Cell Structure 3 Flashcards
What makes up the cytoskeleton?
- Microfilaments: Support/organize plasma membrane, cell shape/division/motility
- Microtubules: Organize cytoplasm, intracellular transport, cell division, cilia/flagellae motility (related to centrosome)
- Intermediate filaments: Strengthen cytoplasm/tissues, support nucleus, epidermal appendages (nails, hair)
* built from smaller protein subunits that can be rapidly assembled and disassembled
What are two main cytoskeletal proteins?
Actin and Tubulin–filaments formed from both proteins have a polarity–> ends are sterically different
What is actin?
- Medium sized globular protein
- Half a dozen actin genes, muscle/cytoplasmic subtypes
- Present in HIGH concentrations in all cells
- Can bind and hydrolyze ATP to ADP
- Interacts with a large number of other proteins
What is Tubulin?
- Medium sized protein
- Over A DOZEN tubulin genes, divided into three subtypes, alpha beta gama
- Expressed in all cells, especially neurons
- can hydrolyze GTP to GDP
- Interacts with a fair number of other proteins
What are intermediate filament proteins?
- vary in size from under 50 to over 200
- over 50 gene products categorize 4 subfamilies
- present in most but not all cells, expressed in tissue specific manner
- Polymerization–regulated by phosphorylation, polymerize when unphosphorylated, depolymerize when phosphorylated
- Interact with a few other proteins
DO NOT HAVE INHERANT POLARITY
What is dynamic instability?
The constant oscillation between rapid depolymerization and growth. In microtubules diassembly is called catastrophe, re-establishment of GTP cap is rescue.
Describe the polymerization/depolymerization of actin and tubulin.
- Soluble subunits are associated with ATP (actin) or GTP (tubulin) which favor addition to polymer.
- Incorporation of ATP/GTP enhances hydrolysis and makes subunits of polymer more unstable.
- Subunits are added or fall off from ends of polymers.
- Microtubules/filaments have polarity and one end (+) is more likely to be added, the other removal (-END).
- If the end subunits are associated with unhydrolyzed nucleotide (ATP/GDP) addition is favored and the polymer grows. If nucleotide hydrolysis catches up with the end, subunits rapidly disassemble.
PHOSPHORYLATION- DEPOLYMERIZAITON
DEPHOSPHORYLATOIN- POLYMERIZATION
What is responsible for most tubulin polymerization and what are the two other modulating factors?
- Nucleotide hydrolysis
- Two factors:
- Proteins, XMAP215 associate with the POSITIVE ends of microtubules to influence tendency to grow or disassemble
- NEGATIVE ends are anchored and stabilized by distinct cytoplasmic structure called centrosome. Nucleation of tubulin polymerizaiton is main function of centrosome.
How does actin polymerization form 3D network?
- Signals activate ARP
- Activated ARP serve as seeds to initatie actin polymerization (+end outward)
- New microfilaments elongate, pushing plasma membrane forward.
- Capping proteins arrest elongation
- ATP is hydrolyzed to ADP destabilizing the microfilament (chop it into pieces and create more plus ends)
- ADF/cofilin servers microfilaments creating more free ends and promoting actin depolymerization
- Profilin binds to actin monomers and catalyzes exchange of ADP to ATP
What do Phalloidin and Taxol do?
binds to actin/microfilaments, stabilize them, disrupt polymerization/depolymerization is fatal to the cell
Taxol is similar to phalloidin but works on microtubules–stabilizes polymerized microtubules and prohibits depolymerization
What affects microfilament organization?
- Small G proteins of the Rho family
Three types: - Rho- stress fibers (parallel bundles assosiated with myosin)
- Rac- lamellipodia (meshworks stabilized with filamin)
- Cdc42- filopodia (parallel bundles stabilized with cross-linking proteins, spiky)
What are actin associated proteins?
- Dictate the function of the microfilament cytoskeleton
- Motor proteins
- Cross-linking proteins (filamin, a actin)
- linking proteins (spectrin, dystrophin, vinculin)
- Severing proteins (cofilin, glesolin)
- Monomer- binding proteins (profilin, thymosin)
- Capping proteins (actin capping proteins
Help give shape and involved in motor proteins
What are myosins?
motor proteins for the actin coytoskeleton
- type 1- lacks tail, find next to membranes where it can move components of a membrane
- type 2- have ability to polymerize into filaments, can give rise to sliding filament phenomenon in muscle contraction
How does myosin 2 walk along an actin filament?
Depends on binding of ATP.
Power stroke arises from conformational change in phosphate head–when phosphate group is released, results in comf change in myosin head
How do cells move?
- lamelipodia–actin polymerization drives membrane forward (Rac regulated)
- behind lamelipodia- filaments reorganize into stress fibers and focal ocontacts form (rho regulated)
- rear of cell- myosin 2 interacts with stress fibers to pull back end or cell forward
- Integrin–links microfilaments iwth extracellular matrix (form car to grip substrate)
