The Kinesin Motor: A Stunning Example of Cellular Nanotechnology
One of the most amazing examples of cellular nanotechnology is a molecular motor protein known as kinesin. Kinesin is responsible for transporting molecular cargo -- including chromosomes (e.g. during cell division), neurotransmitters and other important material -- along microtubule tracks from one region of the cell to another. It is driven by ATP hydrolysis, thereby converting chemical energy into mechanical energy which it can use for movement. A kinesin molecule typically possesses two tails on one end, which attach to the cargo, in addition to two globular heads (often called "motor domains") on the other end. Some readers may recognize this elegant protein from the now-famous Harvard animation, Inner Life of the Cell (time 1:59).
The sheer number of processes needed to be undertaken by such a motor protein makes the appearance of intelligent design seem almost beyond rational denial. Of course, many people resist this conclusion despite the evidence. As one Science Daily article in October 2010 put it,
"Our results show that a molecular motor must take on a large number of functions over and above simple transport, if it wants to operate successfully in a cell," says Professor Matthias Rief from the Physics Department of the TU Muenchen. It must be possible to switch the motor on and off, and it must be able to accept a load needed at a specific location and hand it over at the destination. "It is impressive how nature manages to combine all of these functions in one molecule," Rief says. "In this respect it is still far superior to all the efforts of modern nanotechnology and serves as a great example to us all." [emphasis added]One of my favorite descriptions of the workings of this nano-motor is in the following animation:
Kinesin proteins characteristically march towards the plus end of the microtubule (that is, towards the periphery regions of the cell), while a similar motor protein (called dynein) walks in the direction of the minus end (i.e. towards the centrosome near the nucleus). As a result of this singular and opposite directionality of the motor proteins, materials can be moved either toward or away from the cell's center.
Previous research by Mallik and Gross (2004) has revealed that, though kinesin is simple and efficient (in contrast to its counterpart dynein, which is structurally complex and cumbersome), its simplicity limits the cell's ability to regulate its activity: kinesin can only exist in two functional states, either fully active or fully inactive. Conversely, the structural complexity of dynein provides the cell with the capacity to dynamically regulate its activity, literally shifting gears in response to the load, and regulating its speed in response to the cell's needs. The trade-off between efficiency and regulative capacity allows dynein and kinesin to work together in a unique way. This phenomenon seems, at least on its face, to be better explained as a product of design than as that of a purely material-driven evolutionary process devoid of foresight.
Mallik and Gross themselves write,
We propose that kinesin and myosin are robust and highly efficient transporters, but with somewhat limited room for regulation of function. Because cytoplasmic dynein is less efficient and robust, to achieve function comparable to the other motors it requires a number of accessory proteins as well as multiple dyneins functioning together. This necessity for additional factors, as well as dynein's inherent complexity, in principle allows for greatly increased control of function by taking the factors away either singly or in combination. Thus, dynein's contribution relative to the other motors can be dynamically tuned, allowing the motors to function together differently in a variety of situations.In addition, Hammond et al. (2010) discuss how autoinhibition is caused, during the inactive state when it is devoid of cargo, by a folded conformation that enables nonmotor regions to directly contract and inhibit the enzymatic activity of the motor domain. They write, "The C-terminal tail interferes with microtubule binding and the coiled-coil segment blocks the motor's processive motility."
An interesting research paper has just appeared in the journal Science (Kaan et al. 2011), elaborating on how this molecular motor has the capability to enter into an "energy saving mode" when not in use! You can also read the Science Daily report here.
The research offers one possible means by which the kinesin motor is able to conserve energy when not in use: That is, the ability to fold in upon itself in order to prevent the loss of ATP.
The Science Daily report notes:
Kinesin's heads are typically joined together at one spot, called the hinge. In the new structure, the heads swing in toward each other and are bridged by the tail domain, effectively cross-linking the heads at the site of tail binding. This double lockdown -- at the hinge and at the bridge -- prevents the heads from separating. Because the heads need to be separate from each other to break down ATP, the double lockdown effectively stops the molecule from generating fuel to power the motor.So there you have it: This molecular motor protein elegantly enters into an "energy saving" mode when it isn't in use, thus ensuring very high energy consumption efficiency. If this doesn't constitute positive evidence for design, I don't know what does!