Cells are like miniaturized cities, with elaborate transportation systems ferrying their cargo to and fro (see Feb. 25 headline). Just like a city may have railroads, busses, cars and monorails, the cell has multiple kinds of transport motors: dyneins, kinesins, and myosins. Scientists have learned that most of the roadways are like one-way monorails: actin filaments and microtubules, upon which the vehicles travel in one direction. But what if a passenger needs to jump from one system to another? ’ No problem; the cell has mastered the art of ridesharing with its own park-and-ride system.
In the Dec. 2 issue of Current Biology1, this is described by Marcus Maniak in a Dispatch entitled “A park-and-ride system for melanosomes.” Melanosomes are organelles (somes) that carry melanin, the pigment chemical that allows some organisms, including fish and amphibians, to change their skin color to match their surroundings. For this to work, the melanosomes need to hitch rides either to the exterior of the cell or the interior. He pulls together several recent findings to describe how this all works:
Together these findings suggested how melanosomes might move on actin filaments and showed that this type of motility is required for the even distribution of melanosomes within the cell. From these main observations, it became clear that, during aggregation, a cytoplasmic dynein motor carries melanosomes on the radially arranged microtubules towards the cell center (Figure 1B), while during dispersion a kinesin transports the granules to the periphery (Figure 1C), where they engage via a myosin V molecule with short actin filaments to be distributed further (Figure 1D). This switching of transport systems is a kind of miniature edition of modern urban traffic, where millions of workers leave the city centers in the evening on trains and board their cars at park-and-ride stations to complete their daily journey within the green peripheral belt. (Emphasis added in all quotes.)
As if that were not amazing enough, it appears that the drivers “talk” to each other with a communication system:
Although the work of Rodionov et al. has moved the field a large step further, there are obviously several issues that remain to be investigated. Exciting new findings addressing the coupling of motor molecules to the melanosome surface in other experimental animals open the possibility to speculate how the motors may talk to each other on a molecular level. At least for Xenopus there is now clear evidence that both dynein and kinesin couple to melanosomes via the dynactin complex. Moreover, both motors compete for the same protein component; this could allow one motor to gain access to the microtubule while the other is prevented from engaging successfully.
He describes how this “tug-of-war” competition is actually a kind of way for the motors to negotiate the right-of-way. Additional factors that attach to the vehicles or trackways may assist in making sure the rules of the road are obeyed. “Thus,” he concludes, “further exciting results are on the way to complete the picture of how melanosomes switch from one transport system to the other.”
1Marcus Maniak, “Dispatch: Organelle Transport: A Park-and-Ride System for Melanosomes,” Current Biology Vol 13, R917-R919, 2 December 2003.
Maniak uses the word motor 22 times in his article, which is replete with other urban metaphors: transport system (see 09/26/2002 headline), etc. Moreover, there is no mention of evolution, Darwin, or of any mechanism that might explain how this elaborate, coordinated, interconnected system could have originated. Surprised?
Every muscle move you make, every breath you take, every beat of your heart, and every one of your senses are dependent on molecular machines. The study of biological motors and molecular machines is the “biology of the future” that Bruce Alberts, President of the National Academy of Sciences and editor of Molecular Biology of the Cell has stated more than once (see 01/09/2002 headline). It was also a biology the likes of which Charles Darwin and his followers could not have imagined. Had Darwin seen into the future at what Year 2003 biologists would be detecting in life’s fundamental unit, the cell, and even in the simplest micro-organisms, he well could have developed cold shudders (see 01/29/2002 headline) severe enough to have frozen his evolutionary speculations to death.