Exam 4 - Reading Flashcards
Read and UNDERSTAND. #Believe
Where are IFs found?
Only in animal cells
What is plectin?
- Elongated dimeric protein that acts as a cross bridge (connects IF to other cytoskeletal filaments).
- Has a binding site for an IF at one end, and binding site for another IF, microfilament, or MT at the other (depending on isoform).
What is similar among the polypeptides of all IFs?
All contain a central, rod-shaped, alpha-helical domain of similar length and homologous amino acid sequence.
What is the basic subunit involved in IF assembly?
Tetramer formed by two dimers that become aligned side by side in a staggered fashion with their N and C termini pointing in opposite directions.
IFs tend to be less _____ and more _____.
Less sensitive to chemical agents and more difficult to solubilize.
What is meant by an anti-parallel arrangement?
The dimers that make up IFs point in opposite directions.
IF assembly and disassembly are controlled primarily by what?
Phosphorylation and dephosphorylation of the subunits.
How are new subunits incorporated into IF?
Directly into the IF interior, into an existing IF network (exchanging)
What are keratins?
Primary structural proteins of epithelial cells (including epidermal cells, liver hepatocytes, and pancreatic acinar cells).
What are the connections used by keratins?
Found through the cytoplasm, tethered to the nuclear envelope in the center of the cell and anchored at the outer edge of the cell by connections to the cyoplasmic plaques of desmosomes and hemidesmosomes.
Various physical connections of IFs allow for what?
IF network is able to:
- Organizing and maintaining cellular architecture
- Absorbing mechanical stresses applied by extracellular environment.
What is EBS?
Epidermolysis bullosa simplex
- Mutation of K14 or K5 (forms dimers with K14)
- Leaves animal extremely sensitive to mechanical pressure.
What is desmin and what does it do?
IF Polypeptide
- Key structural role in maintaining the alignment of the myofibrils of a muscle cell.
- Absence of desmin (and Keratin) makes cells extremely fragile.
Is vimentin mutation as severe?
No, leads to relatively minor abnormalities.
What is DRM?
Desmin related myopathy
- Mutation in the gene encoding for desmin.
- Leads to skeletal muslce weakness, cardiac arrhythmias, and eventual congestive heart failure.
How many groups of IFs are there?
5 groups
Plant cells rely primarily on _____ because ______.
Microfilaments for long-distance transport of cytoplasmic vesicles and organelles because MTs are limited in plant cells.
What are the roles of microfilaments?
- Determining shapes of cells.
- Provide structural support for various types of cellular projections.
- Cell’s motile processes (locomotion, cytokinesis, phagocytosis)
Microfilaments are composed of what?
Globular subunits of the protein actin (most abundant protein in most cells).
In the presence of ATP, actin monomers _____.
Polymerize to form a flexible, helical filament.
What are the synonyms for microfilament?
Actin filament and F-actin
Actin filaments can be what two things?
- Highly ordered arrays, loose ill-defined networks
2. Tightly anchored bundles.
How can we easily identify the presence of actin filaments?
- Cytochemical test using fragmented myosin (S1)
2. F-actin will interact in a highly specific manner with the protein myosin.
What information does the S1-actin complex provide?
The direction in which the microfilament is likely to be moved by a myosin motor protein.
Are actin molecules conserved or highly variable?
Conserved (amino acid sequence of actin from a yeast cell to rabbit skeletal muscle are 88% identical).
Just as tubulin is a GTPase, Actin is a ____.
ATPase.
1. ATP-actin is hydrolyzed to ADP = ADP-actin subunits = bulk of actin filament.
Which end is the fast growing end (actin)?
Barbed (plus) end is fast-growing end
Which end is the slow growing end (actin)?
Pointed (minus) end is slow-growing tip
What is treadmilling?
When subunits are being added to the plus ends and removed from the minus ends of each filament at steady state (rate added = rate removed), the relative position of individual subunits within each filament is continually moving.
What does cytochalasin do?
Drug obtained from a mold: blocks plus ends of actin filaments, allowing depolymerization at minus end.
What does phalloidin do?
Drug from poisonous mushroom: binds to intact actin filaments and prevents their turnover.
What does latrunculin do?
Drug from sponge: binds to free monomers and blocks their incorporation into the polymer.
Myosin head contains what?
- Site that binds an actin filament.
2. Site that binds and hydrolyzes ATP to drive myosin motor.
Myosin heads are conserved or divergent?
Conserved/Similar
Myosin tails are conserved or divergent?
Highly divergent
What two broad groups are myosin’s divided into?
Conventional (type II) and unconventional (type I & III-XVIII)
Which classes of myosin are only found in plants?
Myosins VIII and XI
Which class of myosin is found only in vertebrates?
Myosin X
Which class of myosin is the primary motor for muscle contraction?
Myosin type II
What is myosin II responsible for?
- Splitting a cell in two during cell division
- Generating tension at focal adhesions
- Cell migration
- Turning the behavior of growth cones
What direction does myosin II move?
Towards the plus (barbed) end) of an actin filament.
Which myosin is known to move in the minus direction?
Myosin VI
What is meant by bipolar heads?
Myosin II molecules assemble so that the ends of tails point toward the center of the filament, globular heads point away from center = reversal of polarity at the filament’s center.
- Because they are bipolar, Myosin heads at the opposite ends of myosin filament have the ability to pull actin filaments toward one another (as occurs in a muscle cell).
What is unique about myosin I?
- Single head
- Serves as a cross-link between actin filaments of the cytoskeleton and lipid bilayer of plasma membrane.
- Can exert tension on plasma membrane (helps processes that require movement or deformation of the membrane).
What is unique about unconventional myosin’s?
- Aren’t capable of filament formation
- Operate primarily as individual protein molecules.
- Move processively along actin filament
What group is myosin V in?
Unconventional myosin
The movement of vesicles and other membranous carriers over long distances within animal cells occur on ____.
MTs.
Why would vesicles switch over to actin filament tracks?
Periphery of the cell is actin-rich (use myosin’s for travel).
What does the Rab family of proteins do?
- Regulate vesicle trafficking
2. Attachment of myosin and kinesin motors to membrane surfaces.
What is unique about Myosin VI?
- Moves toward pointed (minus) end
- Located at base of stereocilia.
- Thought to be involved in formation of clatrhin-coated vesicles at the plasma membrane
- Movement of uncoated vesicles to early endosomes
- Maintenance of Golgi morphology.
What are myofibrils?
A longitudinal section of a muscle fiber consisting of a cable made up of hundreds of thinner, cylindrical strands.
What is a sarcomere?
A repeating linear array of contractile units found in myofibril = striated appearance.
- Consist of thin and thick filaments partial overlapping.
The I band contains only ____.
Thin filaments
The H zone only has ___.
Thick filaments
The A band represents the ___.
Region of overlap and contains both types of filaments.
All skeletal muscles operate by _____.
Shortening.
Thin filament is _____ arranged around each thick filament.
Hexagonally
What does the sliding-filament model suggest?
Muscle contraction was not the shortening of individual sarcomeres, but rather the sliding over one another. Sliding of thin filaments toward the center of the sarcomere results in increase in overlap between filaments and decreased width of I and H bands.
Thin filaments of a skeletal muscle contain what other two proteins?
- Tropomyosin
2. Troponin
Thick filaments contain a lot of ____.
Several hundred myosin II molecules together with small amounts of other proteins.
What is Titin and its purpose?
- Prevent sarcomere from becoming pulled apart during muscle stretching.
- Maintains myosin filaments in their proper position within the center of the sarcomere during muscle contraction.
- Giant protein originates at M line and terminates at Z line.
- Highly elastic protein that stretches like a molecular spring as certain regions within the molecule become uncoiled (I band portion)
What are nebulin molecules for?
Molecular ruler
1. Regulates the number of actin monomers that are allowed to assemble into a thin filament.
