Biological complexity: how vitamin A enters a cell (Introduction)

by David Turell , Thursday, August 25, 2016, 20:08 (3012 days ago) @ David Turell

It has to be very careful as too much vitamin A is toxic to cells, but A is vital to vision so the system has to work properly. This finding is currently tentative, but reasonable:-http://phys.org/news/2016-08-electron-microscopy-reveals-vitamin-cell.html-"Using a new, lightning-fast camera paired with an electron microscope, Columbia University Medical Center (CUMC) scientists have captured images of one of the smallest proteins in our cells to be "seen" with a microscope. 
The protein—called STRA6—sits in the membrane of our cells and is responsible for transporting vitamin A into the cell interior. Vitamin A is essential to all mammals and is particularly important for making light receptors in our eyes, and to ensuring normal fetal development.-"Images of the protein, which revealed several unusual features, were published in the August 26th issue of the journal Science,...-***-"STRA6 only interacts with Vitamin A via an intermediary protein that carries the greasy vitamin A in the bloodstream. Revealing the structure of STRA6 may not only give the researchers insight into Vitamin A transport, but may also provide clues about how other related transporters work.-"A new type of camera technology was key to obtaining the images of STRA6. When paired with an electron microscope, the camera allows biologists to see tiny, previously unseen structural details of the inner machinery of our cells.-***-"The researchers used approximately 70,000 individual pictures of STRA6 to generate a 3-dimensional map of the protein, which was used to construct an atomic model accurate to the smallest detail. The images and model revealed that STRA6 is "a bit of a freak," says Dr. Clarke. Even more surprising was the fact that STRA6 does not work alone, but is instead tightly associated with another protein, calmodulin, which plays a key role in calcium signaling.-"Although Vitamin A moves through STRA6 to enter the cell, there is no channel in STRA6 like most transporters. Instead, vitamin A enters the top of STRA6, but then appears poised to exit through a side window that opens directly into the cell membrane, not the cell interior.-"Though this needs to be verified, the mechanism may be a way to protect cells from absorbing too much vitamin A. "Vitamin A is actually somewhat toxic," says Dr. Mancia. "Trapping vitamin A inside the membrane may keep control of the amount that gets into the cell."-"The new model of STRA6 advances the understanding of a critical cellular function and may help researchers understand how other, still mysterious, cellular components work."-Comment: Once again a very complicated transport molecule is seen to fill a very exact purpose. How did 'purposeless' Darwin evolution understand the need to do this? Vitamin A has to be used at exact levels or it becomes dangerous. How did vision develop in view of this difficulty. Again a strong suggestion of saltation, and what is the mechanism for saltation? God is a good answer.