FUNCTIONAL-ANALYSIS OF DYNACTIN AND CYTOPLASMIC DYNEIN IN SLOW AXONAL-TRANSPORT

The neuron moves protein and membrane from the cell body to the synaps e and back via fast and slow axonal transport. Little is known about t he mechanism of microtubule movement in slow axonal transport, althoug h cytoplasmic dynein, the motor for retrograde fast axonal transport o f membranous organelles, has been proposed to also slide microtubules down the axon. We previously showed that most of the cytoplasmic dynei n moving in the anterograde direction in the axon is associated with t he microfilaments and other proteins of the slow component b (SCb) tra nsport complex. The dynactin complex binds dynein, and it has been sug gested that dynactin also associates with microfilaments. We therefore examined the role of dynein and dynactin in slow axonal transport, We find that most of the dynactin is also transported in SCb, including dynactin, which contains the neuron-specific splice Variant p135(Glued ), which binds dynein but not microtubules. Furthermore, SCb dynein bi nds dynactin in vitro. SCb dynein, like dynein from brain, binds micro tubules in an ATP-sensitive manner, whereas brain dynactin binds micro tubules in a salt-dependent manner. Dynactin from SCb does not bind mi crotabules, indicating that the binding of dynactin to microtubules is regulated and suggesting that the role of SCb dynactin is to bind dyn ein, not microtubules. These data support a model in which dynactin li nks the cytoplasmic dynein to the SCb transport complex. Dynein then m ay interact transiently with microtubules to slide them down the axon at the slower rate of SCa.