Page 142 - Engineering in Nature

Engineering in Nature

and dynein perform different tasks apart from linking the micro-tu-
bules together. (In the micro-hair are other connectors as well as dy-
nein and nexin.) If the proteins dynein and nexin did not have these
mutually complementary properties, the hairs could not move.

A Microscopically Small Engine
Another detail makes this interconnected structure even more
complex and convoluted. The structure that enables the micro-hairs
to move, and which resembles a motor, lies not in the cells to which
they belong, but in the micro-hairs themselves. Were just one of the el-
ements in these engines to be absent – the protein dynein, for instance
– the hair would be unable to move.
To acquire a better understanding of this structure, scientists set up
a model that can be compared to a continuation of the example of the
food tins we gave earlier.
Two columns of tins, one on top of the other, are joined by loose
wires. A tiny engine is attached to one tin, and a motor arm to the ad-
jacent tin beside it. When the engine is set in motion, the motor arm
slides down, pushing down the tube to which it is attached. Since the
columns are interconnected, the loose wires begin to contract. As the
motor arm pushes the tube opposite it, the tension caused by the wire
causes both tubes to lean over to a specific degree. The movement of
separation is thus turned into bending.
Let's express this analogy in biochemical terms.
The dynein protein arms between two micro-tubules set the oppo-
site tube in motion. The biological energy known as ATP is used for
this movement. When this takes place, the two micro-tubules begin
moving together. Were it not for the nexin—in our analogy, the loose
wire in between—both tubes would continue to move away from one
another. However, the nexin protein's mutual links prevent the micro-

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