1. cytoskeleton

• The cytoskeleton of the cell is not a rigid or permanent structure
• The cytoskeleton can dismantle itself in certain cases, reorganize itself in other parts of the cell, and coordinate processes like the division of the cell (mitosis)
• The cytoskeleton is composed of small subunits, namely Actin and Tubulin, that form filaments that become a part of larger structures
• Actin and Tubulin react and organize in different ways
• Small actin subunits coalesce together to form actin filaments
• There is an extracellular signal that initiates a signaling cascade leading to the disassembly of actin filaments
• Actin filaments allow division and locomotion in cells
• Protofilaments are made up of α-tubulin and β-tubulin heterodimers with each binding GTP
• Multiple protofilaments are stronger and thermally stable due to the increased number of weak non-covalent bonds between the dimers and bonding interactions
• Nucleation is the "critical mass" or number of proteins required to initiate the construction of filaments
• Treadmilling involves the growth and shrinkage of filament proteins
• Tubulin has 2 copies of GTP, of which only one is hydrolyzable
• Dynamic instability in microtubules involves rapid changes in the addition and removal of subunits
• Catastrophe is the rapid loss of subunits at the "plus" end, while rescue is the rapid growth of subunits at the "plus" end
• The "plus" end of the microtubule is capped with tubulin subunits that have GTP attached
• The microtubule grows rapidly when the "plus" end is capped with GTP-bound tubulin subunits
• The switch between growth and shrinkage (catastrophe) in microtubules is dependent on the concentration of tubulin and GTP in the cytoplasm
• When the concentration of tubulin is high, the ends of the microtubules are tapered, and when it is low, the ends are blunt
• When the concentration of tubulin is low (0.8 μM), the ends of the microtubules are blunt
• Rates of addition (k_on,MT) and removal (k_off,MT) of tubulin subunits at the microtubule ends are relatively slow
• The "minus" end of the microtubule is more stable and doesn't undergo rapid growth and shrinkage
• Filaments (AFs, MTs, IFs) are important for the structure and are dynamic and under control of the cell
• Accessory proteins modify cytoskeletal dynamics
• Accessory proteins are involved in treadmilling and dynamic instability
• 1. tubulin (gamma) is highly conserved and accelerates microtubule formation
• 1. tubulin is insoluble
• 1. tubulin is part of the gTuRC (tubulin ring complex)
• Nucleation by g-tubulin protein complex occurs at the (-) end
• Nucleation of microtubules occurs at the (-) end
• Integration between alpha and beta tubulin with the gamma ring complex (gTuRC)
• MT initiation also occurs in the cytosol (cytoplasm) in plants
• MTs nucleate daughter MTs allowing for branching MTs
• Centrosome contains 2 cylindrical centrioles organized at a right angle from each other
• Centrioles organize PCM (Pericentriolar Material)
• PCM is a cloud of gamma-tubulin where MT assembly is initiated
• Centriole lumen and PCM contain g-tubulin
• Centrosome is near the nucleus
• (-) ends of MTs are anchored inside the centrosome interacting with gamma-tubulin
• The gamma-tubulin complex organizes subunits crucial for assembling microtubule structure
• Ensures the formation of necessary protofilament arrangement consisting of 13 adjacent protofilaments