Question

a.)Do you agree or disagree with this statement? Explain your answer.

• Minus-end directed microtubule motors (like dyneins) deliver their cargo to the periphery of the cell, whereas plus-end directed microtubule motors (like kinesins) deliver their cargo to the interior of the cell.

b.)• There are no known motors that move on intermediate filaments. Propose an explanation for this.

c.)• Cytochalasin is a drug that caps actin filament plus ends, thus preventing actin polymerization. Phalloidin is a drug that binds to and stabilizes actin filaments, preventing actin depolymerization. Even though these drugs have opposite effects on actin polymerization, the addition of either of these drugs instantaneously freezes the cell movements that depends on actin filaments. Explain why drugs that have opposite effects on actin filaments can have a similar effect on cell movements.

Answer 1

(a) Disagree

The plus end of the microtubules are towards the plasma membrane, i.e, periphery and the minus end is towards the centrosome. This is because the centrosome nucleates the microtubules at the minus end lying towards the center of the cell.

Thus, plus ended microtubules transfers cargo towards the periphery and minus ended microtubules towards the center.

(b) Intermediate filaments do not have a fixed polarity and the two ends are not distinguishable chemically. So, a motor protein bound at the center of an intermediate filament cannot define a specific direction towards which to move.

(c) The actin filaments starts to grow (polymerises) near to the cell membrane and pushes it outwards such that the cell can move forward. Depolymerisation of the actin filaments occur which moves the actin filaments away from the plasma membrane.

Thus, both polymerisation and depolymerisation is required for cell movements. So, drugs which affect either polymerisation or depolymerisation of actin has the same effect on cellular movement.