Intermediate filaments

Intermediate filaments are important components of the cell's cytoskeletal system. They may stabilize organelles, like the nucleus, or they may be involved in specialized junctions. They are distinguished from "thin filaments" by their size (8-10 nm) and the fact that thin filaments are obviously motile. However recent evidence indicates that Intermediate Filaments may also have dynamic properties. We will be reading a paper this week that shows examples of these. Below is a menu that shows how this topic fits into the overall cytoskeletal system unit.  

This figure shows the rope-like characteristics of IFs.  Note that there is some organizational pattern.

You can also see organization in the cartoon. By the end of the unit, you will be able to identify the regions in this pattern:




Test yourself:  How much do you know about Intermediate filaments? 

In the cartoon, note the concentration of filaments at cell junctions or at the base. Note the filaments around the nucleus.  What is the significance of these sites?

Read Lodish et al, pp 836-845; 969-979

Answer the following questions:

bullet1. Intermediate filaments are one of three components of the cytoskeletal system. Name all three.
bullet2. What are basic characteristics of Intermediate Filaments?
bullet3. Name the five major types of Intermediate filaments and give examples of tissues that contain each type.
bullet4. Describe the basic structure of an Intermediate filament monomer
bullet5. Describe the formation of an Intermediate filament protofilament.  How do these organize to form the fibrils.
bullet6. How could you regulate the assembly (or disassembly) of intermediate filaments?
bullet7. If you added protofilaments of an intermediate filament to the cytoplasm, could they be added to the existing cytoskeleton?  How could you prove this?
bullet8. Do Intermediate filaments associate with other proteins?  What are some functions associated with this?
bullet9. Where are lamins found?  How are they unique?
bullet10. What might happen if you added antibodies to lamins during metaphase or prophase?
bullet11. A number of different forms of keratins are found?  How is this useful for tumor diagnoses? 
bullet12. What types of junctions are formed with keratin or desmin as the intermediate filaments? Describe the components and structure of each junction. 
bullet13. Patients who make antibodies to cadherins found in epithelial cells of skin have defects in what type of junction?
bullet14. Types III and IV Intermediate filaments are found in what types of tissue?  What extra proteins are found that add strength to the neuro filaments?

  Read the following article for Thursday's class:

Han Yoon, Kyeong, Yoon M, Moir, R.D., Khuon S, Flitney F W, and Goldman R.D. 2001 Insights into the dynamic properties of keratin intermediate filaments in living epithelial cells. J Cell Biology 153: 502-516

Discussion questions:

    1) What problem were they trying to solve?
    2) What was their hypothesis?
    3) Describe "Fluorescence recovery after photobleaching" (FRAP) techniques
    4) Why did fewer cells transfected with homodimers of keratin only make cytoskeletal elements than those transfected with heterodimers of keratin?
    5) When GFP-keratins and YFP-vimentins were used to transfect the cells, the pattern of fluorescence was different.  What does this signify? Discuss Figure 1.
    6) Compare and contrast FRAP analysis of GFP and YFP-tagged keratin or vimentin (Discuss Figures 2 and quantification in Figure 4)
    7) How does the FRAP analysis give an estimate of turnover rates in different cell types?
    8) Are there differences in tonofibril uptake of the keratin protofilaments with the size of the tonofibril? (Discuss Figure 3).
    9) Discuss the movement of vimentin and keratin squiggles (Figure 5).  Do they require energy for movement?  What experiments were done to test this?
    10) Was the movement of tonofibrils (keratin fibrils) and squiggles dependent on thin filaments (microfilaments)?  How did they test this? (Figure 6)
   11)  Was the movement of tonofibrils and squiggles dependent on microtubules? (Figure 7) How was this tested?
   12) Why did they test for dynein involvement? (Figure 8).
   13) Figure 9 shows a possible mechanism behind the movement of tonofibrils. What is it and how did they test it? What is the effect of shearing stress on these intermediate filaments (Figure 10).

Last updated: 04/02/03
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Gwen V. Childs, Ph.D.
text copyright 2001 Gwen V. Childs, Ph.D.