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• Microtubule Polarity


  -Microtubules are polar structures:

  - +ve ends (beta-tubulins exposed)

  grow rapidly.

  - -ve ends grow slowly or not at all.
• Structure of Microtubule


  - polymers of tubulin dimers alpha and beta tubulin

  -a tubulin and b tubulin forma stable dimer and do not come apart

  -25 nov diameter
• Microtubule Formation


  - in test tube = spontaneous polymerisation

  -In the cell tubulin concentration is too low for spontaneous polymerisation.

  -Cells use a template made from gamma-tubulin and other proteins to speed up the process of polymerisation - called nucleation.
• Gamma-tubulin location in cells

  -In non-dividing cells on centrosomes.

  -In ciliated cells on basal bodies of cilia.

  -Ciliated cells have an extra set of microtubules in the cilia, which are nucleated by the basal body.
• Microtubule Ends locations


  -Microtubule +ve ends are at the cell periphery (outer areas)

  -Microtubule -ve ends are at the cell centre, at the centrosome.
• Microtubule Dynamics


  -Microtubules are dynamic - each one grows and shrinks independently of its neighbours.

  -Can switch between growing and shrinking = dynamic instability.
• Role of GTP in microtubule Growth

  -GDP-bound tubulin cannot polymerise, GTP-bound tubulin can.

  -In the microtubule, GTP is gradually hydrolysed to GDP.

  -The protein EB1 binds preferentially to GTP tubulin, marks growing microtubules.

  -GTP-tubulin dimers bind more tightly to each other than GDP-tubulin dimers due to their shape is slightly different.
• GTP Capping


  - GTP capped = growth

  - no GTP cap = depolymerisation
• Actin Filaments Structure


  -Major component of the muscle.

  Found in contractile bundles (e.g. epithelial cells) or non-contractile bundles (e.g. fibroblasts) in almost all other cell types.

  -Assembled from monomeric actin.

  -Thin, flexible, helical filaments, 7nm in diameter.
• Actin Polarity and ATP activity

  -Have +ve and -ve ends.

  -Actin filaments hydrolyse ATP after assembly - actin is an ATPase.
• Assembly and Disassembly of Actin

  -In a cell, capping proteins often bind to -ve ends to prevent depolymerisation.

  -Assembly occurs from +ve end, disassembly occurs from -ve end.
• many proteins bind actin filaments and alter their organisation and dynamics.


  -Some control the balance between monomer and polymer.

  -Nucleating proteins promote polymerisation - cells use them to control where it will happen.

  -Other proteins alter filament length or dynamic - monomers, nucleating or branching.

  -Some proteins alter filament organisation

  -some control or drive movement along actin
• Alteration of Polymerisation by Natural Small Molecules

  -Phalloidin stabilises actin filaments.

  -Cytochalasin caps filament ends, preventing actin polymerisation from existing ends.

  -Latrunculin binds to actin monomers, preventing actin polymerisation.