Cytoskeleton #1

Exam #3

What are the 3 basic types of cytoskeletal elements?

1. Microtubules (MT)
2. Actin filaments
3. Intermediate filaments (IFs)

Microtubules (MT) structure of the cytoskeleton

 Long, hollow, unbranched tubes made of subunits of the protein tubulin.

Actin filaments structure of the cytoskeleton

Solid, thinner structures, are often organized into a branching network.

Intermediate filaments (IFs) structure of the cytoskeleton

Tough, ropelike fibers made of a variety of related proteins

5 Functions of the cytoskeletal elements

* Support
* Positioning
* Movement of materials and organelles within cells
* Anchor mRNA/translation
* Cell movement of the cell itself

Microtubule structure

1. The outer diameter of 25 nm and a wall thickness of 4nm and may extend across the length of a cell.
2. Microtubules have 13 protofilaments side by side in a circular pattern within the wall with noncovalent interaction between the protofilaments which are important for the structure.
3. ==Overall structure:==


1. Alpha, B tubulin, GTP, GDP
2. 13 protofilaments in the ring (picture)
* Alpha and beta come together to form each of the 13

Microtubule function: what are 3 things that microtubules do? (S. A. C.)

1. Stability
2. Assembly
3. Crosslinking

Microtubule structure: arrangement

The microtubule wall is made of globular proteins __arranged__ in longitudinal rows called __protofilaments__ that are aligned parallel to the long axis of the tubule.

Microtubule structure: protein subunits

* Each protofilament is assembled from dimeric __building blocks made of one α-tubulin and one β-tubulin subunit__.
* The __2 kinds of globular subunits__ have similar 3D structure and fit together.
* The tubulin dimers are organized in a linear array along the length of each protofilament.
* Since each assembly unit has 2 nonidentical components __(heterodimer)__, the protofilament is asymmetric with an __α-tubulin at one end and a  β-tubulin at the other end.__

What is the difference between the plus and minus end of a microtubule in terms of structure and function?

* The entire polymer has polarity because all of the protofilaments of a microtubule have the same polarity.
* One end of a microtubule is known as the __plus end__ and is terminated by a row of __β-tubulin subunits__.
* The opposite end is the __minus end__ and is terminated by a row of __α-tubulin subunits__.

What is the function of MAPs (Microtubule-Associated Proteins)?

* __MAPs generally increase the stability of microtubules and promote their assembly by linking tubulin subunits together, making it harder for them to fall apart.__
* The microtubule-binding activity of some MAPs is controlled by the addition and removal of phosphate groups from particular amino acid residues. An abnormally high level of phosphorylation of one particular MAP, called tau, has been implicated in neurodegenerative disorders like Alzheimer’s disease.

Discuss how the arrangement of microtubules is different in round, flattened cells in culture

* The distribution of cytoplasmic microtubules in a cell helps determine the shape of that cell
* In cultured animal cells, the $microtubules extend in a radial array outward from the area around the nucleus$, giving these cells their __round, flattened shape__.
* Structural support

Discuss how the arrangement of microtubules is different in tall columnar epithelial cells

* The distribution of cytoplasmic microtubules in a cell helps determine the shape of that cell.
* In contrast, the microtubules of __columnar epithelial cells__ are typically oriented with their __long__ axis parallel to the long axis of the cell. This configuration suggests that microtubules help support the cell’s __elongated shape.__
* Structural support

What are motor proteins?

The motor proteins of a cell ==convert chemical energy (stored in ATP) into mechanical energy== which is used to general force, as occurs when a muscle cell contracts or to move cellular cargo attached to the motor.

What are 3 basic kinds of motor proteins?

1. Kinesins


1. Work on microtubules
2. Dyneins


1. Work on microtubules
3. Myosins


1. Work on actin filaments; myosin it is a motor protein, but not necessary cytoskeletal, __but force-generating working on actin__

Where do the 3 basic kinds of motor proteins act?

1. Kinesins and dyneins move along microtubules.
2. Myosins move along actin filaments.
3. No motor protein uses intermediate filament tracks because intermediate filaments are not polarized and wouldn’t provide directional cues to the motor.

Kinesin structure

* A plus end-directed motor protein that moves membranous vesicles and other organelles along microtubules through the cytoplasm.
* Head, neck, stalk, light chain around tail portion, dimer of the heavy chain forming a helical structure

Kinesin function and how does it accomplish this? How is specificity of action accomplished?

* It’s function is to bind to a microtubule and act as ATP-hydrolyzing.
* The motor proteins of all KRPs have related amino acid sequences, reflecting their common evolutionary ancestry and their similar role in moving along microtubules.
* Function/mechanism
* The head attaches to a microtubules and cargo attaches at the other end; carries it along the microtubule
* Specificity
* There's more than one kinesin with different functionality

Cytoplasmic dynein structure

* A really large protein with 2 identical heavy chains and a variety of intermediate and light chains, a large globular head with an elongated projection stalk.
* Don’t really need to know the subparts, but NOTICE:
* There are stalks where the microtubules are
* 2 Motor heads
* How it generates force is a little different; generates force towards the minus end

Cytoplasmic dynein function

* Its functions in the movement of chromosomes during mitosis and also as a minus-end directed microtubule motor for the movement of vesicles and membranous organelles through the cytoplasm
* Function
* Part recognizes tubule and other part associated with cargo…

What is a MTOC?

* Microtubule Organizing Centers
* A variety of specialized structures that exert a role in initiating microtubule formation.

Describe the structure of centrosomes and basal bodies

* Centrosomes
* 2 Barrel-shaped centrioles surrounded by amorphous, electron-dense pericentriolar material. The specific molecule called B-tubulin helps MTOCs nucleate microtubules.
* 9 Triplets that go around the pair
* Microtubule growth at centrosomes after nocodazole treatment
* Basal bodies

What specific molecules help MTOCs nucleate microtubules?

* How does it occur?
* Alpha beta dimers connect on to the ==gamma-tubulin== and grow out of it: nucleation
* At what end of the microtubule to they grow?
* They grow out the plus end; plus end growth

Compare the stability of microtubules in dividing cells, neurons, and cilia

* __Microtubules__ of mature __neurons are much less labile__, and those of __centrioles, cilia, and flagella, are highly stable__.
* These __differences in microtubule stability are determined by__ microtubule-interacting proteins including MAPs (figure 9.4), which stabilize microtubules; __proteins known as +TIPs__, which bind to the plus end of growing microtubules; __and an enzyme called katanin__ (named after the samurai sword) which severs microtubules into shorter pieces
* ==What regulates stability:==
* MAPs
* +TIPs
* Katanin (microtubule severing enzyme)
* Post-translational tubulin modifications

How can the cell control the stability of microtubules?

Microtubule __stability is regulated by__ posttranslational modifications to the tubulin subunits like the covalent attachment of multiple glutamates on the C-terminus of tubulin.

Describe the assembly of microtubules

* The assembly of microtubules of aB-tubulin dimers occurs in two distinct phases
* __Slow phase of nucleation__
* __Rapid phase of elongations__
* Process
* Alpha/beta dimers (have GTP associated with them) come in onto the plus end (adds on to plus end)- step 1
* Role of GTP
* GTP is converted to GDP
* Which end is growing?
* As long as their is microtubulin at the end, it will be
* As it closes, it is not as stable due to the strain; undergoes catastrophe and microtubule starts falling apart
* MICROTUBULES ARE GROWING AND SHORTENING (REPEAT)

Describe the disassembly of microtubules

* __Living cells can be subjected to a variety of artificial treatments__ that lead to the __disassembly of labile cytoskeletal microtubules without disrupting other cellular structures__.
* Disassembly can be __induced by:__ cold temperatures; hydrostatic pressure; elevated Ca2+ concentration; and a variety of chemicals (colchicine, vinblastine, vincristine, and nocodazole)
* The drug taxol stops the dynamic activities of microtubules by a very different mechanism