October 25, 2010 | David F. Coppedge

Automatic Turnstiles Found in the Cell

One of the things students learn about in high school biology classes is active transport: the ability to control flow through a semi-permeable membrane.  Contrary to osmosis, in which the flow goes naturally from high concentration to low concentration, cell membranes employ active mechanisms to push or pull the molecules through their membranes according to what they need – even against the concentration gradient.  Students were often presented these concepts as simple facts of life, but only in the last decade or two have researchers developed the tools to see how cells do it.  The results are amazing.  Cells have whole families of transporters that employ a variety of mechanisms to shuttle cargo through their membranes.  The structure of one of the last “transporter” machines has finally been cracked and reported in Nature.1  Although many questions remain, it appears to act as a rocking turnstile activated by a flow of cations (positive ions).
    This transporter, called NorM, a member of the MATE family (multidrug and toxic compound extrusion), exists in all domains of life, from simple bacteria to humans.  “Cellular export of toxins and substrates is a fundamental life process,” the authors said.  Its importance is profound: in plants, it determines crop yields according to its ability to pump toxins out from the soil.  In the liver of animals, as well as in microbes, it pumps toxins out of the body.  It does such a good job that, unfortunately, it confers multi-drug resistance on some germs we would like to destroy by allowing them to pump out the medicine, or prevents targeted medicines from reaching good cells doctors would like to help.  Understanding this transporter gateway enzyme is therefore of crucial importance.
    The team’s findings, using X-ray crystallography, show NorM to be made up of 12 intermembrane helices of amino acids, arranged in two groups of six to form a cup-shaped V pattern facing outward.  When a sodium ion (Na+ enters the cup, it binds to a particular site that has “three evolutionarily conserved amino-acid side chains that can carry a negative charge due to the presence of a carboxylic acid group.”  Somehow, this binding causes the cup to flip into an inward-facing conformation just long enough for the toxin inside the cell to bind to the cup.  When the sodium ion is released, the cup flips back out, tossing the toxin outside the cell.
    Hendrick W. van Veen, commenting on this paper in Nature,1 said that this automated turnstile can pump anywhere from 14 to 1,500 molecules out per minute.  Try to imagine this little cup flipping over up to 25 times per second, each time shuttling out its unwanted cargo.  Even that, though, is slow compared to some other specialized transporter families that can pump 100,000 ions per minute, he said.  The difference is that NorM and other MATE transporters have to shuttle a wide variety of larger molecules.  The sodium ion gradient indicates that this little machine, what’s more, runs on an electrical current.
    A number of questions still remain about how NorM achieves its mechanical effectiveness.  He et al said nothing about evolution – nothing about how this mechanism came to be.  Hendrick van Veen said very little: “Over the past decade, crystallographic evidence has been obtained supporting the general concept of alternating access for a variety of membrane transporters, demonstrating that this mechanism has been evolutionarily conserved” (i.e., unevolved).  Such a statement clearly begs the question of evolution.  It also leaves unanswered the question of how the first primitive microbe could have avoided death by poison without the ability to actively, effectively, pump toxins outside its protective walls.

1.  He, Szewczyk et al, “Structure of a cation-bound multidrug and toxic compound extrusion transporter,” Nature 467, pp. 991?994, 21 October 2010, doi:10.1038/nature09408.
2.  Hendrick W. van Veen, “Structural biology: Last of the multidrug transporters,” Nature 467, pp. 926?927, 21 October 2010, doi:10.1038/467926a.

Cell biology needs evolution like a king needs a jester.  The jester may not win wars or feed the peasants, but provides some comic relief, standing there juggling balls and telling jokes about where the leopard got its spots (10/20/2010).  Bring it on, Chuck: tell us how an unguided, purposeless process arranged twelve precisely-arranged intermembrane helices of amino acids into a cup shape, provided a precise pocket for a sodium ion, figured out how to invert the cup to the inside just long enough to recognize and bind a toxic molecule, and then flip over and shuttle it out, and do this 25 times a second.  Did this all happen in a single chance miracle or a string of chance miracles?  Tell us also how all the other tens of thousands of molecular machines essential for life “emerged,” like the topoisomerases that fold DNA into compact shapes and separate the chromosomes, the winches that pull chromosomes apart, the molecular highways that transport the good cargo all throughout the cell, the DNA repair molecules, and much, much more.
    Neo-Darwinism is utterly incapable of standing up to these findings about precision molecular machines in the cell.  The Old Guard Darwinists are like the old guard communists in the former Soviet Union, who clung to their worn-out Marxist-Leninist talking points when it was clear to everyone that they didn’t work.  The Darwinist talking points, with their ridiculous notions of conservation and convergence, are a ball and chain to molecular biology.  This is the age of systems biology, reverse engineering, nanotechnology, biomimetics, information science, machine language.  Take off the ball and chain.  Run toward the future with intelligent-design gusto.

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