Cytoskeleton II: Actin – motility & contractility; Intermediate filaments

 

Describe the general structure of an actin filament.

• Basic unit: actin monomers assembled head to tail

• Overall structure: 2 coiled filaments

What gives an actin filament its polarity, and why is this polarity important?

• Actin monomers have polarity

• This polarity means that the two ends of the actin filament will be different (different shape NOT charge)

• The different ends are called the plus end or minus end. Growth is fastest at the plus end.

 

Actin is an ATPase—how is that key to actin filament formation and dynamics?

• Actin is an ATPase – hydrolyses ATP to ADP (like a G-protein e.g. tubulin but uses ATP instead of GTP)

• Only ATP-bound actin can be added to plus end of actin filament

• When actin hydrolyses ATP to ADP, it is now ADP-bound actin. This weakens binding and can cause disassembly from minus end of actin filament.

Describe treadmilling of actin filaments.

• ATP-bound actin is added to plus end to grow actin filament

• Actin hydrolyses ATP to ADP – to become ADP-bound actin – weaker binding – lose actin from minus end of actin filament

• This process looks like a treadmill – actin filament looks like it is sliding through cell

Compare and contrast treadmilling of actin filaments with dynamic instability of microtubules.

Difference:

• The minus end of actin filaments is NOT anchored to a scaffold as actin filaments have no organizing center in contrast to microtubules where minus end is anchored to MTOC. 

Similarity:

• Both ATP-bound actin or GTP-bound tubulin is added to plus end of actin filament or microtubule, respectively, to grow and create strong binding.

• ADP-bound actin or GDP-bound tubulin (after ATP hydrolysis by actin or GTP hydrolysis by tubulin, respectively) weakens binding causing actin to be lost or microtubules to shorten

Describe the steps involved in cell crawling. How are actin filament assembly and motor protein activity used in this process?

1.      Actin branches push out membrane (actin filament grows)

2.      Maintains new position by forming contacts with the environment

3.      Contraction at rear of cell – requires myosin motor proteins to pull actin filaments forward

4.      Let go of “rear” attachments

Describe the general structure of an intermediate filament. How does this structure relate to polarity?

• Made up of diverse protein subunits

• Coiled dimers make staggered tetramers that ultimately make an intermediate filament

• Intermediate filaments have high tensile strength due to large number of non-covalent interactions

How does the structure of an intermediate filament relate to polarity?

• Intermediate filaments have NO POLARITY - two ends are not structurally different

• This is a major difference to the other cytoskeleton filaments - microtubules and actin filaments - which do have polarity

What primary function do intermediate filaments serve in animal tissues, particularly keratin in the epidermis (skin)?

• Function: support against mechanical stress

• Keratin has high tensile strength and provides strong structural support against mechanical stress to prevent the skin from breaking