lect 12: cell motility Flashcards
what are the learning objectives of this lecture?
-microtubules’ structure and functions
-microtubule-organizing centers (MTOCs): centrosomes
-motor proteins: kinesins and dyneins
-cilia and flagella movement: as mediated by microtubules
what is the overview of the cytoskeleton?
composed of a network of three filamentous structures: intermediate filaments (IF), actin filaments (F-actin or microfilament), microtubules(MT)
-determine cell shape
-position membrane-enclosed organelles within the cell
-provide tracks for vesicles and organelle transport within a cell
-involved in mitotic spindles formation and cell division
what is the structure and composition of microtubules?
-hollow, relatively rigid, tubular structures made of the protein tubulin (tubulin alpha/beta)
-one end of a microtubule is attached to microtubule-organizing center called centrosome while the other end spans the cytoplasm
-MT have 13 protofilaments aligned side by side in a circular pattern within the wall of the tubule
where are microtubules found? what is the structure and composition (continued)?
MT are found in the cytoskeleton, the mitotic spindle, centrioles and the core of cilia and flagella
-MT have 13 parallel protofilaments aligned side by side in a circular pattern within the wall of the tubule
-each protofilament is structurally polar resulting in a structurally polar microtubule
what are MTOCs? what do they control?
-microtubules in cells grow from specialized organizing centers (MTOCs)
-they do so by providing premade rings upon which new microtubules can readily assemble
-the most prominent MTOC is the centrosome
MTOCs control:
-number of microtubules
-polarity of microtubules
-number of protofilaments (13)
-time and location of assembly
what are centrosome (MTOC)?
-a centrosome is a major site of microtubule initiation (nucleation) in animal cells and remains at the center of the cell’s microtubule network
-it contains two barrel-shaped centrioles surrounded by amorphous (gel-like matrix of proteins)
-the amorphous is where the gamma-tubulin ring complexes are distributed from which microtubules grow outwardly
what are the dynamic properties of microtubules?
-some microtubules are highly stable such as those in cilia and neurons, while others are subject to disassembly such as those forming the cytoskeleton or mitotic spindle
-microtubules of the cytoskeleton are dynamic polymers subject to polymerization, depolymerization and repolymerization when and where they are needed in the cell-this is referred to as dynamic instability
-the dynamic instability of microtubules stems from the intrinsic capacity of tubulin dimers to hydrolyze GTP to GDP
what is the graph of centrosomes?
fast microtubule nucleation at the centrosome in a human cell
what is microtubule assembly?
during assembly (growth) of MT:
1. each free tubulin dimer contains one GTP molecule tightly bound to beta-tubulin (beta-tubulin is a structural protein and a GTPase)
2. GTP-tubulin dimer is added to the plus end of MT
3. shortly after, GTP is hydrolyzed to GDP
4. the resulting GDP remains tightly bound to beta-tubulin
5. if tubulin dimers are added to the end of the MT faster than the GTP they carry is hydrolyzed, MT grow
6. this is because GTP-associated dimers bind more strongly to their neighbors in the microtubule than do dimers that bear GDP
what is the disassembly of microtubules?
during disassembly (shrinking) of MT:
1. occasionally, the tubulin dimers at the plus end of the microtubule hydrolyze their GTP before the addition of a new GTP-tubulin dimer
2. this means that the plus ends of protofilaments are now composed of GDP-tubulin
3. GDP-bearing dimers associate less tightly, tipping the balance in favor of disassembly
4. MT start to shrink rapidly and continuously and may even disappear (catastrophic disassembly)
5. GDP-tubulin dimer enters the soluble pool, the GDP is replaced by a new GTP and is ready to serve as a building block for polymerization
what are microtubule-associated proteins?
-microtubules are typically associated with additional proteins, called microtubule-associated proteins (MAPs)
-MAPs increase stability and promote microtubule assembly by facilitating MT nucleation (gamma-tubulin) and by linking tubulin subunits together
-MAPs also link MT to other filaments or cell structures such as the cell cortex
-an abnormally high level of tau phosphorylation disables its binding to microtubules which has been implicated in Alzheimer’s disease development
what are the steps of microtubule-associated proteins?
- nucleating
- branching
- linking to other filaments or cell structures
- polymerizing (depolymerizing)
- stabilizing
- severing
- motor proteins
how can microtubules act as agents of intracellular motility?
-MT provide means for directed movement of vesicles, organelles and molecules (RNAs, ribosomes, and cytoskeletal elements) within the cell
-defects in transport along microtubules can result in neurological diseases
along a nerve cell axon, directed movement relies on a highly organized arrangement of microtubules and other cytoskeletal components
-nerve cell’s axon is long
-can stretch from spinal cord to fingertip or toe
-directed movement is crucial for delivering neurotransmitters and other essential material in both directions
what are motor proteins?
motor proteins traverse the microtubular cytoskeleton
-motor proteins convert chemical energy (stored in ATP) into mechanical energy, to generate force to, for example, move cellular cargo
-microtubules and actin filaments serve as tracks for a variety of motor proteins that generate forces required to efficiently move objects within a cell
-move unidirectionally along their cytoskeletal track in a stepwise manner from one binding site to the next
what are the superfamilies of motor proteins?
motor proteins can be grouped into three broad superfamilies
-kinesin and dynein which move along microtubules
-myosin which moves along the actin filament
microtubule-associated motor proteins belong to two families:
- kinesins: move toward the plus end of a microtubule (outwardly towards cell periphery)
- cytoplasmic dyneins: move toward the minus end of a microtubule (inwardly towards the cell center)
what is the structure of kinesins and cytoplasmic dyneins?
-most kinesins and cytoplasmic dyneins are dimers that have two globular ATP-binding heads and a single tail
-the heads of kinesin and cytoplasmic dynein interact with microtubules in a stereospecific manner, so that the motor protein will attach to a microtubule in only one orientation
-the tail of a motor protein binds stably to vesicle or organelles
what is the hand-over-hand model
motor proteins traverse the microtubular cytoskeleton
-globular heads of kinesin and dynein are enzymes with ATP-hydrolyzing (ATPase) activity
-each motor protein requires the hydrolysis of a single ATP molecule/step
-a cycle of binding, release, and rebinding of motor heads along the microtubule drives motor protein movement
what is the graph of kinesins and cytoplasmic dynein?
different kinesins share similar head structure but differ in their tails, why?
what else do microtubules act as?
structural supports and organizers
-MT distribution helps determine the cell shape
-kinesins bind the endoplasmic reticulum (ER) and pulls it outwardly along MT
-cytoplasmic dyneins pulls the golgi apparatus inwardly along MT and towards the cell center (centrosomes)
disassembly of MT changes the ER and golgi localization and distribution
-ER collapse
-golgi dispersion
what is kinesin-mediated organelle transport?
-kinesins are force-generating agents that drive the movement of cargo and organelles (such as mitochondria)
-they tend to move vesicles and organelles in an outward direction toward the cell’s plasma membrane (towards the plus end of MT)
what are basal bodies?
a basal body is another type of MTOC that forms at the base of a cilium or a flagellum
basal bodies:
-are identical in structure to centrioles
-can turn into centrioles and vice verse
what is the structure of eukaryotic cilia and flagella?
cilia and flagella share a similar internal structure (axoneme)
axoneme:
-is the core of cilium/flagellum
-contains an array of microtubules that runs longitudinally through the entire organelle
-the axoneme of a motile cilium or flagellum consists of nine peripheral doublet microtubules surrounding a central pair of single microtubules (known as the 9 + 2 array)
what does each axoneme contain?
-peripheral doublets with an A tubule (complete microtubule) and a B tubule (incomplete microtubule)
-central tubules enclosed by the central sheath, connect to the A tubules by a set of radial spokes
-doublets are connected to one another by an interdoublet bridge
-inner arm and an outer dynein arms that project from the A tubules
what does the cilium or flagellum emerge from?
-a longitudinal section reveals the continuous nature of the microtubules and the discontinuous nature of the other elements
-a cilium or flagellum emerges from a basal body
-if a cilium or flagellum is sheared from the surface of a living cell, a new organelle is regenerated as an outgrowth of the basal body
what is the first step of the mechanism of ciliary and flagellar locomotion?
-dynein generates the bending motion of the cilium/flagellum
-the dynein anchored along one tubule (A) in a doublet attach its arms to binding sites on another tubule (B) in an adjacent doublet
-dynein undergoes a conformational change (power stroke) that causes the one doublet to slide against the other doublet
what is the second step of the mechanism of ciliary and flagellar locomotion?
-because of the multiple links that hold the adjacent microtubule doublets together, the sliding force between adjacent microtubules is converted to a bending motion
-other ciliary components control dynein activity leading to the complex wave seen in cilia and flagella
