5: Cytoskeleton Overview

intricate network of protein filaments that extend throughout the cytoplasm

one side is alpha (-) end one side is beta (+) end

alpha and beta tubulin which are 55kDa each. these form a stable dimer of 8nm

25nm up to 100s of µm long (project out of cell - flagella)

protofilaments when B subunit is in GTP subunit

no seam forms

B tubulin can be hydrolyzed from GTP to GDP

• singlet: most common, found in cytoplasm made of 13 protofilaments • doublet: found in cilia and flagella, A MT (13 PF) + B MT (10 PF) • triplet: basal bodies & centrioles: A MT (13 PF) + B MT (10 PF) + C MT (10 PF) - stable tend to not polymerize and depolymerize

cytoplasmic: singlet nerve axon cytoplasm MT axonemal: doublet cilia/flagella MT

MT organizing centre (MTOC) which is associated with (-) end and (+) end grows away

main cellular MTOC that contains centrioles and pericentriolar matrix

• spindle poles in mitotic apparatus • basal body: MTOC for cilia/flagella • nerves: MT are not continuous (axon + dendrites: orientation varies)

animals (not plants)

2 barrel-like triplet MT that are perpendicular to each other

space around centrioles where proteins gamma tubulin and augmin are found

proteins to make singlet cytoplasmic MTs gamma tubulin: starts polymerization of singlet MT augmin: increases polymerization both form gamma tubulin ring complex

pericentriolar matrix also replicates and they make a mother and daughter MT which are different

nucleating sites for MTs = this is the minus end

where polymerization of MT can occur on

MT branching

Take flagellar nucleus (piece of flagella) which acts as a nucleating site, which is a doublet MT = stable and won’t depolymerize Add tubulin diners to the nucleus until critical concentration for polymerization is reached

depolymerization

plus bc minus end has gamma tubulin ring complex = capped = blocking polymerization and depolymerization

nucleus presence