WASHINGTON, April 19 (Xinhua) -- A protein that plays an important role in embryonic development and nervous system wiring in humans might be borrowed from bacteria, American scientists found.
In a study published Thursday in the journal Cell, scientists from the University of Chicago and Stanford University described for the first time the three-dimensional structure of proteins called teneurins.
Proteins play a crucial role in helping cells communicate with each other.
The multi-functional teneurins sit on the surface of cells and bind to other proteins on the surface of other cells, working on embryonic development, guiding nerve cell axons to the right place to make connections with other nerve cells, and helping these connections, or synapses, form.
WHY WE BORROWS BACTERIAL TOXIN?
Scientists found that, instead of resembling other proteins that perform similar functions, its structure most closely matches that of a bacterial toxin, making it the only known human protein with such a structure.
"No other eukaryotic protein was similar to this, so its resemblance to a bacterial toxin is unusual," said Demet Arac, assistant professor of biochemistry and molecular biology at the University of Chicago and senior author of the study.
Based on its genetic sequence, researchers suggested that teneurins resembled the poison molecules by which bacteria use to attack and comprise host cells.
Arac and Georgios Skiniotis from Stanford University used high-resolution electron microscopy to define the structure of the human version of the teneurin protein. When they compared it to reference databases of other protein structures, it most closely resembled a bacterial toxin with a barrel shape and propeller-like protrusion.
The researchers said that this toxin may have been incorporated into multicellular organisms very early in evolutionary history.
Choanoflagellates, one of the earliest single-celled organisms believed to evolve into multicellular eukaryotes, have a teneurin-like protein. It's likely that bacteria transferred some of their genes to early choanoflagellates, which co-opted them for their own genomes.
As these simple creatures evolved into more complex species, the original toxin-like protein has been conserved ever since.
HOW WE USE THIS BORROWED PROTEIN?
Arac and her colleagues suspect that the protein borrows some of the same tools bacteria use to infect cells and employs them to attach to and communicate with other cells instead.
According to Arac, a genetic process called alternative splicing may also help teneurins perform their various tasks.
Alternative splicing, or differential splicing, is a process that results in a single gene coding for multiple proteins. During this process, small segments of a gene called exons may be included or excluded in the final messenger RNA (mRNA) produced from that gene.
These different mRNAs in turn translate into alternate versions of proteins that can perform various functions.
The researchers generated two different, alternatively spliced teneurins and tested their functionality.
It turned out that just a tiny discrepancy or seven out of 2,500 amino acids made a difference.
One version of the teneurin lacking seven specific amino acids could adhere to a cell receptor that is important for signaling between cells.
Another version of the teneurin that includes those seven amino acids couldn't adhere to this receptor. Instead, it promoted formation of synapses between nerve cells.
"Humans are invaded by all these bugs. We are not just human," said Arac.
