I saw a train traveling down the street in front of my house the other day…no, not really. But imagine the surprise if something like that actually happened!
Recently a team of researchers saw the biochemical equivalent of my hypothetical scenario. They observed the molecular motor kinesin moving cargo along actin tracks in plant cells instead of along microtubules as expected. This new discovery uncovers a novel mechanism for the transport of organelles in plants and at the same time highlights the elegant design of biochemical systems.
Actin and microtubules form part of the cytoskeleton, a three-dimensional network of filaments that attaches to the plasma membrane and organelles. The cytoskeleton imparts structural integrity to the cell and forms the framework for the cell’s shape and movement.
There are three types of cytoskeleton filaments.
- Microtubules: long, slender tubes, 20 to 25 nanometers in diameter and composed of the protein tubulin
- Intermediate filaments: about 8 to 10 nanometers in diameter, found only in animal cells, and consisting of intertwined rope-like assemblies of protein fibers
- Microfilaments: only 6 nanometers in diameter and made up of the protein actin
The cytoskeleton is not permanent; rather it assembles and disassembles as needed at the various locales within the cytoplasm. The cytoskeleton also serves as a “railway system” of sorts, used by the cell’s machinery to ferry organelles and other cellular cargo from place to place within the cell.
Several different proteins function as molecular motors transporting cargo along the cytoskeletal tracks. This list includes myosin, dynein, and kinesin. Myosin moves cellular material along actin filaments or microfilaments. Both dynein and kinesin haul their loads on microtubule tracks.
Kinesin’s structure resembles two golf clubs with their shafts intertwined around one another in a helical fashion. Attached to kinesin’s rod-like region are two lobe-shaped structures that resemble the heads of golf clubs.
The kinesin heads interact with microtubules. The rod-like shaft binds the cellular cargo that kinesin will transport along the microtubules. Kinesin moves along the microtubules by “walking” along the microtubule with the heads attaching and detaching to the microtubule in an alternating fashion.
Biochemists are still learning how the kinesin motor operates. It looks as if kinesin functions as a Brownian ratchet, man-made machines designed to power movement in nanodevices. Accordingly, the kinesin heads randomly diffuse . Once one head binds to the microtubule, it restricts the movement and binding of the other head. This restriction causes kinesin to move in a single direction along the microtubule. (Check out this video of kinesin moving a vesicle along microtubules.)
In plant cells, kinesin plays a key role in optimally positioning chloroplasts as light conditions change. This function is called photorelocation. Chloroplasts are the organelles responsible for photosynthesis (a process that uses sunlight to convert water and carbon dioxide into sugars). Under low light conditions, kinesin motors transport chloroplasts along the cytoskeleton toward the light source. This movement is called photoaccumulation. When light conditions are high, kinesin moves the chloroplasts away from the intense light to avoid photodamage to these organelles in a move known as photoavoidance.
Scientists expected that kinesin proteins would ferry the chloroplasts along microtubules. However, researchers from Japan were surprised to learn that kinesin appears to interact with actin during photorelocation, not microtubules. This is the biochemical equivalent of a train traveling down the highway instead of the tracks!1 The researchers also discovered that these kinesin motors were retooled to bind to actin instead of microtubules, which is analogous to redesigning a train to travel on asphalt.
This study calls attention to how much information about the operation of biochemical systems remains to be discovered. It also highlights the fact that what we do know about biochemistry points to the work of a Creator.
In this case, an elegant molecular logic undergirds the photorelocation process suggesting it has been designed by an intelligent Agent. So, too, does the machine-like operation of kinesin and specifically its direct correspondence to Brownian ratchets. (Click here for a detailed discussion of how molecular motors like kinesin evoke and reinvigorate William Paley’s Watchmaker argument for design.)
As biochemists learn more and more about the cell’s chemistry, the evidence for design keeps truckin’ on.