Chapter 16- The Cytoskeleton Flashcards
Cytoskeleton
A system of protein filaments- basically acts like a skeleton for the cell. It gives the cell shape and integrity, supporting the membrane and helping the cell to withstand external forces. Proteins in the membrane are often bound to the cytoskeleton, anchoring the membrane in place. Rearrangements of the cytoskeleton allows the cell to change shape and facilitate movement (leukocyte crawling, phagocytosis, and muscle contraction). The cytoskeleton guides cell growth and is responsible for the cell’s spatial arrangement and movement of organelles or vesicles throughout the cell
3 filaments making up the cytoskeleton in animals
- Intermediate filaments
- Microtubules
- Actin filaments
Intermediate filaments
Provide mechanical strength
Microtubules
Position organelles and direct intracellular transport. They act as “tracks” for organelles or vesicles that are moving in the cell.
Actin filaments
Determine the shape of cell surface- actin is the filament that is positioned just under the membrane, in the cytoplasm. Therefore, rearrangements of actin will help to change the shape at the cell surface. Necessary for whole cell locomotion
Accessory proteins
These proteins are often associated with cytoskeleton filaments. They link cytoskeleton filaments to cell components and to each other. Accessory proteins are also responsible for the controlled assembly of filaments and bundling them together. Includes motor proteins, which are important for moving things like organelles and vesicles along microtubule tracks (a cytoskeleton filament).
Types of subunits
Filaments are formed from subunits. Actin and microtubules (tubulin) subunits are compact, globular (round), and soluble, and they come together to form larger filamentous polymers. Intermediate subunits are fibrous and come together to form an even larger fibrous structure.
Assembly of subunits
All 3 types of subunits will come together in helical assemblies. The subunits self-associate and form side to side and end to end contacts, forming the length of the filament. Differences in subunit structures and the strength of contacts create differences in the stability and mechanical properties of each type of filament
Subunit linkage
The linkage of subunits to form filaments is non-covalent, in contrast to other polymers like DNA, RNA, and proteins. However, these non-covalent linkages allow for rapid assembly and disassembly of subunits without the need to break covalent bonds- disrupting covalent bonds can be difficult. Accessory proteins regulate the building and dynamic behavior of filaments, responding to extracellular/intracellular signals. Filaments are constantly growing and shrinking.
Protofilaments
Subunits assemble end to end into long filament assemblies called protofilaments. The protofilaments will then associate laterally (side to side) to form a mature filament
Filament nucleus
Forms if enough subunits are present to form a large aggregate of the filament. Rapid filament elongation ensues
Nucleation
The initial rate of nucleus filament assembly, when the filament is growing in size
FtsZ
A bacterial tubulin (microtubule homolog) that also acts as a structural protein. It forms filaments that assemble into a circular Z-ring structure, which is important for forming a septum during cell division (binary fission)
MreB & Mbl
Bacterial homologs to actin in animal cells. Form dynamic patches that move along the length of the cell. Their function is unclear, but this is part of what gives B. subtilis structure. If the proteins are removed, bacteria will start to clump together and will not have a proper structure
ParM
A bacterial actin homolog. It binds to ParR proteins at the origin of replication in DNA- it helps to separate the circular bacterial chromosomes (original and replicated) to opposite parts of the cell prior to cell division
TubZ
A bacterial tubulin homolog. It serves the same function as ParM in other bacterial cells. TubZ is found in some bacterial species while ParM is found in other bacterial species
Crescentin
A bacterial intermediate filament homolog. Found in the species Caulobacter crescentus. Filamentous, provides strength & shape. Crescentin also gives these species a crescent shape
Structure of tubulin subunits
Heterodimer (made of α and β-tubulin). The heterodimer associates through non-covalent bonding. The α and β-tubulin each have a bound GTP molecule. Especially on the β subunit, GTP is important for the filament dynamics. It allows the filaments to grow and fall apart consistently
Structure of tubulin filaments
Consists of multiple heterodimers (α and β subunits) that associate end to end. The chain of numerous heterodimers builds to form a protofilament. However, there are lateral interactions between α-α domains and β-β domains that helps to form the mature filament. The mature filament contains 13 parallel protofilaments which are arranged around a hollow cylinder. The β-tubulin has an “up” position with the α-tubulin below it, giving the filaments a structural polarity
Structure of actin filaments
Actin monomers are true globular monomers, with an ATP binding site. Therefore, the monomers are bound to an ATP molecule. Actin protofilaments form when numerous actin monomers come together end to end. A mature filament is 2 parallel protofilaments that form a right handed helix. Actin filaments also have a polarity (directionality), with a minus and plus end. These filaments are flexible and more easily bent compared to microtubules. Many filaments are bundled and crosslinked together by accessory proteins, making them much stronger than individual filaments
Polarity of filaments
Actin and microtubules have a polarity due to their arrangement, they both have a plus end and minus end. The plus end is more dynamic and is where growth and shrinking can occur quickly. The minus end is more fully assembled, so growth and shrinkage occurs at a much slower rate. When filaments are referred to as growing or shrinking, that is always occurring at the plus end
Filament ATP/GTP hydrolysis
Tubulin and actin subunits catalyze GTP and ATP hydrolysis. Free energy released by hydrolysis stored in polymer lattice. Less free energy is needed for subunits to dissociate. There are 2 binding forms- T form (bound to ATP or GTP) or D form (ADP or GDP). The T form is more likely to grow- when GTP or ATP are bound to the plus end, the filament will be more likely to grow in size. The D form occurs when ATP or GTP is hydrolyzed (to ADP or GDP). It is more likely to shrink, and a higher concentration of monomers is needed for the D form to grow. There will also be an ATP or GTP cap on the growing plus end of the filament
Filament treadmilling
An intermediate monomer concentration occurs if higher than critical concentration for T form but lower than the critical concentration for D form. In this situation, filament treadmilling occurs. Subunits are recruited to plus end (T form) and shed from minus end (D form) in a balanced manner, so the filament is moving along like a treadmill. ATP/GTP hydrolysis must occur if there are D forms in the filament
Dynamic instability
Also called catastrophe and rescue. The idea that when ATP or GTP is at the plus end, we are growing the filament, but the filament is falling apart if ADP or GDP is at the plus end, and therefore the filament shrinks. The loss of the T form results in shrinking, gaining the T form results in growing- this cycle repeats over and over. The filaments are dynamic and are constantly growing and shrinking due to if ATP/GTP are present or hydrolyzed
Filament dynamics
Dynamic instability and treadmilling are common events in the cell. Microtubules switch between growth and shrinkage every few minutes. Likewise, actin filaments have rapid turnover. Individual filaments persist from a few tens of seconds to a few minutes
Where are intermediate filaments found?
All eukaryotic cells have actin filaments and microtubules. However, Intermediate filaments are found only in some metazoans- vertebrates, nematodes, and mollusks. These filaments also are not present in every cell. Oligodendrocytes (make myelin of CNS) lack intermediate filaments. They are more prevalent in cells subjected to mechanical stress, like epithelial cells. Similar to nuclear lamins in that they are structurally related
Formation of intermediate filaments (5 steps)
- Two α helical monomers come together and coil around one another. That forms a coiled-coil dimer
- 2 dimers associate in an antiparallel fashion, forming a staggered tetramer. Many tetramers come together to form an intermediate filament
- Two tetramers pack laterally to form a protofilament
- 8 parallel protofilaments are twisted into a ropelike filament
- In a mature intermediate filament, there are 32 individual α coils
Structure of intermediate filaments
In contrast to the other filaments, intermediate filament monomers are fibrous (not globular). Intermediate filaments are just extended α helical monomers. 2 coiled- coil dimers form a staggered tetramer. This tetramer is a subunit analogous to the tubulin α and beta subunits and actin subunits. Many tetramers come together to form an intermediate filament. There is no binding site for nucleoside triphosphate (no ATP or GTP will be bound). The 2 dimers point in opposite directions, so there is no polarity.
Intermediate filament dynamics
Intermediate filaments are easily bent and difficult to break. They can be dynamic and are probably regulated by phosphorylation, like lamins. When phosphorylated, lamins come apart, when they are dephosphorylated, lamins come back together. This is likely how intermediate filaments work
Types of intermediate filaments (4)
Actin and tubulin are mostly the same cell to cell, but there are many types of intermediate filaments.
1. Keratins
2. Neurofilaments
3. Nuclear lamins
4. Vimentin-like- Myocytes, astrocytes, Schwann cells (myelin for the PNS)
