Cytoskeleton Flashcards
What is a cytoskeleton? (4 functions)
protein structure that provides shape and helps maintain internal structure. Also involved in cell mobility, division, and intracellular trafficing
Microtubule Properties
25nm in diameter, Function as scaffolds and assist with trafficking. All negative ends attach to centrosome.
Microtuble Formation
Composed of Alpha and Beta tubulin dimers. Both bind ATP, alpha GTP covered after dimerization. Dimers bind together to make protofilaments which associate to make microtubules.
Polymeriation occurs from beta end where GTP is hydrolyzed. GTP cap stabilizes growing tubule.
Intermediate Filament Properties
Types vary based on cell type. (Keratin in epithelia, Vimentin in connective tissue)
Provide mechanical stabilization to the cell and accessory proteins provide cross-linking
Intermediate Filament Formation
Non-polarized, alpha helical, monomers that dimerize and coil. Dimers form staggered tetramers in anti-parallel fashion (C and N termini). Tetramers stack to from complex ropes.
Microtubule stabilization
Growth stabilized by capping proteins
Microtubule de-polymerization
Severing hexamers recognize C-terminal tails and utilize ATP for cleavage.
Microtubule severing proteins (3)
Katanin, Spastin, Figetin
Spastin mutation associated with Hereditary Spastic Paraplegia
Molecular Motors
Move cargo along microtubules. Form homo- or hetero-dimers.N terminus binds ATP and microtubule, A terminus binds adapter which binds cargo. ATP hydrolysis produces power stroke.
Kinesin
Molecular motor moves in + direction away from centrosome toward PM
Dynein
Molecular motor moves in - direction toward centrosome away from PM
Microtubules in Mitosis (3 types)
3 microtubules develop in spindle following centrosome duplication.
Astral tubules form in all directions, stabilizing
Kinetocore tubules bind chromosomes
Overlap tubules attach to opposite overlap and help move chromosomes apart
Taxol
Drug that prevents spindle assembly
Hereditary Spastic Paraplegia
Spastin mutations reduce cleavage of excess microtubule branches in axons and can lead to cell death
Keratin mutations
Disease occurs when all keratins in cell are mutated, most common in liver or kidney where only keratin 8 and 18 are expressed
Intermediate filament diseases
Failure to cleave membrane binding Cas domains of laminins causes Progeria
Actin Function
Involved in epithelial polarity, muscle contraction, cell motility, cell divison
What do microtubules traffic?
mRNA, proteins, other microtubules (overlaps)
Actin Formation
Helical filament of polar tetramers. Actin monomers (g-actin), do not form tetramers spontaneously (nucleation) and require either FH2 or Arp2/3 assistance.
Elongation and shrinking occur at either end but - end has exposed ATP that is needed for shrinking whereas + end ATP is covered.
Regulation of Actin formation (3)
g-actin concentration, ADP-ATP exchange, capping/severing of filaments
FH2
Helps with nucleation of elongating filaments. Mimics 2 g-actin for tetramer formation. ATP-independent, only needs activation by small GTPase
Arp2/3
Helps with nucleation of branching filaments. Mimics 1 g-actin in pseudonucleation center. ATP-independent, only needs activation by small GTPase
Epithelial Polarity
Actin forms tight and adherin junctions within cell that localizes appropriate proteins to appropriate membranes.
Also helps form microvili to improve absorption.
Molecular Motion
Actin associates with myosin thick filaments where N-terminal head of myosin dimers perform power stroke with ATP hydrolysis
Myosin 5
Irregular myosin, motor protein moving toward + end of actin
Cell Movement
Leading edges of cells (lamellipodia) are covered in actin filaments produced by Arp2/3. Actin polymerization (via GTPase activation) elongates cell and causes binding to ECM through beta units associated with integrins. Contraction causes movement.
Actomyosin ring in cell division
This contracts and pinches off cytoplasm to separate daughter cells. Localization of ring determined by Ect2 on astral filaments. At points where opposite astral filaments meet, Ect2 concentration is high enough to activate RhoGTP which activates ROCK which phosphorylates myosin and leads to ring contraction
Asymmetric Cell Division
Erythrocytes cut out nucleus. Megakaryocyte divides but only splits off platelets. Ring canals form in spermatogonia. Sperm cells mature by releasing residual body. Epithelial cells divide only sideways to preserve monolayer
