Microtubule Structure

Microtubules: Structure, Assembly, and Function

• Microtubules (MTs) are the largest structural elements of the cytoskeleton, involved in various cell functions.
• Two types of microtubules exist: cytoplasmic microtubules and their functions in maintaining axons, formation of spindles, and cell shape.
• Tubulin heterodimers are the protein building blocks of microtubules, consisting of protofilaments with α and β-tubulin.
• Protofilaments have inherent polarity with plus and minus ends, and microtubules form through reversible polymerization of tubulin dimers.
• Microtubule assembly involves nucleation, elongation, and critical concentration of tubulin dimers.
• Addition of tubulin dimers occurs more quickly at the plus ends of microtubules, leading to treadmilling.
• GTP hydrolysis contributes to the dynamic instability of microtubules, involving GTP-tubulin and dynamic instability with catastrophe and rescue phases.
• Microtubules originate from microtubule-organizing centers (MTOCs) and centrosomes containing γ-tubulin ring complexes (γ-TuRCs).
• MTOCs organize and polarize microtubules within cells, nucleating and anchoring MTs with fixed polarity.
• Microtubules facilitate various cellular functions, including maintaining cell shape and facilitating intracellular transport.
• Microtubule-based drugs such as taxanes and vinca alkaloids target microtubule dynamics for cancer treatment.
• Microtubule-associated proteins (MAPs) regulate microtubule dynamics and stability, influencing their functions in the cell.

Cytoskeleton Proteins: Actin and Intermediate Filaments

• Myosin subfragment 1 (S1) binds to actin microfilaments (MFs) in a distinctive arrowhead pattern
• The plus end of microfilaments is called the barbed end, and the minus end is called the pointed end
• Actin monomers in the cytosol bind ATP and are converted to ADP after complexing ATP
• Microfilaments show polarity with more rapid addition or loss of G-actin at the plus end
• Cells can dynamically assemble actin into various structures using actin-binding proteins
• Thymosin β4 binds a large amount of free G-actin, while profilin competes for G-actin binding
• ADF/cofilin binds ADP-G-actin and F-actin to increase turnover of ADP-actin at the minus end
• Capping proteins like CapZ and tropomodulins regulate the growth of microfilaments
• Filamin is important in forming loose networks of crosslinked actin filaments
• Actin may be bundled by proteins like α-actinin and fascin into highly organized arrays
• Microfilaments are connected to the plasma membrane by crosslinks made of myosin I and calmodulin
• Actin can form a dendritic network through the Arp2/3 complex, activated by proteins like WASP and WAVE/Scar

Microtubule and Microfilament-Based Intracellular Movement

• Microtubules act as tracks for organelle and vesicle transport inside cells
• Kinesins and dyneins are motor proteins that move along microtubules
• Kinesins move toward the plus ends of microtubules and are involved in fast axonal transport
• Dyneins move cargo toward the minus ends of microtubules and are linked to cargo by dynactin protein complexes
• Microtubule motors are crucial for shaping the endomembrane system and transporting vesicles
• Cilia and flagella are involved in cellular movement and consist of axoneme connected to a basal body
• Doublet sliding within the axoneme causes cilia and flagella to bend, generating forces for movement
• Myosins, a large superfamily of ATP-dependent motors, interact with and exert force on actin microfilaments
• Myosins function in muscle contraction, cell movement, phagocytosis, and vesicle transport
• Type II myosins are well understood and responsible for pulling arrays of actin filaments together, resulting in cell contraction
• Muscle contraction is a familiar example of mechanical work mediated by intracellular filaments
• Skeletal muscles are composed of parallel muscle fibers responsible for voluntary movement

DNA Structure and Packaging

• The double helix model of DNA was developed by Watson and Crick, with critical evidence from X-ray diffraction data by Rosalind Franklin
• The model revealed 10 base pairs per helical turn and a 0.34 nm distance per nucleotide pair
• Purine-pyrimidine pairing adheres to Chargaff’s rules, with hydrogen bonding between complementary bases (adenine-thymine, guanine-cytosine)
• The double helix model suggested a mechanism for DNA replication, where the strands could act as templates for new complementary strands
• DNA length is measured in base pairs (bp), with larger stretches in kilobases (kb)
• DNA can form right-handed (BDNA) and left-handed (Z-DNA) helices, with Z-DNA's biological significance not fully understood
• DNA can be interconverted between relaxed and supercoiled forms, with positive and negative supercoils
• Supercoiling occurs in both linear and circular DNA, mediated by topoisomerases which induce and relax supercoils
• Strand separation (denaturation) can be induced by raising temperature or pH, while reformation (renaturation) occurs by lowering the temperature to permit hydrogen bonds to reform
• DNA packaging is a challenge for all forms of life, especially in eukaryotes where DNA interacts with histones to form chromatin
• Bacterial chromosomes are bound to proteins, negatively supercoiled, and folded into loops held in place by RNAs and proteins
• Bacteria may contain plasmids, small circular DNA molecules with genes for replication and cellular functions, including virulence factors and metabolic enzymes.

Test your knowledge of microtubules with this quiz on their structure, assembly, and functions. Explore topics such as tubulin heterodimers, microtubule nucleation, GTP hydrolysis, and the role of microtubules in maintaining cell shape and facilitating intracellular transport. Also, discover how microtubule-based drugs are used in cancer treatment and the influence of microtubule-associated proteins on their dynamics and stability.