomers. They have a diameter of 5-9 nanometers.

• Actin filaments are involved in cell motility, shape, structural support, and muscle contraction. They are highly concentrated just beneath the plasma membrane.

• Intermediate filaments (IFs) are rope-like fibers, strong, and flexible. They have a diameter of 10-12 nanometers.

• IFs are found in animal cells, providing mechanical strength where cells are subjected to physical stress.

• IFs are chemically heterogeneous, existing in multiple classes based on various factors (e.g., cell type, biochemical, genetic). About 70 different genes encode them.

• IF assembly does not involve ATP or GTP hydrolysis.

Intermediate Filaments (continued)

• IFs extend throughout the cytoplasm in a variety of animal cells, often interconnected by wispy cross-bridges.
• Plectin is a protein involved in cross-bridging IFs to other cytoskeletal filaments.
• A single plectin molecule has a binding site for an intermediate filament at one end and, depending on the isoform, a binding site for another intermediate filament, microfilament, or microtubule at the other end.
• IF assembly involves the association of eight tetramers in a lateral arrangement, forming filaments with a length of approximately 60 nanometers.
• Unit lengths of filaments then associate end-to-end to form longer intermediate filaments.
• IF assembly does not involve ATP or GTP hydrolysis. Polarity doesn't exist in IFs.

Actin and Myosin

• G-actin is the globular form of actin, and F-actin is a filamentous form of actin.
• In the presence of ATP, G-actin monomers polymerize into F-actin filaments via a flexible helical structure.
• The ATP binding cleft is oriented in the same direction in all actin subunits (monomers) within the filament.
• Plus and minus ends of actin filaments have different polymerization rates.
• The minus end of the filament contains an exposed binding cleft (pointed end) and the plus end (also called the barbed end) is where the minus end of G-actin binds.
• Only ADP-actin filaments dissociate.
• In vivo, polymerization only occurs at the plus end of ATP-actin filaments.

Actin Binding Proteins

• Many accessory proteins bind to actin, regulating actin polymerization and organization.
• Cofilin promotes depolymerisation at the minus end of actin filaments.
• Profilin binds to ADP-actin at the plus end, affecting actin monomer binding.
• Thymosin sequesters G-actin to prevent polymerization.
• CapZ caps plus ends, preventing further growth and capping at the minus end prevents loss of subunits during F-actin.

Myosins

• Myosins are actin-based motor proteins.
• All myosins have a characteristic motor domain that have ATPase activity.
• Different types of myosins are involved in different tasks, such as muscle contraction, cytokinesis, and cell migration.
• Type II myosins are best studied in muscle contraction.
• Conventional myosins (Type II) consist of two heavy chains, each with a globular head and a tail; two light chains are part of each heavy chain's head, forming a highly asymmetric structure.
• Myosin II assembly is into fibers with tails pointing toward the center and globular heads pointing outwards.

Muscle Organization and Contraction

• Muscle consists of parallel fibers (cells) joined by tendons.
• Muscle fibers are multinucleate cells formed during embryogenesis, specializing in contraction.
• Myofibrils are thinner cylindrical strands composing each muscle fiber.
• Myofibrils consist of repeating sarcomeres.
• A sarcomere includes the area between adjacent Z lines.
• Thin filaments (actin) extend from one Z line to the next, and thick filaments (myosin) are also found between adjacent Z lines.
• A sarcomere is composed of an A band (where thick filaments are present), and I bands (where thin filaments are located).
• The H zone is a region of the sarcomere where only thick filaments are present.
• The M line consists of proteins that connect adjacent thick filaments.
• Titin is a protein extending through the myosin filaments (thick filaments) and attaches to the Z line, preventing muscle tearing.
• Tropomyosin blocks myosin binding sites on actin.
• Troponin is a complex with three subunits (TnT, TnI, TnC) that regulates the interaction of troponin with tropomyosin and actin.

Calcium ions

• Calcium ions trigger muscle contraction by binding to troponin.
• This binding releases the tropomyosin blockage, allowing myosin to bind to actin, initiating contraction.

Other Myosin-Related Pathologies

Myosin mutations can cause diseases ranging from deafness to familial hypertrophic cardiomyopathy.

Contractile cycle

• Myosin head undergoes conformational changes driven by ATP hydrolysis.
• ATP binding causes the myosin head to detach from actin.
• Hydrolysis activates the myosin head for a new power stroke.
• The power stroke causes actin filaments to slide relative to each other.
• In the absence of ATP, rigor mortis occurs due to the inability for myosin heads to detach from actin filaments.

###Actin in endocytosis

• Actin plays a crucial role in endocytosis, including membrane invagination, pinching, and scission.
• Components are recycled and the process of endocytosis occurs with different phases.

Actin polymerization regulation

• Actin polymerization is regulated by several proteins, allowing for diverse cellular functions and complex organization.

Rho family GTPases

• Rho family of small GTPases regulate the actin cytoskeleton, influencing cellular functions like membrane remodeling and motility.
• GDI (guanine nucleotide dissociation inhibitor) protein associates with these GTPases and may affect their functions at the cellular membrane.

Test your knowledge on the cytoskeleton, actin filaments, and intermediate filaments. This quiz covers the structure and function of these critical components in cell motility and mechanical strength. Ideal for students studying cell biology.