Textbook Chapter 9: Microfilaments Flashcards
Actin filaments
- solid, thinner structures, often organized into a branching network.
cytoskeleton (3)
Composed of+each filament is….+construction
- composed of three well-defined filamentous structures—microtubules (MT), actin filaments, and intermediate filaments (IFs)—that together form an elaborate interactive and dynamic network.
- Each of the three types of cytoskeletal filaments is a polymer of protein subunits held together by weak, noncovalent bonds.
- This type of construction lends itself to rapid assembly and disassembly, which is dependent on complex cellular regulation
Microtubules (5)
The physical description+found in+Role(2)+consist of+intramolecular interaction
- long, hollow, unbranched tubes composed of subunits of the protein tubulin
- found in the cytoskeleton, the mitotic spindle, centrioles, and the core of cilia and flagella
- Roles in cell support and movement of materials within a cell
- microtubules are seen to consist of 13 protofilaments aligned side by side to form a tubel.
- Noncovalent interactions between adjacent protofilaments are thought to play an important role in maintaining microtubule structure.
Intermediate filaments
- tough, ropelike fibers composed of a variety of related proteins.
Five function of cytoskeleton:
- Structural support (cell shape, resisting forces)
- Transport of materials and organelles within a cell
- Contraction and motility
- Spatial organization (hold secretory lysosome ontop)
- Cell division
Each protofilament is assembled from dimeric building blocks consisting of one —— and one —– subunit. The two types of globular tubulin subunits have a similar threedimensional structure and fit tightly together. The tubulin dimers are organized in ———. Because each assembly unit contains two nonidentical components (a heterodimer), the protofilament is asymmetric. All of the protofilaments of a microtubule have ——. Consequently, the entire polymer has —–. One end of a microtubule is known as the plus end and is terminated by a row of ——–. The opposite end is the minus end and is terminated by a row of ——-.
- α-tubulin
- β-tubulin
- in a linear array along the length of each protofilament
- the same polarity
- polarity
- β-tubulin subunits
- α-tubulin subunits
Microtubule-associated proteins (or MAPs) (2)
What they do+ controlled by
- increase the stability of microtubules and promote their assembly by linking tubulin subunits together, making it harder for them to fall apart.
- activity of some MAPs is controlled by the addition and removal of phosphate groups from particular amino acid residues
Structures and materials traveling from the cell body toward the terminals of a neuron are said to move in ——- direction
an anterograde direction
Microtubules as structural supports (3):
What+ help determine+maintains
- Microtubules provide mecanical support: Are stiff enough to resist compression or bending forces
- Help determine the shape of a cell
- Maintains intracellular location of organelles
Microtubules as agents of intracellular motility (2)
Two types of stuff it transports
- Transport of membrane vesicles from one membrane compartment to another
- Transport non membrane bound cargos (RNAs, ribosomes, cytoskeletal elements)
How do the motor proteins of a cell generate force?
convert chemical energy (stored in ATP) into mechanical energy, which is used to generate force, as occurs when a muscle cell contracts, or to move cellular cargo attached to the motor
Collectively, motor proteins can be grouped into three
broad superfamilies:
kinesins, dyneins, and myosins
Kinesins and dyneins move along ——, whereas myosins move along —–
- microtubules
- actin filaments
No motor protein is known that uses intermediate filament tracks. This is not surprising considering that intermediate filaments are ……
not polarized and thus would not provide directional cues to the motor.
Kinesins (4)
Family+constructed from+head+tail
- kinesin will refer only to members of the kinesin-1 family
- Each kinesin-1 molecule is a tetramer constructed from two identical heavy chains and two identical light chains
- a pair of globular heads that bind a microtubule and act as ATP-hydrolyzing, force-generating “engines.”
- Each head (or motor domain) is connected to a neck, a rodlike stalk, and a fanshaped tail that binds cargo to be hauled
The motor portions of all KRPs have related amino acid sequences, reflecting their common evolutionary ancestry and their similar role in moving along microtubules. In contrast, the tails of KRPs have ——, reflecting the ——-. A number of different proteins
have been identified as potential adaptors that link specific KRPs and their cargoes. Surprisingly, the motor domains of kinesins are strikingly similar in structure to those of myosins, despite the fact that kinesins are much smaller proteins and the two types of motors operate over different tracks. Kinesins and myosins almost certainly evolved from a common ancestral protein present in a primitive eukaryotic cell.
- diverse sequences
- variety of cargo these motors haul
In an axon, where all of the microtubules are oriented with their minus ends facing the cell body, kinesin transports vesicles and other cargo toward —–
the synaptic terminals
A single kinesin molecule moves along a single —– of a microtubule at a velocity ——
- protofilament
- proportional to the ATP concentration
Each step of the kinesin molecule is approximately —- in length, which is also the length of ——-, and requires the hydrolysis of —– ATP molecule
- 8 nm
- one tubulin dimer in a protofilament
- a single
The movement of kinesin molecules, both in vitro and in vivo, is highly processive, meaning that——. This is because……
- the motor protein tends to move along an individual microtubule for considerable distances (over 1 µm) without falling off.
- A two-headed kinesin molecule can accomplish this
feat because at least one of the heads is attached to the microtubule at all times
Explain the kinesin movement:
- Leading head binds one ATP: Hydrolysis and release of ADP + Pi = powerstroke that swings trailing head forward
members of the kinesin superfamily tend to move vesicles and organelles (e.g., peroxisomes and mitochondria) in an outward direction toward the cell’s —–
plasma membrane
Dynein (6)
what it is responsible for+ composed of chains+structure+head function+stalk+projection
- the protein responsible for the movement of cilia and flagella
- a huge protein composed of two identical heavy chains and a variety of intermediate and light chains
- Each dynein heavy chain consists of a large globular head with an elongated projection (stalk)
- The dynein head, which is larger than a kinesin head, acts as a force-generating engine.
- Each stalk contains the all-important microtubule-binding site situated at its tip.
- The longer projection, known as the stem (or tail), binds the intermediate and light chains
Dyein moves towards
- moving toward the minus end of the track
Both motor proteins are attached to the vesicle membrane by an intermediary:
- kinesin can be attached to vesicles by a variety of integral and peripheral membrane proteins
- dynein by a protein complex called dynactin
The number of protofilaments in the microtubule is apparently dictated by —–. In this model, only the —–tubulin of the heterodimer can bind to a ring of γ-tubulin subunits. Thus, the—- determines the polarity of the entire microtubule and also forms a cap at its minus end, preventing the gain or loss of tubulin subunits.
- the number of γ-tubulins around the ring
- α
- γ-TuRC (Tubulin ring complex)
How are microtubule initiated?
- γ-TuRC is a helical array of γ-tubulin (brown)
subunits, each bound with a set of accessory proteins. This provides a place for microtubules to assemble and speeds it up.
The core of the cilium, called the —–, contains an array of ——- through the
entire organelle.
- axoneme
- microtubules that run longitudinally
Structure of the axoneme (2)
Structural+dynein
- 9 peripheral doublet microtubules around a central pair of single microtubules (9+2 array)
- dynein tails are anchored to one of the tubules in each pair (A tubules, dark blue)
