unit 3 week 2 pt 1 Flashcards
What are intermediate filaments (IFs)?
Intermediate filaments are solid, unbranched cytoskeletal filaments with a diameter of 10–12 nm that provide mechanical strength to animal cells.
In which type of cells are intermediate filaments found?
Intermediate filaments have only been identified in animal cells.
What is the primary function of intermediate filaments?
They provide mechanical strength to cells that experience physical stress, such as neurons, muscle cells, and epithelial cells lining body cavities.
How do intermediate filaments differ from actin filaments and microtubules?
Unlike actin filaments and microtubules, IFs are a chemically heterogeneous group of structures encoded by approximately 70 different genes in humans.
-also more permanent
How are intermediate filament subunits classified?
IF polypeptide subunits are divided into five major classes based on their cellular location and biochemical, genetic, and immunologic properties.
Which classes of intermediate filaments are involved in cytoplasmic filaments?
Classes I–IV are involved in the construction of cytoplasmic filaments.
Where are type V intermediate filaments found?
Type V IFs, known as lamins, are part of the inner lining of the nuclear envelope.
How are intermediate filaments interconnected with other cytoskeletal elements?
IFs are linked to other cytoskeletal filaments by thin, wispy cross-bridges.
What protein forms the cross-bridges that connect intermediate filaments to other cytoskeletal elements?
Plectin, an elongated dimeric protein, forms these cross-bridges.
What is the function of plectin?
Plectin has a binding site for an intermediate filament at one end and, depending on its isoform, a binding site for another intermediate filament, actin filament, or microtubule at the other end.
Do all intermediate filament (IF) polypeptides have the same amino acid sequence?
No, IF polypeptides have diverse amino acid sequences but share a similar structural organization.
What is the structural organization of IF polypeptides?
All IF polypeptides contain a central, rod-shaped, α-helical domain of similar length and homologous amino acid sequence, flanked by globular domains of variable size and sequence.
How do IF subunits differ from tubulin and actin subunits?
IF subunits have a long fibrous α-helical domain, while tubulin and actin subunits are globular proteins.
What is the first step in IF assembly?
Two IF polypeptides wrap around each other to form a ropelike dimer, approximately 45 nm in length.
Do IF dimers have polarity?
Yes, IF dimers have polarity because their polypeptides align in the same orientation, with one end defined by the C-termini and the opposite end by the N-termini.
What is the basic building block of an IF filament?
The basic building block is a rodlike tetramer formed by two dimers that align in an antiparallel, staggered fashion.
Does the IF tetramer have polarity?
No, because the dimers in the tetramer are arranged in opposite directions, the tetramer itself lacks polarity.
How are intermediate filaments formed from tetramers?
Eight tetramers associate laterally to form a unit-length filament (~60 nm), which then assembles end-to-end to create the elongated intermediate filament.
Do IF assembly steps require ATP or GTP?
No, none of the IF assembly steps require ATP or GTP.
How does the lack of polarity in IFs distinguish them from other cytoskeletal elements?
Unlike microtubules and actin filaments, which have polarity and directional growth, IFs lack polarity, affecting their assembly dynamics.
How resistant are IFs to chemical agents?
IFs are less sensitive to chemical agents than microtubules and actin filaments and are more difficult to solubilize.
What happens to IFs when a cell is treated with ionic detergents?
Almost everything inside the cell is extracted except for the intermediate filaments, due to their insolubility.
Are IFs static or dynamic structures?
Despite their insolubility, IFs are dynamic and incorporate new subunits in vivo.
How do new subunits integrate into existing IFs?
Labeled keratin subunits injected into cells incorporate into the interior of existing IFs rather than at the ends.
What mechanism controls IF assembly and disassembly?
IF assembly and disassembly are primarily regulated by phosphorylation and dephosphorylation of subunits.
How does phosphorylation affect IFs?
Phosphorylation of IFs, such as vimentin filaments by protein kinase A, leads to their disassembly.
What are keratin filaments, and what is their function?
Keratin filaments are the primary structural proteins in epithelial cells. They form a network that extends from the nuclear envelope to desmosomes and hemidesmosomes, connecting with microtubules and actin filaments. This network provides mechanical strength and helps organize the cellular architecture, absorbing mechanical stresses.
What are neurofilaments, and where are they found?
Neurofilaments (NFs) are intermediate filaments found in neurons, particularly in the axon. They are composed of three proteins: NF-L, NF-M, and NF-H (type IV IFs). NF-H and NF-M have sidearms that help maintain the spacing between neurofilaments.
How are genetically engineered mice used to study IF function?
Genetically engineered mice that fail to produce a particular IF polypeptide or produce an altered version are used to study IF function. These studies show the critical roles of intermediate filaments in specific cell types.
What happens when the keratin K14 gene is knocked out in mice?
Mice that lack the K14 keratin gene suffer from extreme sensitivity to mechanical pressure, causing severe blistering of the skin or tongue.
What is the consequence of a desmin knockout in mice?
Mice that lack the desmin polypeptide experience significant cardiac and skeletal muscle abnormalities, linked to desmin-related myopathy.
What happens when the vimentin gene is knocked out in mice?
Mice lacking the vimentin gene show only minor abnormalities, suggesting that vimentin is not as essential as other intermediate filaments in certain cell types.
What does research on knockout mice tell us about the role of IFs?
Research on knockout mice indicates that intermediate filaments have tissue-specific functions, crucial for mechanical strength in certain cells.
What are some examples of cellular motility that rely on actin?
Examples include the migration of neural crest cells, movement of white blood cells, motility of epithelial cells at wound edges, and growth of axons.
How does actin function in plant cells?
In plant cells, actin is primarily responsible for the long-distance transport of cytoplasmic vesicles and organelles.
What is the structure of actin filaments?
Actin filaments are approximately 8 nm in diameter and composed of globular actin subunits that polymerize in the presence of ATP.
What are the terms used to refer to actin filaments?
The terms actin filament, F-actin, and microfilament are used interchangeably.
How is the polarity of actin filaments identified?
The polarity can be identified using a proteolytic fragment of myosin, called S1, which binds to actin filaments.
How is actin localized in cells for observation?
Actin can be localized using fluorescently labeled phalloidin, fluorescently labeled actin, or anti-actin antibodies.
What is the role of actin in muscle cells?
Actin plays a key role in muscle contraction, working in conjunction with myosin to generate force.
How conserved is actin across different species?
Actin has been highly conserved throughout evolution, with amino acid sequences from different sources showing high similarity.
How do motor proteins interact with actin filaments?
Most cellular processes involving actin require the activity of motor proteins, particularly myosin.
How does ATP relate to actin monomer incorporation into a filament?
An actin monomer binds to ATP before incorporating into a filament, after which ATP is hydrolyzed to ADP.
What is the process of actin polymerization in vitro?
Actin polymerization in vitro shows slow nucleation followed by rapid elongation, with the barbed end incorporating monomers faster than the pointed end.
-steps: nuleation, elongation, steady state
What is the critical concentration in actin filament assembly?
The critical concentration refers to the minimum concentration of ATP-actin monomers required for filament elongation.
What happens during the steady-state phase of actin filament elongation?
In the steady-state phase, the barbed end continues to grow while the pointed end stops elongating, maintaining a constant filament length.
What is treadmilling in actin filaments?
Treadmilling refers to the continuous movement of actin subunits within the filament, with subunits added at the barbed end and removed at the pointed end.
How do accessory proteins influence actin filament dynamics in the cell?
Accessory proteins and local environmental changes influence the assembly and disassembly of actin filaments.
Which drugs can disrupt actin filament dynamics?
Drugs like cytochalasin, phalloidin, and latrunculin can affect actin dynamics.
What are the effects of these drugs on actin-mediated processes?
These drugs disrupt the dynamic nature of actin assembly and disassembly, halting processes like cell movement and division.
What is the primary role of myosin in relation to actin filaments?
Myosin is a molecular motor that moves toward the barbed end of an actin filament.
Where was myosin first isolated, and where is it found in eukaryotic cells?
Myosin was first isolated from mammalian skeletal muscle tissue and is present in virtually all eukaryotic cells.
What are the key features shared by all myosins?
All myosins share a characteristic motor (head) domain that binds to actin filaments and hydrolyzes ATP.
How are myosins classified?
Myosins are classified into conventional (type II) myosins and unconventional myosins, further subdivided into at least 17 classes.
What is the structure of myosin?
Myosin has a characteristic motor (head) domain that binds to actin filaments and hydrolyzes ATP. The head domains are similar, while the tail domains are highly divergent.
How are myosins classified?
Myosins are classified into two broad groups: conventional (type II) myosins and unconventional myosins, with the latter further subdivided into at least 17 classes based on amino acid sequences.
How many myosin classes are found in humans, and what are their functions?
Humans have about 40 different myosins from at least 12 classes, each with specialized functions, with conventional (type II) myosins being the best understood.
What is the role of myosin II in cells?
Myosin II is primarily involved in muscle contraction and also functions in cell division, migration, and generating tension at focal adhesions.
How many myosin II heavy chains are encoded in the human genome, and what is their function?
The human genome encodes 16 different myosin II heavy chains, 3 of which function in non-muscle cells, involved in processes like cell division and migration.
What is the structure of myosin II?
Myosin II is composed of six polypeptide chains: a pair of heavy chains and two pairs of light chains, featuring a pair of globular heads, necks with associated light chains, and a long rod-shaped tail.
How does the myosin II molecule function in vitro?
In vitro, isolated myosin heads can slide attached actin filaments, demonstrating that the machinery for motor activity is contained within the myosin head.
What is the function of the fibrous tail in myosin II?
The fibrous tail of myosin II plays a structural role, allowing the protein to form bipolar filaments crucial for muscle contraction.
What is the significance of the bipolar structure of myosin II filaments?
The bipolar structure allows myosin heads to pull actin filaments toward each other, essential for muscle contraction and cellular movements.
Do myosin II filaments assemble and disassemble?
Yes, myosin II filaments can assemble and disassemble in response to cellular needs.
How were unconventional myosins first discovered?
In 1973, a unique myosin-like protein called myosin I was described, extracted from Acanthamoeba, which was smaller than muscle myosin and couldn’t form filaments in vitro.
What is the role of myosin I in the cell?
Myosin I serves as a cross-link between actin filaments and the plasma membrane, potentially playing a role in membrane movement or deformation.
Can unconventional myosins form filaments?
No, unconventional myosins cannot form filaments and typically function as individual protein molecules.
How do unconventional myosins, such as myosin V, move along actin filaments?
Unconventional myosins like myosin V move processively along actin filaments in a ‘hand-over-hand’ fashion.
What experimental method was used to study the movement of myosin V along actin filaments?
Atomic force micrographs were used to study myosin V’s movement along actin filaments by placing molecular obstacles in its path.
How does myosin V move along actin filaments?
Myosin V moves in a ‘hand-over-hand’ fashion, with one head detaching, swinging forward, and reattaching ahead of the other head.
Why is the long neck of myosin V important for its movement?
The long necks of myosin V act like levers, allowing it to take large steps essential for traveling along helical actin filaments.
What role do unconventional myosins play in vesicle and organelle movement?
Unconventional myosins are involved in the transport of cytoplasmic vesicles and organelles, acting as tethers or transporters.
How do unconventional myosins contribute to the movement of vesicles within the cell?
They transport vesicles along microfilament tracks in the actin-rich periphery, switching to actin filaments for localized movement.
How do microtubules and microfilaments cooperate in pigment cells?
In melanocytes, pigment granules are transported by myosin V to hair follicles, where they are incorporated into developing hair.
What happens when myosin Va is defective in mice and humans?
Mice lacking myosin Va cannot transfer melanosomes, leading to lighter coat color, while humans with defects have Griscelli syndrome, causing partial albinism.
What is the relationship between Rab27a and Griscelli syndrome?
Some Griscelli syndrome patients lack a functional Rab27a gene, which regulates vesicle trafficking and attaches motors to membranes.
How are inner ear hair cells important for studying unconventional myosins?
Inner ear hair cells have stereocilia supported by actin filaments, crucial for studying unconventional myosins and sound detection.
What is the structure of stereocilia in inner ear hair cells?
Stereocilia are supported by bundles of parallel actin filaments, undergoing dynamic turnover.
What role do unconventional myosins play in stereocilia?
Unconventional myosins help maintain the structure and function of stereocilia, essential for sound detection.
What is Usher 1B syndrome, and how is it related to myosin VIIa?
Usher 1B syndrome is caused by mutations in the myosin VIIa gene, resulting in deafness and blindness due to malformed inner ear hair cells.
What is unique about myosin VI, and how does it function?
Myosin VI moves in the ‘reverse direction’ toward the pointed end of an actin filament, playing roles in membrane trafficking and maintaining cellular morphology.
What diseases are associated with mutations in myosin VI?
Mutations in myosin VI are linked to inherited diseases affecting the inner ear, resulting in hearing loss.
What can we learn from the interaction between actin and myosin?
The interaction mediates complex cellular activities, providing insight into cellular dynamics and the role of unconventional myosins.
