Biological complexity:maintaining membrane integrity (Introduction)

by David Turell , Tuesday, December 06, 2016, 19:25 (2909 days ago) @ David Turell

Cells in the body are constantly turning over and replaced. This is a study as to how membranes are maintained:

https://www.sciencedaily.com/releases/2016/12/161206110102.htm

"Scientists from the National Centre for Biological Sciences (NCBS), Bangalore, and the Babraham Institute in UK have recently found a critical player essential for proper membrane recycling. Using the light-sensitive membranes of fruit-fly eyes as a model system, the researchers have discovered that the enzyme Phospholipase D or PLD is necessary for membrane recycling to sustain normal sight.

"The resetting of rhodopsin molecules begins with a process called endocytosis, where the cell pinches off parts of its surface membranes into structures called endosomes. The rhodopsin in these endosomes is eventually recycled back onto the cell surface for further events of light detection. Since a photoreceptor's sensitivity depends on how many rhodopsin molecules it has on its surface, membrane turnover in these cells is critical in preserving normal eyesight.

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"Using fruit fly photoreceptor cells as a model system, the team has found that when these cells are exposed to light, PLD is switched on, and that its activity is essential in coupling endocytosis with recycling of rhodopsin back to the cell surface.

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"'The enzyme PLD converts a molecule called phosphatidylcholine into phosphatidic acid or PA, which is implicated in membrane turnover. However, PA is also produced by other enzymes, and our study conclusively shows that the PA regulating membrane turnover was produced by PLD," says Rajan Thakur, a researcher from Padijat's group and the primary author of a publication in the journal eLife that reports these results.

"Despite identifying a key player in the membrane turnover process, Padinjat's team believe that many more gaps need filling in understanding the phenomenon. For example, the work shows that the PLD activity in photoreceptors is light-activated, but how this happens is still unresolved.

"'The light receptor, rhodopsin is on the membrane surface, and can detect light when the surface of the cell is illuminated. But PLD, which is also light-activated, is somewhere inside the cell. So how is the information about perceived light conveyed to PLD for it to get activated?" asks Thakur. "We also need to fill in blanks about how PA actually affects membrane recycling and the turnover process. Our finding has opened up more questions to answer in the membrane turnover process," he adds.

"But the results from this study are not limited only to membrane turnover in the light-sensitive membranes of the eyes. Membrane turnover is a critical mechanism that maintains cell surface area. In cells that have expanded cell surfaces such as those lining the airways of the lungs or the nutrition-absorbing cells of the gut, maintaining cell surface area is essential for their normal function. Even processes such as cell migration have extensive endocytosis and membrane recycling events that must be tightly regulated. (my bold)

"'Therefore, regardless of what the cell type is, there need to be mechanisms to couple endocytosis with recycling of membrane," says Padinjat. "And that is the importance of our work -- we define a mechanism by which cell membrane size is regulated," he adds."

Comment: Two points: first, the need for tight regulation is obvious as noted in the bold area. Secondly, once again a giant enzyme molecule is used. How did the evolutionary process find it? Not by chance!