Microtubule Structure and Function Quiz

Microtubules: Structure, Assembly, and Function

• Microtubules are the largest structural elements of the cytoskeleton, involved in various cellular functions.
• There are two types of microtubules: cytoplasmic and nuclear, responsible for different functions such as maintaining axons and cell shape, and forming spindles.
• Microtubules are composed of tubulin heterodimers, forming straight, hollow cylinders of varied lengths.
• Protofilaments, made of α and β tubulin heterodimers, give microtubules inherent polarity with plus and minus ends.
• Microtubules form through the reversible polymerization of tubulin dimers in the presence of GTP and Mg2+, with distinct lag and elongation phases.
• Microtubule assembly depends on the concentration of tubulin dimers and the critical concentration, leading to growth or disassembly.
• The plus end of microtubules grows faster due to the addition of tubulin dimers, while treadmilling occurs when free tubulin concentration varies at the plus and minus ends.
• GTP hydrolysis contributes to the dynamic instability of microtubules, leading to periods of growth, shrinkage, catastrophe, and rescue.
• Microtubules originate from microtubule-organizing centers (MTOCs) containing γ-tubulin, which nucleates the assembly of new microtubules.
• MTOCs organize and polarize microtubules within cells, with the minus ends anchored in the MTOC, leading to dynamic growth and shrinkage at the plus ends.
• Centrosomes, containing γ-tubulin ring complexes, are MTOCs that nucleate the assembly of new microtubules away from the centrosome.
• Loss of γ-TuRCs prevents a cell from nucleating microtubules, impacting cellular functions.

Actin and Intermediate Filaments: Structure and Function

• Myosin subfragment 1 (S1) binds and decorates actin microfilaments in a distinctive arrowhead pattern, defining the plus and minus ends.
• Actin monomers in the cytosol bind ATP, which is converted to ADP upon complex formation, reflecting the polarity of microfilaments.
• Actin-binding proteins regulate the polymerization, length, and organization of actin, controlling nucleation, elongation, severing, and network association.
• Thymosin β4 and profilin compete for G-actin binding, regulating the availability of ATP-bound G-actin for microfilament assembly.
• ADF/cofilin binds ADP-G-actin and F-actin, increasing turnover at the minus end of microfilaments and severing filaments to create new plus ends.
• Capping proteins like CapZ and tropomodulins bind to the plus and minus ends of microfilaments, respectively, regulating filament growth.
• Filamin, a protein, is crucial for the formation of loose networks of crosslinked actin filaments, splicing and joining intersecting microfilaments.
• Actin can be bundled into highly ordered arrays like focal contacts or focal adhesions, with proteins like α-actinin and fascin playing prominent roles.
• Microfilaments are connected to the plasma membrane by crosslinks made of myosin I, calmodulin, fimbrin, and villin, exerting force during cell movement.
• Proteins like band 4.1, ezrin, radixin, moesin, spectrin, and ankyrin link microfilaments to the plasma membrane, facilitating indirect force exertion during cell movement.
• Actin forms dendritic networks with the help of the Arp2/3 complex, nucleated by proteins like WASP and WAVE/Scar.
• Intermediate filaments (IFs) are abundant in many animal cells, with keratin being an important IF in structures growing from animal skin, supporting the entire cytoskeleton.

Microtubule-Based and Microfilament-Based Movement Inside Cells

• Microtubules act as tracks for organelle and vesicle transport, with inbound and outbound traffic
• Kinesins and dyneins are microtubule-associated motor proteins that enable movement
• Fast axonal transport involves transporting proteins at a rate of about 2 μm/sec
• Kinesin is involved in ATP-dependent transport towards the plus ends of microtubules
• Kinesins are a large family of proteins classified based on amino acid sequence
• Cytoplasmic dynein moves cargo towards the minus ends, aided by protein complexes called dynactin
• Microtubule motors shape the endomembrane system and facilitate vesicle transport
• Cilia and flagella are structures that aid in movement through fluid environments
• Cilia and flagella consist of an axoneme connected to a basal body
• Doublet sliding within the axoneme causes cilia and flagella to bend
• Myosins, a large superfamily, interact with actin microfilaments and are involved in various cellular events
• Muscle contraction is mediated by intracellular filaments, such as thin and thick filaments in skeletal muscle cells

DNA Structure and Packaging

• X-ray diffraction data from Rosalind Franklin revealed DNA's long thin helical structure
• Watson and Crick produced the double helix model based on Franklin's data
• The double helix model has 10 base pairs per turn and a diameter of 2 nm
• Purine-pyrimidine pairing in the double helix is consistent with Chargaff’s rules
• The double helix model suggested a mechanism for DNA replication
• DNA length is measured in base pairs (bp) with larger stretches in kilobases (kb)
• DNA can exist in right-handed (BDNA) and left-handed (Z-DNA) helical forms
• DNA can be interconverted between relaxed and supercoiled forms
• Supercoiling helps make chromosomal DNA more compact
• Topoisomerases induce and relax supercoils in DNA
• Strand separation (DNA denaturation) can be induced by raising temperature or pH
• Very long DNA molecules must be packaged into the cell's nucleus in eukaryotes