Posts Tagged ‘microtubes’

Lesson 7 of Cell Signaling 7 Motility: Tubulin and Tutorial Quizes for #TUBiol3373

Stephen J. Williams, Ph.D.

This lesson (lesson 7) will discuss the last type of cytoskeletal structure: microtubules and tubulin.  In addition I want to go over the last quiz answers and also introduce some new poll quizes.

I had given the lecture 7 over Canvas and each of you can download and go over the lecture but I will highlight a few slides in the lecture.

Let’s first review:

Remember that microtubules are the largest of the three cytoskeletal structures:

actin microfilaments < intermediate filaments < microtubules

This is very important to understand as the microtubules, as shown later, shuttle organelles and cellular structures like synaptic vesicles, as well as forming the centrisome and spindle fibers of mitosis.














Now remember the quiz question from last time

Remember that actin monomers (the G actin binds ATP)  while tubulin, the protein which makes up the microtubules binds GTP {although it is a little more complex than that as the following diagram shows}













































See how the growth at the plus end is dependent on tubulin heterodimer GTP while when GDP is only bound to tubulin (both forms) you get a destabilization of the plus end and removal of tubulin dimers (catastrophe) if there is no source of tubulin GTP dimers (alpha tubulin GTP with beta tubulin GTP).





Also remember that like actin microfilaments you can have treadmilling (the plus end  continues growing while minus end undergoes catasrophe).  The VIDEO below describes these processes:




Certain SNPs and mutants of tubulin are found and can result in drastic phenotypic changes in microtubule stability and structure. Below is an article where a mutation in tubulin can result in microtubule catastrophe or destabilization of microtubule structures.


From: A mutation uncouples the tubulin conformational and GTPase cycles, revealing allosteric control of microtubule dynamics;, E.A. Geyer et al..; elife 2015;4:e10113


Microtubule dynamic instability depends on the GTPase activity of the polymerizing αβ-tubulin subunits, which cycle through at least three distinct conformations as they move into and out of microtubules. How this conformational cycle contributes to microtubule growing, shrinking, and switching remains unknown. Here, we report that a buried mutation in αβ-tubulin yields microtubules with dramatically reduced shrinking rate and catastrophe frequency. The mutation causes these effects by suppressing a conformational change that normally occurs in response to GTP hydrolysis in the lattice, without detectably changing the conformation of unpolymerized αβ-tubulin. Thus, the mutation weakens the coupling between the conformational and GTPase cycles of αβ-tubulin. By showing that the mutation predominantly affects post-GTPase conformational and dynamic properties of microtubules, our data reveal that the strength of the allosteric response to GDP in the lattice dictates the frequency of catastrophe and the severity of rapid shrinking.



Remember the term allosterism: change in the affinity for binding of a ligand or substrate that is caused by the binding of another ligand away from the active site (for example like 2,3 DPG effect on oxygen binding to hemoglobin


Cellular transport of organelles and vesicles: a function of microtubules


















Now the above figure (figure 9 in your Powerpoint) shows the movement of organelles and vesicles in two different types of cells along microtubules.

Note the magenta arrow which goes from the nucleus toward the plus end of the microtubule (at cell membrane) is referred to as anterograde transport and is movement away from center of cell to the periphery.  Retrograde transport is movement of organelles and vesicles from periphery of cell to the center of the cell.

Note that kinesin is involved in anterograde transport while dyenin is involved in retrograde transport

Also refer to the Wiki page which shows a nice cartoon of this walking down a microtubule on the right hand side of the page









Cilia; a cellular structure of microtubules (we will talk about cilia later)

for more information on structure of Cillia please see https://www.ncbi.nlm.nih.gov/books/NBK21698/

This is from a posting by Dr. Larry Bernstein of Yale University at https://pharmaceuticalintelligence.com/2015/11/04/cilia-and-tubulin/








Defective cilia can lead to a host of diseases and conditions in the human body, from rare, inherited bone malformations to blindness, male infertility, kidney disease and obesity. It is known that these tiny cell organelles become deformed and cause these diseases because of a problem related to their assembly, which requires the translocation of vast quantities of the vital cell protein tubulin. What they didn’t know was how tubulin and another cell organelle known as flagella fit into the process.

Now, a new study from University of Georgia shows the mechanism behind tubulin transport and its assembly into cilia, including the first video imagery of the process. The study was published in the Journal of Cell Biology.

Cilia are found throughout the body, so defects in cilia formation affect cells that line airways, brain ventricles or the reproductive track.  One of the main causes of male infertility is the cilia won’t function properly.

The team used total internal reflection fluorescence microscopy to analyze moving protein particles inside the cilia of Chlamydomonas reinhardtii, a green alga widely used as a model for cilia analysis.

The team exploited the natural behaviour of the organism, which is to attach by its cilia to a smooth surface, such as a microscope glass cover. This positions the cilia within the 200-nanometer reach of the total internal reflection fluorescence microscope allowing for the imaging of individual proteins as they move inside the cilia.  A video explaining the process was published along with the study.

Tubulin is transported by this process called intraflagellar transport, or IFT.  Though it has long been suspected in the field and there was indirect evidence to support the theory, this is the first time it has been shown directly, through live imaging, that IFT does function as a tubulin pump.  The team observed that about 400,000 tubulin dimers need to be transported within 60 minutes to assemble a single cilium. Being able to see tubulin moving into cilia allowed for first insights into how this transport is regulated to make sure cilia will have the correct size.

The new findings are expected to have wide implications for a variety of diseases and conditions related to cilia defects in the body.  The team state that they are on the very basic side of this research.  But because more and more diseases are being connected to cilia-related conditions, including obesity and even diabetes, the number of people working on cilia has greatly expanded over the last few years.


So here are the answer to last weeks polls

  1. Actin filaments are the SMALLEST of the cytoskeletal structures.  As shown in this lecture it is tubulin that binds GTP.  Actin binds ATP.
  2.  ARP2/3 or actin related proteins 2 and 3 are nucleating proteins that assist in initiating growth of branched chain micofiliment networks.  Formins are associated with unbranched actin formations.
  3.  The answer is GAPs or GTPase activating proteins.  Remember RAS in active state when GTP is bound and when you hydrolyze the GTP to GDP Ras is inactive state






4.  Okay so I did a type here but the best answer was acetylcholinesterase (AchE) degrading acetylcholine.  Acetylcholinesterase degrades the neurotransmitter acetylcholine into choline and acetate not as I accidentally put into acetylCoA.  The freed choline can then be taken back up into the presynaptic neuron and then, with a new acetyl group (with Coenzyme A) will form acetylcholine.


Synthesis of the neurotransmitter acetylcholine




The neuromuscular junction





















Thanks to all who took the quiz.  Remember it is for your benefit.





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Silk Biomaterials Produced from 3D Bone Marrow Generate Platelets

Reported by: Irina Robu, PhD

The team used silk protein scaffolds that silk is a very biocompatible material that is amenable to many manipulations to customize it for a specific use, while also avoiding any cell-specific signaling. They formed silk scaffolds with thickness ranging from 2 to 5 micrometers and stiffness combined with growth factors, to test the success of megakaryocyte adhesion and the formation of pro-platelets—the parts of the megakaryocytes that fragment into platelets.  After determining the best combination of scaffolds with appropriate thickness and stiffness, the researchers attached the silk scaffolds to a plastic framework to guide the growth of cells. The next step is to grow endothelial primary cells on one side of the silk scaffold and megakaryocytes on the other side, partly because endothelial primary cells are known to secrete growth factors that help megakaryocytes mature.

In order to mimic the microvasculature and environment, the  researchers form silk sponges around the porous microtubes. The culture media with necessary nutrients is being pumped to mimic the flow of blood which leads to higher numbers of platelets generated than was previously possible; and most importantly, the platelets were functional.This is the first time researchers were able to create the complete micro environment where platelets are formed.


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