Cell Bio - Cell Structure and Movement

Cytoskeleton, Motor Proteins, Extracellular Membrane, Plasma Membrane

Cytoskeleton

Fibers that provide structure to a cell (microfilaments, microtubules, intermediate filaments)

Microfilaments

Thin, structural cytoskeleton fibers made of actin
Have a plus and a minus end
Found in all eukaryotic cells

Intermediate Filaments

Intermediate, structural cytoskeleton fibers made of different proteins

Do NOT have a plus or minus end
Not found in all eukaryotic cells (found mainly in cells that require physical strength)

Microtubules

Thick, structural cytoskeleton fibers made of alpha and beta tubulin
Have a plus and a minus end
Found in all eukaryotic cells
Used for intercellular movement (motor proteins, cilia, flagella) and mitosis

Actin

Polymer that builds microfilaments
Binds with ATP/ADP

G Actin

Globular Actin, not yet polymerized

F Actin

Filamentous Actin
Has been polymerized

Minus End (microfilaments)

ADP actin falls of microfilaments

Plus End (microfilaments)

ATP actin adds onto microfilaments

ADP Actin

Low affinity
Minus end of microfilament

ATP Actin

High affinity

Plus end of microfilament

Polymerization of Microfilaments

ATP actin is added to the plus end of a microfilament → ATP is hydrolyzed → ADP actin falls off the minus end → exchange of ADP to ATP → ATP actin is added to the plus end

Cofilin

Actin binding protein

Helps microfilaments shrink at the minus end
Can be used to split microfilaments

Profilin

Actin binding protein

Helps microfilaments grow at the plus end

Formin

Actin binding protein
Helps to start the formation/polymerization of microfilaments with actin monomers (nucleation)

Capping Protein

Actin binding protein
Stabilize microfilaments by protect the minus and plus ends

Tropomyosin

Actin binding protein
Stabilize microfilaments by wrapping around the filament
Can regulate skeletal muscle contractions

Actin Bundle

Parallel filaments attached together by fimbrin

Network Actin

Mesh-like filaments attached together by filamin

Arp2/3

Actin binding protein
Used to make branches in microfilaments

Fimbrin

Actin binding protein
Crossing linking protein
Bind actin bundles together

Filamin

Actin binding protein
Cross linking protein
Bind actin networks together

Cytochalasin

Affects microfilaments
Fungal metabolite
Binds to G actin
Blocks actin polymerization
Destabilizer

Phalloidin

Affects microfilaments
From amanita mushrooms
Binds to F Actin
Blocks depolymerization
Stabilizer

Tubulin

Monomer of microtubules
Dimeric protein - alpha and beta subunit
Binds to GTP/GDP

Minus End (microtubules)

Tubulin falls off of the microtubule

Plus End (microtubules)

Tubulin adds to the microtubule

Beta Tubulin

Faces the plus end of the microtubules
Switches between being bound to GTP/GDP

Alpha Tubulin

Faces the minus end of microtubules
Always bound to GTP

Centrioles

Nucleate microtubules during cell division
Contain gamma tubulin

Gamma tubulin

Unique form of tubulin found in centrioles

Microtubule Polymerization

Nucleation → protofilament assembly → sheet assembly → microtubule elogation

Elongation (microtubules)

High concentration of GTP beta tubulin
Tubulin added to the plus end

Shrinkage (microtubules)

Microtubules can fray at both ends when there is a low concentration of tubulin
There can be GDP beta tubulin at the plus end, which does not have a high affinity

Polymerase

Microtubule-associated protein (MAP)
Adds tubulin to the plus end

Depolymerase

Microtubule-associate protein (MAP)
Depolymerizes GTP cap causing shrinkage of microtubules

CLASP Proteins

Microtubule-associated protein (MAP)
Rescue microtubules from calapse

Pacitaxel (Taxol)

Stabilizes microtubules
Prevents depolymerization
Isolated from pacific yew tree

Colchicine

Changes shape of tubulin, preventing polymerization
Destabilizes microtubules
Isolated from false autumn crocuses
Used to treat gout

Motor Proteins

Microtubule-associated proteins (MAPs)

Kinesins (move toward plus end)

Dyneins (move towards minus end)

Tau

Microtubule-associated protein (MAP)
Stabilizes microtubules
Mutant tau believed to cause alzheimer’s
Works with MAP2 in axons

Intermediate Filament Monomer

Protein monomer with N-terminus head, central rod domain, and C-terminus tail

Intermediate Filament Structure

Monomer → coiled-coil dimer → tetramer → protofilament → filament

Keratin

Type of intermediate filament found mainly in epithelial cells

Nuclear Lamins

Type of intermediate filament that phosphorylates to form the nuclear lamina

Desmin

Type of intermediate filament found in smooth muscles that resist overstretching

Neurofilaments

Clusters of intermediate filaments in axons that provide strength and flexibility

Adapter Proteins

Proteins that connect to the cytoskeleton

Integrins

Proteins that connect to the ECM via adaptor proteins

Adherens Junction

Microfilament bundles that connect cells together via adapters called cadherins

Desmosomes

Tight intermediate filament bonds between cells

Epidermolysis Bullosa (EB)

Blistering condition due to defect in intermediate filaments

Motor Proteins

Proteins that travel across microtubules that allow for intercellular movement
Involved in muscle contrac