Cytoskeleton I: Microtubules

 

What are the three types of cytoskeletal filaments in animals?

• Microtubules: made of tubulin

• Actin filaments: made of actin

• Intermediate filaments: made of various subunit proteins

Complete the cytoskeleton table over next two classes and use it to compare and contrast their structural and functional differences. PART 1

Complete the cytoskeleton table over next two classes and use it to compare and contrast their structural and functional differences. PART 2

Why is it advantageous for cells to assemble cytoskeletal filaments from small, compact protein subunits? Why is it important that they're held together by non-covalent interactions?

• Small protein subunits can respond quickly to cellular changes via rapid diffusion

• Non-covalent interactions allow the filaments to break apart quickly with little energy cost into small protein subunits and then allow quick reassembly into filaments at needed sites

Describe the general structure of a microtubule.

• Basic unit: tubulin dimers assembled head to tail. Dimers have polarity (2 parts different).

• Overall structure: hollow tube of multiple parallel protofilaments

What gives a microtubule its polarity, and why is this polarity important?

• Tubulin dimers have polarity (heterodimer made up of 2 different subunits).

• This polarity means that the two ends of the microtubule 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.

Why do cells need MTOCs to initiate the formation of new microtubules?

• Microtubules require a scaffold to grow from called a microtubule organizing center (MTOC)

• The minus end of a microtubule gets anchored at MTOC and then the plus end can grow out to form a new microtubule

• Centrosome is the main MTOC in animal cells while MTOC in specialized cells is called a basal body

Tubulin is a G-protein – how is that key to microtubule formation and dynamics?

• Tubulin has 2 conformations – 2 functions

• GTP-bound tubulin can only be added to growing microtubule at plus end.  Creates strong microtubule

• GDP-bound tubulin (tubulin itself hydrolyses GTP to GDP, GTPase) causes weaker binding, microtubule breaks apart

Describe dynamic instability. What role does a GTP cap play in cycles of catastrophe and rescue?

• Dynamic instability: repeated cycles of polymerization (growing microtubule) and depolymerization (breaking apart microtubule)

• GTP cap: GTP-bound to tubulin dimer at plus end of microtubule creates strong microtubule

• If GTP cap is lost (e.g. GTP hydrolysis to GDP) and now there is GDP-bound tubulin dimer then microtubule will be weak and undergo rapid shortening (depolymerization) = catastrophe

• If GTP cap is regained (add GTP-bound tubulin on plus end) then microtubule can grow again (polymerization) = rescue

Why does the endomembrane system require microtubules?

• Microtubules are required to transport vesicles through the endomembrane system

• Proteins made in ER are transported via vesicles to Golgi and then to plasma membrane using microtubules and motor proteins that go in a specific direction

• Also microtubules are important for shape and positioning of ER and Golgi

If a defect prevented tubulin dimers from forming:

A) Intermediate filaments would not form

B) Plasma membrane proteins would not be produced

C) Actin filaments would not form

D) Nuclear proteins would not be produced

E) Two of these are true

Answer: B) Plasma membrane proteins would not be produced

• Plasma membrane proteins are made in the endomembrane system which require microtubules (subunit: tubulin dimers) to transport vesicles (containing plasma membrane protein) through endomembrane system (from rough ER to golgi to plasma membrane)