5 The cytoskeleton and motor proteins
5.1 The role of the cytoskeleton in intracellular transport
In Section 3 we explained how vesicles bud from donor membranes and fuse with target membranes, which may be quite distant. In such cases, vesicles are actively transported from one site to another, a process that involves motor proteins attached to the vesicle, which propel the vesicle along the cytoskeleton. The microtubule network in particular acts as a trackway for long-range movement of transport vesicles, and this is evident in the movies that show the dual staining of tubulin and secretory vesicle components.
How is the microtubule network arranged in relation to the Golgi network and the ER and how might this facilitate long-range movement of transport vesicles? What implications does this arrangement have for the direction of travel of vesicles along the microtubules?
The microtubule network extends from the MTOC, which is located near the nucleus, and the Golgi network is also located near the nucleus.
The implication of these observations is that the microtubule network can act as a semi-permanent link between compartments, but that the directionality of migration is very carefully controlled – the network is doing much more than speeding up diffusion – vesicle migration is directional and depends on the vesicle and its cargo as well as on the orientation of the cytoskeletal elements and the action of motor proteins.
Microfilaments also affect vesicle movement, especially for vesicles positioned near the cell surface. For example, the mobility of caveolae located beneath the plasma membrane is affected by latrunculin, a drug that inhibits microfilament assembly, whereas their movement across the cell is blocked by nocodazole, a drug that causes microtubules to disassemble. For caveolae, the mobility of vesicles near the membrane appears to be limited by their interaction with the microfilaments.