Week 11 Flashcards
Structure of myosin II thick filaments
- Coiled coil of two a helices
- Neck or hinge region on N-terminus
- Light chain on N-terminus
- Exists in dimer form with light chains and heavy chains
Myosin motor domains walk along what type of protein?
Actin filaments.
Why do myosin II thick filaments have a bare zone in the middle?
Because the filaments are aligned in an anti-parallel fashion, there is a bare zone in the filament.
Why is it important that muscle fibers are multinucleated?
This allows the fiber (cell) to be extremely large.
What is the dark band in sarcomeres composed of?
Myosin thick filaments
What is the light band in sarcomeres composed of?
Actin thin filaments
During contraction, how do the sarcomere, myosin filaments, and actin filaments move?
Sarcomere shrinks but length of myosin filament and actin filaments unchanged.
What is the physical basis of sarcomere contraction?
The walking of myosin heads on actin is the basis of contraction. As myosin walks, it pulls its filament with it, resulting in contraction.
Role of titin
A pair of titin proteins holds myosin thick filament in the center.
Role of nebulin
Nebulin determines the length of the actin filaments. It coats the actin filament.
Role of CapZ in sarcomere
CapZ caps the plus ends of actin filaments.
Importance of capping actin in the sarcomere
Half-life of actin in these filaments is days, rather than seconds to minutes like for filaments in other cells. The actin filament on both sides must be capped on plus and minus sides to prevent disassembly
Role of tropomodulin in sarcomere
Tropomodulin caps the minus end of the actin filament
How is troponin C related to calmodulin?
Like calmodulin, troponin C binds Ca2+
tropomyosin in absence of Ca2+
In absence of Ca2+, tropomyosin prevents myosin heads from interacting with actin filaments.
tropomyosin in presence of Ca2+
Binding of Ca2+ to troponin C results in tropomyosin snapping out of the way, letting myosin heads make contact with the actin filaments and to allow contraction.
Role of sarcoplasmic reticulum on myofibrils
Sarcoplasmic reticulum forms an extensive net-like structure that covers the myofibrils, allowing Ca2+ released from it to flood the entire muscle cell.
The inside of the T tubule is a part of what environment?
The inside of the T tubule is part of the external environment.
After an action potential, how is Ca2+ restored to resting levels?
Ca2+ in cytosol restored to resting levels in 30 ms via Ca2+ pumps (Ca2+ - ATPase) after an action potential
Relationship between voltage-gated Ca2+ channels in T-tubule and in SR
Voltage-gated Ca2+ channels in T-tubule membrane open in response to depolarization.
They are physically coupled to the Ca2+ channels in the sarcoplasmic reticulum.
Structure of microtubules
- Asymmetric, just like actin.
- All subunits stacked in same orientation so alpha always facing minus end and beta always facing plus end.
- Can be several mm long.
- Grows more rapidly at the plus end just like actin
Difference between minus and plus ends of microtubule
- Minus end has a-tubulin facing cytoplasm
- Plus end has B-tubulin facing cytoplasm
- This is the basis of the asymmetry (polarity) of microtubules
Only GTP in which type of tubulin subunit is hydrolyzable to GDP?
B-tubulin
Role of gamma-tubulin ring complex
Gamma-tubulin ring complex acts as a nucleating factor for microtubules, like Arp2/3 complex does for actin.
Role of two y-tubulin small complex
The two y-tubulin subunits can promote growth of microtubules.
Dynamic instability vs treadmilling
Since microtubules are almost always capped at the minus end in cells, you get dynamic instability rather than the treadmilling that occurs in actin filaments
Why does a GTP cap form at the plus end of a microtubule?
GTP in tubulin is hydrolyzed almost immediately after getting incorporated into a microtubule – Results in a GTP cap at the plus end
Depolymerization rates of GDP-tubulin vs GTP-tubulin
Microtubules depolymerize 100 times faster from an end that contains GDP-tubulin than from once containing GTP-tubulin
Cycle of microtubule growth and shrinkage
- Rapid growth with GTP-capped end
- Random loss of GTP cap (Catastrophe)
- Rapid shrinkage
- Regain of GTP cap (Rescue)
- Repeat
Role of centrosomes
Centrosomes serve as microtubule organizing centers (MTOCs) in some cells
MAPs and their role
Microtubules-associated proteins
- Regulate microtubule dynamics
- Regulate microtubule packing and arrangement
Role of MAP2 and tau
Both of these proteins regulate microtubule packing. However, microtubules are more closely packed in axons because of tau (has shorter arm than MAP2).
What determines the size of the space between microtubules?
The space between two microtubules is dictated by the length of the arm of the protein attached to it.
Localization of MAP2 and tau
MAP2 is localized to the dendrites and tau to the axons.
Taxol
A drug that affects microtubule dynamics. Used as part of cancer treatment because it interferes with mitosis.
Direction kinesin motors move in
The plus end of a microtubule filament. In many cells, traffic organelles and vesicle move away from the cell center and out to the periphery.
Direction dynein motors move in
The minus end of microtubules.
Processes cytoplasmic dynein-1 is responsible for
Cytoplasmic dynein-1 responsible for all processes that require minus-end directed movement including organelle and mRNA trafficking, organelle positioning, and construction of microtubule spindles.
Difference in number of dyneins and kinesins needed for transport
Single dynein responsible for most minus-end directed transport, while about 15 different kinesins are used for plus-end directed transport.
Dynein and kinesin activity in pigment granules (melanosomes) in pigment cells of fish
Kinesin activity drops when cAMP levels decrease, allowing dynein activity to predominate
In melanosomes, when dynein wins the “tug-of-war”, what state is the cell in?
When dynein wins, the melanosome is aggregated.
In melanosomes, when kinesin wins the “tug-of-war”, what state is the cell in?
When kinesin wins, the melanosome is dispersed (a rise in cAMP activates kinesin motors)
What type of motors allow for cilia and flagella movement?
Axonemal dyneins
Axoneme
The microtubule-based core inside a cilium or a flagellum
Role of nexin in axonemes
Nexin molecules connect the doublets of microtubules.
How do microtubule doublets allow for bending of cilia and flagella?
Dynein attached to one of the doublet microtubules walks along another doublet.
Symptoms of primary ciliary dyskinesis (Kartagener’s syndrome)
- Immotile sperm
- Chronic lung infection
- Sinus infection
- Situs inversus
Role of intermediate filaments
They impart strength, such as in hair, nails, and axons.
Component polypeptides of nuclear intermediate filaments
Lamins A, B, and C
Component polypeptides of epithelial intermediate filaments
Type I keratins (acidic) and type II keratins (neutral/basic)
Component polypeptides of axonal intermediate filaments
Neurofilaments proteins (NF-L, NF-M, and NF-H)
Why are intermediate filaments symmetrical (non-polar)?
Intermediate filaments bind in an anti-parallel fashion, thus are symmetrical on both sides.
The cell cycle is generally divided into what four stages?
G1: Gap 1
S: Synthesis
G2: Gap 2
M: Mitosis
G0 phase
When cells have exited the cell cycle (maybe temporarily or permanently)
Role of gap phases in the cell cycle
Gap phases allow for growth, as well as provide time for the cell to monitor internal and external environment to ensure suitable conditions and preparations are complete before cell commits to S or M phase.
In which phase does DNA replication occur?
S phase
How is BrdU labeling used to study the cell cycle?
- BrdU (bromodeoxyuridine) is an artificial thymidine analog.
- Brief exposure to BrdU allows cells that are replicating their DNA to incorporate BrdU into newly synthesized DNA.
- Cell nuclei that have incorporated BrdU are then revealed by staining with anti-BrdU antibodies.
How is DNA content analysis with flow cytometry (FACS) used to study the cell cycle?
Cells that have been stained with a dye that binds DNA become fluorescent.
The fluorescence is directly proportional to the amount of DNA in each cell.
Three important model systems for studying the cell cycle
- Budding yeast
- Frog embryo
- Fission yeast
Three major regulatory transitions in the cell cycle
- Start transition: Enter cell cycle and proceed to S phase
- G2/M transition: enter mitosis
- Metaphase to anaphase transition: trigger anaphase and proceed to cytokinesis
Three characteristics of cell cycle control system
- Series of binary (on/off) switches
- Reliable and robust (backup mechanisms)
- Adaptable and modifiable
