BIOL 2020 Lecture Notes - Lecture 7: Treadmilling, Intermediate Filament, Cell Surface Receptor
Microfilaments
Dynamic Instability
o Activates tubulin and binds onto the plus end of the microtubule
o As they bind on, the tubulin GTP forms a GTP cap
o As the microtubule is elongating, it has a tubulin cap and GTP in an exchangeable site
o Tubulin adds on a certain rate, and as they do that, the GTP gets hydrolyzed to GDP
o If you reach a rate of GTP hydrolysis that is faster than the rate of addition of tubulin,
the cap will fall off
o Catastrophe is the process of the microtubule breaking down
o If it stops and begins to reassemble, it is called "rescue"
o This may use a lot of energy and hydrolyzing GTP but it allows for constant
organization
o **In textbook
Intermediate Filaments
o Microtubules are 25nm in diameter (know this!)
o Intermediate filaments are 10nm, they are smaller than microtubules and they are filaments
and thus don't have a hollow core
o Microtubules are red, and intermediate filaments are blue
o Intermediate filaments are hooked to either microtubules or microfilaments by a
protein called plectin (green in the photo)
o The main function of intermediate filaments is to reinforce the cell and provide
mechanical structure to it
Intermediate Filament Distribution
o Keratin is a protein that is in hair and in intermediate filaments
o Antibody that specifically recognizes keratin intermediate filaments
Intermediate Filament Distribution
o Intermediate filaments prevailed the entire cytoplasmic cell and they attach to desmosomes
and hemidesmosomes so they have a direct connection
o Hemidesmosomes sit on the extracellular matrix
Intermediate Filament Proteins
o Chemically heterogeneous group of proteins
o We can recognize that they are different by their sequences, their reactions to antibodies
o They are different in their tissue distribution
o They are extended sorts of proteins, unlike tubulin which is globular
o Intermediate filaments are elongated and all 5 families of them share a common
region in the middle
o This is a coiled region made by heptad repeats (7 amino acids repeated over and over
again) and all intermediate filaments have this.
o The amino and carboxy end is different from each protein group
o This is what determines the 5 families
Intermediate Filament Assembly
o Coiled coil dimer come together in a parallel fashion
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o It is still a polar protein as a dimer (remember - polar means different at two ends)
o Then the two dimers together in an antiparallel arrangement to become a tetramer
o The tetramer is non polar
o The tetramer is a building block of intermediate filaments and
o Protofibril stage
o They line up head to tail and this is non polar
o No GTP or ATP involved in this assembly
Phosphorylation Kinases Disassembly
o Assembly and disassembly of a protein is regulated by phosphorylation and
dephosphorylation
o Done with kinases, it will take an organic phosphate group and add it to a proteins
o Kinase adds a phosphate - kinases are major signalling group in the cell
o Phosphatase will take away a phosphate
o However, the kinase's main role is structural
Intermediate Filaments
o If you have a malfunction in a keratin protein, it is a mutation of one amino acid. It is called
epidermolysis bullosa simplex
o When you disrupt a keratin, you disrupt the strength of a cell
o Ex. If you wanted to put on a shirt, you would get blisters wherever the shirt was and
that if they break open, they are exposed to potential infection and if this happens, yu
are not likely to survive very long
Microfilaments
o Diameter between 6-8 nm
o They move a lot in the cell and play a role in moving the cell from place to place
o They help determine the shape of the cell
o And they also provide structural support when the cell makes projections
Actin
o Actin is the main structural protein in microfilaments
o Similar in size and number of amino acids
o Similar amino acid sequences, they very often are in the same group of proteins
o Copolymerize - suggest that actin is very similar to one another, they can even come
together to make the same filament
o In all likelihood, they all came from the same common ancestor gene
o So similar that they can do things together
o There are two forms of actin
o G-actin (globular) monomers
o F- actin (filamentous) which is a polymer of G actin monomers
o Microfilaments
G- Actin
o Once it has an ATP bound to it, it is fairly stable
o Three dimensional structure
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