w9 txtbk Flashcards
cytoskeleton structure and function
intricate network of protein filaments that extends throughout the cytoplasm
-in animals cells plays role in supporting cell cytoplasm
-controls location of organelles
-highly dynamic structure that continuously reorganizes itself as a cell changes shape, moves or divides
–w/o it, wounds can’t heal, muscles can’t contract, sperm can’t reach egg
cytoskeleton structure
made up of 3 types of protein filaments: intermediate filaments, microtubules and actin filaments
-each type has its own mechanical properties and formed from diff. protein subunit
–family of fibrous proteins entwine to form
intermediate filaments
–globular tubulin subunits form microtubules
–globular actin subunits form actin filaments
intermediate filament function
enable cells to withstand mechanical stress that occurs when cells become twisted or deformed
-toughest and most durable out of the 3
–explains why hair and nails that are made of
these remain intact even after organism is
dead
where are intermediate filaments found?
-cytoplasm of animal cells
form a network thru cytoplasm, surrounding nucleus and extending out of cell
-there, they’re anchored to plasma membrane
at cell-cell junctions called DESMOSOMES, where plasma membrane of 1 cell connected to another cell
intermediate filaments in animal cells
-found in their nucleus
there, they form meshwork called NUCLEAR LAMINA, which underlies and reinforces nuclear envelope
IF strengthens cells and protects them from tearing
structure of intermediate filament
resembles a rope in which many long strands are twisted together to provide tensile strength (withstand tension)
-strands each contain a central elongated rod domain with distinct unstructured domains at either end
–domain has an extended a-helical region that lets pairs of filament proteins form stable dimers by wrapping around each other in a coiled-coil config.
coiled-coil dimers in intermediate filament
2 coiled-coil dimers run in opposite directions, associate to form a staggered tetramer
-these dimers and tetramers are the soluble subunits of IF
-associate with each other side by side and then assemble to generate the final ropelike intermediate fil.
benefit to having paired dimers run in opposite directions
both ends of the staggered tetramer are the SAME, as are the 2 ends of assembled IF
-this feature distinguishes IF from microtubules and actin filaments, whose structural polarity is important for their function
-almost all interactions depend on NONcovalent bonding
what gives IF their tensile strength
combined strength of the overlapping lateral interactions along the length of subunit proteins
where do IF appear most in cells
in cytoplasm of cells that undergo a lot of mechanical stress
-provide internal reinforcement to long, thin cell extensions by distributing the effects of locally applied forces, keeping cells and their membranes from tearing in response to mechanical shear
4 classes that IF can be grouped into
Keratin filaments - in epithelial cells (cyto.)
Vimentin and vimentin-related filaments - in cells of connective tissue, muscle cells and supportive glial cells of nervous system (cyto.)
Neurofilaments - in nerve cells (cytoplasm)
Nuclear lamins - strengthen nuclear envelope (found in nucleus)
filaments are formed by polymerization of their corresponding IF subunits
keratin filaments
class of IF abundant in epithelial cells, provides tensile strength and main structural component of hair, feathers and claws
indirectly connected thru desmosomes and the ends are anchored to desmosomes
why is it important that filaments associate laterally with other cell components
the strong cables distribute the stress that occurs when the skin is stretches
mutations in keratin genes interfere with the formation of keratin filaments in the epidermis
=skin is highly vulnerable to mechanical injury and gentle pressure can rupture cells, causing skin to blister
nuclear lamina structure and function
fibrous layer on the inner surface of the inner nuclear membrane formed as a network of IF made from nuclear lamins
disassembles and re-forms at each cell division, when the nuclear envelope breaks down during mitosis and then re-forms in each daughter cell
-controlled by the phosphorylation and dephosphorylation of the lamins
phosphorylation of lamins
phosphorylation of lamins by protein kinases weakens interactions between lamin tetramers and causes filaments to fall apart
dephosphorylation by protein phosphatases at the end of mitosis allows lamins to reassemble
what do defects in the nuclear lamin cause
associated with certain types of progeria (rare disorder that causes affected individuals to age prematurely)
role of microtubules
extend thru cytoplasm and create a system of tracks within the cell, where vesicles, organelles and other macromolecules can be transported
also responsible for positioning membrane-enclosed organelles within the cell
non-permanent structures and can rapidly disassemble in one location and reassemble in another based on cell needs
can also bundle to form stable structures like cilia and flagella
role of microtubules in organizing cytoplasm
depends on their association with accessory proteins, like MOTOR proteins that propel organelles along cytoskeletal tracks
microtubules structure
built from subunits(molecules of tubulin) each is a dimer composed of 2 similar globular proteins called a-tubulin and b-tubulin (alpha and beta), bound by noncovalent interactions
these dimers stack tg to form the wall of the hollow microtubule
-each filament has a structural polarity with
a-tubulin exposed at one end and b-tublin at other
polarity: end with B-tubulin=PLUS end and end eith a-tubulin=MINUS end
why is polarity important for microtubules
crucial for the assembly of microtubules and for their role once formed
-without polarity, they could not guide directional intracellular transport
centrosome structure and function
microtubule-organizing centre that sits near nucleus in an animal cell
-during cell cycle, it duplicates to form the 2 poles of mitotic spindle
centriole structure and function
array of microtubules found in pairs at the center of a centrosome in animal cells
-contain a special form of tubulin called g-tubulin
–each of these g-tubulin ring complexes serves as the starting point or nucleation site, for growth of one microtubule
what happens once a microtubule gets nucleated?
grows outward from the organizing center for many minutes by the addition of aB-tubulin dimers to its free PLUS end
then without warning, microtubule can undergo transition that makes it shrink by losing tubulin dimers from PLUS end
-can start growing again or may disappear completely, to be replaced by new microtubule that grows from same y-tubulin ring complex
dynamic instability
rapid switching between growth and shrinkage carried thru by microtubules
-allows them to undergo quick remodeling and important for their function
–helps them emerge from organizing centres and retract back before shooting in a diff direction
-this allows them to “explore” cell interior and to establish an organized array in the part of cell where its needed
dynamic instability rxn of microtubules
stems from the intrinsic capacity of tubulin dimers to hydrolyze GTP
-this energetically favourable rxn, which generates GDP and inorganic phosphate is similar to hydrolysis of ATP
-each free tubulin dimer contains one GTP molecule tightly bound to B-tubulin, which hydrolyzes the GTP to GDP shortly after dimer is adding to growing microtubule
-GDP remains tightly bound to the B-tubulin
-when polymerization is proceeding rapidly, tubulin dimers add to end of microtubule faster than GTP is hydrolyzed
as a result…end of a rapid growing microtubule is composed entirely of GTP-tubulin dimers, which forms a GTP cap
how does the randomness of chemical processes affect the tubulin dimers
tubulin dimers at free end of microtubule will hydrolyze their GTP before the next dimers are added so that free ends of protofilaments are composed of GDP-tubulin
GDP-bearing dimers associate less tightly, tipping balance in favour of DISASSEMBLY
because the rest of microtubule is composed of GDP-tubulin, once depolymerization starts, it will continue and can disappear
tubulin dimers then get added to cytoplasmic pool and exchange their bound GDP for GTP, allowing them to add to another growing microtubule
what happens with dynamic instability when a cell has differentiated into a specialized cell type
dynamic instability of its microtubules is often suppressed by proteins that bind to the ends or the sides of the microtubules and protect them against disassembly
-the stabilized microtubules then maintain the organization of cell
relation of cell polarity to microtubules
cell’s polarity is a reflection of the polarized systems of microtubules in its interior, which help to position organelles in their required location within cell
AND
guide the streams of vesicular and macromolecular traffic moving b/w one part of cell and another
difference in movement guided by microtubules or by free diffusion
movement by microtubules is way faster and efficient than diffusion
-however, some axons that extend along the limbs of animals are so long that it can take weeks for materials to reach the nerve terminal
-still faster than diffusion (which could take yrs)
microtubule-associated proteins
stabilize microtubules against disassembly, while others link microtubules to other cell components, including cell cortex or other types of cyto. filaments
some nucleate the growth of new microtubules
how do tip-binding proteins stabilize and control positioning of microtubules
they do this by riding the end of a growing microtubule as it probes the cell interior and then attaching itself(and its microtubule) to a target structure
motor proteins
use the energy derived from repeated cycles of ATP hydrolysis to travel steadily along the microtubule or actin filament in a single direction
because they can attach to other cell components, they can transport cargo along the filaments
families of microtubule-associated motor proteins
KINESINS
-move twd the PLUS end of a microtubule (outward from cell body)
DYNEINS
-move twd the MINUS end (twd cell body)
both are dimers that have 2 globular ATP-binding heads and a single tail
-the heads interact with microtubules in a specific manner so that the motor protein will attach to a microtubule in ONLY 1 orientation
how do motor proteins determine the type of cargo that they can trasnport
the tail of a motor protein generally binds stably to some cell component, such as a vesicle or an organelle and determines the type of cargo that the motor protein can transport
how do microtubules and motor proteins position organelles in the cytoplasm
in animal cells, the tubules of the ER reach almost the edge of the cell, whereas the golgi apparatus is located in the cell interior, near centrosome
-ER extends out from its points of connection with the nuclear envelope along microtubules, which reach from the centrosome to plasma membrane
as cell grows, KINESINS attached to outside of ER membrane (vias adaptor proteins) pull the ER outward along microtubules
DYNEINS attached to golgi membranes pull golgi apparatus along microtubules in opposite direction, twd nucleus
dynamic instability in microtubules stems from the intrinsic capacity of tubulin molecules to hydrolyze what?
A. tubulin dimers
B. ATP
C. water
D. GTP
D. GTP
Dynamic instability in microtubules stems from the intrinsic capacity of tubulin molecules to hydrolyze GTP. β-tubulin hydrolyzes its bound GTP shortly after a dimer is added to a growing microtubule.
how would the dynamics of microtubule polymerization change if cells were incubated with nonhydrolyzable analog of GTP
microtubules would polymerize more and grow longer
actin filaments structure and function
polymers of actin and are essential for maintaining cell shape and for movements that involve the cell’s outer membrane
–also have structural polarity (+ and - end)
w/o actin filaments, an animal cell couldn’t crawl along a surface, engulf a large particle by phagocytosis or divide
depending on which protein they associate with, actin filaments can form stiff and stable structures
how can actin filaments grow
by the addition of monomers at either end but their rate of growth is FASTER at + end than - end
naked filament is unstable and can disassemble from both ends
actin filaments vs microtubules
actin filaments are thinner, more flexible and shorter than microtubules
unlike intermediate filaments and microtubules, actin filaments rarely occur in isolation in the cell; they’re generally found in cross-linked bundles and networks, which are stronger than individual filaments
free actin monomers
carry a tightly bound nucleoside triphosphate, in this case ATP
-the actin monomer hydrolyzes its bound ATP to ADP soon after it’s incorporated into the filament
–hydrolysis reduces the strength of binding b/w the monomers, thereby decreasing the stability of the polymer
nucleotide hydrolysis promotes depolymerization, helping the cell to disassemble its microtubules and actin filaments after they have formed
what happens if the conc. of free actin monomers is very high
an actin filament will grow rapidly, adding monomers at both ends
-at intermediate concentrations of free actin, monomers add to the PLUS end at a rate faster than the bound ATP can be hydrolyzed, so the plus end grows
-at the MINUS end, atp is hydrolyzed faster than new monomers can be added bc ADP-actin destabilizes structure, filament loses subunits from minus end at same time its added to plus end
bc these processes happen simultaneously called treadmilling
treadmilling
process by which a protein filament simul. adds subunits at one end while losing them at the other; in the process, an individual subunit will move along the length of the filament
Phalloidin drug function
Binds to filaments and prevents depolymerization
cytochalasin drug function
caps filament plus ends, preventing polymerization and leading to filament depolymerization at minus ends
latrunculin drug function
binds actin monomers and prevents their polymerization
actin-binding proteins
bind to actin monomers in the cytoplasm, regulating when and where actin filaments will form and grow
when actin filaments are needed, actin-binding proteins promote actin polymerization
-at the same time, actin-binding proteins like thymosin, bind to monomers and prevent them from adding to the ends of actin filaments
-by keeping actin monomers isolated until needed, these proteins contribute to the regulation of actin polymerization
myosin
type of actin-binding (motor) protein
-bind to and hydrolyze ATP, which provides the energy for their movement along an actin filament twd PLUS end
myosin 1
-present in all cell types, has a head domain and tail
-head domain binds to actin filament and has ATP-hydrolyzing motor activity that lets it move along filament in continuous cycle of binding, detachment and rebinding
–tail varies among the diff. types of myosin
and what cargo it’s carrying
myosin 2
2-headed myosin motor that interacts with actin filaments to form contractile bundles, causing changes in cell shape
-changes in shape help with cell movement and division (mitosis)
why is the ability to rearrange actin filaments important
1) Directing cell migration
2) Forming cell polarity
-asymmetries that allow one region of cell to be structurally or functionally diff. from another region
cell polarity is crucial for cells to function
-plays key role in reproduction (establishes organisms’ body plan) and development of cells and organisms
-guides the development and helps form a coherent, organized multicellular organism
What statement describes the behavior of an actin filament undergoing treadmilling at the leading edge of a lamellipodium?
A.) the filament grows exp.
B.) the filament depolymerizes and disappears
C.) the filament adds actin monomers to its minus end while losing them from the plus end
D.) the filament remains about the same length, although the subunits that comprise it are replaced
D.) the filament remains about the same length, although the subunits that comprise it are replaced
Treadmilling involves a simultaneous gain of monomers at the plus end of an actin filament and loss of monomers from the minus end; hence, actin filaments tend not to undergo drastic changes in length.
myosin 2 structure
these proteins are dimers, with 2 globular ATPase heads at one end and a single coiled-coil at the other
clusters of myosin ll bind to each other thru their coiled-coil tails, forming a bipolar myosin filament from which the heads project
myosin filament
polymer composed of interacting molecules of myosin ll
interaction with actin promotes contraction in muscle and nonmuscle cells
myosin filament structure
like a double headed arrow (one set binds to actin filaments in 1 orientations and moves filaments one way; other set moves other way)
bc of this, myosin filament slides sets of opp oriented actin filaments past each other
-thus, if organized in a bundle, it can generate a strong contractile force
-seen in muscle contraction and in the contractile ring that pinches dividing cell in 2 by contracting and pulling inward on plasma membrane
