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.