To use an analogy between motor proteins and cars, the cytoskeleton plays the role of road networks along which motor proteins move. Like road networks, cytoskeleton networks are distributed functionally along intracellular transport pathways. Moreover, the degradation and reconstruction of networks frequently occur as needed. Some examples of the cytoskeleton networks formed in cells are discussed below.
One example comparable to expressways is the transport pathway in the axons of neurons (Fig. 6-13A). An axon is a long projection extending from the cell body of a neuron (where the nucleus is located), and forms a synapse with other cells at its tip. A well-developed transport pathway is formed in the axon, because active transport takes place between cell body and synapses. The transport pathway is bundle of microtubules in the axon, and these microtubules are arranged with their plus ends pointing toward the synapse. Kinesins and dyneins with cargo (such as endoplasmic reticulum and mitochondria) come and go actively along the microtubules.
Transport pathways are also formed in general cells. As an example, there is a secretory pathway from rough endoplasmic reticulum (where proteins are synthesized) through Golgi apparatus to the plasma membrane. Conversely, there is an endocytosis (the process of material incorporation) pathway from the plasma membrane to Golgi apparatus or to rough endoplasmic reticulum (Fig. 6-13B). Cytoskeletal filaments run along these transport pathways, and motor proteins carry cargo along the filaments.
Fig. 6-13 Intracellular transport system with polarity
A) The transport system formed in the axon of neurons. The axon contains bundle of microtubules with their plus ends pointing toward the synapse. Kinesins and dyneins with cargo move back and forth along the microtubules.
B) The intracellular transport system of general cells. In the cell, transport pathways with cytoskeleton are formed in the direction of cargo transport. Motor proteins carry cargo along the pathways.