"Model train enthusiasts never had it so good. Imagine five different models of finely-crafted engines, all in perfect working order, and enough track to cover a city. That’s what each of us has, right now, inside our cells.
Review article in the current issue of the journal Cell, by Ronald D. Vale of the Howard Hughes Medical Institute. He begins,
"A cell, like a metropolitan city, must organize its bustling community of macromolecules. Setting meeting points and establishing the timing of transactions are of fundamental importance for cell behavior. The high degree of spatial/temporal organization of molecules and organelles within cells is made possible by protein machines that transport components to various destinations within the cytoplasm."
Vale reviews the five major motor engine families that ferry cargo
Q: What about the switching?
Q: What keeps the engines from colliding on the tracks?
There is an unknown switching mechanism at so-called “turnaround zones” on the microtubules that dynein and kinesin engines travel on.
As a sidelight, another review article in the same issue of Cell by a team from UC San Diego describes how these motors are involved in tugging the chromosomes apart during cell division (mitosis).
Vale’s article is another of many we have reported that seems
schizophrenic. On one side of his brain, he marvels at the engineering and design, and on the other side, attributes it all to chance. Here is Vale’s storytelling about how this coordinated transportation system arose: “The complexity of these ‘Toolbox’ motors expanded in higher eukaryotes through gene duplication, alternative splicing, and the addition of associated subunits, which enabled new cargoes to be transported.” Impressed?
"To achieve law and order on the intracellular highways, the multiple cargo-carrying motors in a single cell must be regulated. In the majority of animal cells, individual organelles switch frequently between anterograde (microtubule plus-end-directed) and retrograde (minus-end-directed) movement …. In most cells, relatively little is known about the regulation and coordination of bidirectional motion. … individual cargoes move primarily unidirectionally in these extended processes, and a switch in direction occurs when cargoes reach the ends of these elongated structures."
There is an unknown switching mechanism at so-called “turnaround zones” on the microtubules that dynein and kinesin engines travel on.
"The microscopic observations of cargo transport in axons and flagella raise a number of similar questions. How do the opposite polarity motors, kinesin and dynein, coordinate their activities? What kind of machinery processes the incoming cargo and switches motor direction at the ‘turnaround’ zones? Molecular answers to these questions are beginning to emerge but are far from complete."
As a sidelight, another review article in the same issue of Cell by a team from UC San Diego describes how these motors are involved in tugging the chromosomes apart during cell division (mitosis).
Vale’s article is another of many we have reported that seems
Q: Well, for crying out loud, how did the motors get there in the first place?"
CEH