Chapter 11: Cytoskeleton Flashcards
• Know the three types of cytoskeletal elements and be able to compare their structures
- micro tubules (tubule polymers): hollow tubes that have 13 columns of tubulin molecules.
- microfilament (actin filaments) 2 intertwined strands of actin, each polymer of actin subunits
- intermediate filaments: fibrous proteins supercoiled into thicker cables
• The general purpose of microtubules and the various functions they’re involved in
primarily responsible for organizing contents of the cell
- integral during mitosis by forming the mitotic spindle, provides force needed to segregate chromosomes into 2 daughter cells , critical for stable structures like cilia and flagella.
• How microtubules grow and where they originate from
• Microtubules grow from centrosome
• Sometimes called microtubule organizing center (MTOC) it’s involved in controlling location, # and orientation of microtubules
• centrioles are surrounded by matrix of proteins w/ hundreds of ring-shaped structures formed by γ-tubulin
(gamma ring)
• The structure of microtubules and what they’re made up of
- Microtubules are made up of polymers =protofilaments
- Protofilaments =made of dimers of alpha tubulin & beta tubulin
- microtubule is hollow tube of 13 protofilaments aligned side by side (parallel)
- The result is = hollow tube w/ polarity, w/ one end showing the α-tubulin (the minus end) & the other showing β-tubulin (the plus end)
- Critical for role in directional movement w/i the cell
• The idea of dynamic instability and the detailed mechanism of polymerization and depolymerization of microtubules
• The function of microtubules in vesicle transport and the mechanism of how vesicles move along them
- Motor proteins are what drives movement along microtubules
- Vesicles that are trafficked from the ER/Golgi don’t just float randomly to the cell membrane or organelles
- Each attaches to a specific microtubule and moves along it to the right organelle/location
- Use energy from ATP hydrolysis to carry cargo along a microtubule in a single direction
• The structure of motor proteins
- Kinesin moves toward the positive end of the microtubule (away from cell body/MTOC)
- Dynein moves toward the negative end of the microtubule (toward cell body/MTOC)
- Both form dimers that have 2 globular ATP-binding heads that bind the microtubule and a single tail that binds a specific cargo for transport
• The general structure of microfilaments and what processes they’re involved in
- Microfilaments are also polymers but only single, globular protein called actin
- Actin filaments carry out various functions in the cell by associating with other proteins
- Depending on which proteins they interact with, they can form:
- stiff/stable structures like microvilli (small intestine)
- amoeboid cytoplasmic protrusions that facilitate crawling
- contractile rings that split cells in two during cytokinesis
- Polymerization will occur at either end of the microfilament, so if the cell wants an actin filament to grow in a certain direction, it must “cap” (block) one end
• The mechanism of treadmilling and how actin filaments grow/shrink
- Free actin carries ATP & hydrolyzes it to ADP shortly after being added to the chain (just like tubulin & GTP)
- If the amount of free actin is at an intermediate level, then treadmilling can occur where the filament shrinks at the minus end and grows at the plus end
- When the rate of ATP hydrolysis at the plus end is slower than subunit addition = growth & polymerization
- When the rate of ATP hydrolysis at the minus end is faster than subunit addition = depolymerization
- Overall, the plus end grows and the minus end shrinks creating “treadmilling”
- If actin concentration is really high in the cell, both ends will grow, but if its really low, both ends will shrink
• The purpose of thymosin and profilin
- Thymosin binds monomers to prevent polymerization
* Profilin binds monomers and helps them associate to speed polymerization
• The structure of myosin and what proteins make up a sarcomere
dimers that have globular head connected to long alpha helix
• Myosin uses atp
• Sarcomere = myosin and actin
• Sarcomeres are repeating units- make up myofibrils
• The detailed powerstroke mechanism involved in muscle contraction
- The myosin heads are locked onto the actin filament when bound to ADP; they push inward toward the midline of the sarcomere
- Binding of ATP to the myosin head causes a conformational change that reduces its affinity for actin
- Hydrolysis of ATP triggers the myosin head to pivot & change positions
- the myosin head is still bound to ADP and Pi
- Weak interactions with actin at this new site cause the myosin head to release Pi
- This release causes the myosin head to tightly bind the actin
- Release of ADP causes the powerstroke mechanism
- Powerstroke = pivoting of the myosin head back into its original position causes the actin filament to move inward and muscle contraction to happen
• The role of tropomyosin and troponin and how Ca stimulates a muscle contraction
• Troponin = a calcium sensitive protein complex that interacts with tropomysosin
• Tropomyosin = rod shaped protein that binds to actin and covers myosin binding sites
• When Ca2+ binds troponin, it causes a conformational change in troponin which, in turn, changes the position of
tropomyosin
• This opens up the myosin binding site and allows for a muscle contraction
• The structure of intermediate filaments and the four different classes
• surround nucleus & extend into pm of cell where they anchor to pm at cell-cell junctions
• They be found w/i nuc where they support nuc mem. by forming nuclear lamina network
• Less dynamic than microtubules or actin filaments (don’t polymerize and depolymerize frequently)
1. keratin filaments
2. vimentum and vimentum containing filaments
3. neurofilaments
4. nuclear lamins
• The purpose of keratin and its function in multicellular organisms
- Keratin filaments provide mechanical strength to these cells by anchoring to sites of cell-to-cell contact called desmosomes
- This allows strong cables of these filaments to exist throughout the epithelium like that of our skin
- As our skin stretches, these filaments distribute the stress and prevent our skin cells from rupturing
