In an effort to solve the mystery of how cells "decide" whenand where to move, scientists have uncovered a molecularstrategy evolved in mammals to assure tight control ofcellular marching orders. But the two-pronged approachappears to be exploited by a pathogenic microbe that hijacksthe cell's molecular motors to power a destructive marchthrough its human host.
The research by biochemists at the University of CaliforniaSan Francisco is reported in the May 14 issue of the journalCell.
Regulated movement at the cellular level is critical forsurvival. Cells divide, migrate toward food and away fromdanger. Nerve axons grow and immune armies attack -- all inresponse to orders from a family of molecules known asorganizing or signaling proteins. These proteins are thoughtto translate selected signals into commands that engage thecell's gears of motion.
The pathogenic, food-borne bacterium Listeria monocytogenes,which can cause meningitis and other serious illnesses, isknown to recruit the motility apparatus of its human hostcells. It uses this engine to power arocket-like tail andpropel itself along a route of infection from one cell tothe next.
By studying the three-dimensional structure of a mouseorganizing protein called "Enabled" and analyzing itschemical interactions with one of Listeria's proteins, theUCSF scientists found that the microbe's success may lie inits ability to mimic a dual molecular docking maneuver thattakes place in mammalian cells, most likely evolved toreduce the chance the cells will march to the wrong orders.
Under normal conditions, the Enabled protein links toreceptors near the cell membrane when it receives anexternal signal to move. Then the organizing proteinrecruits and mobilizes the cell's motility protein, actin.The cell will then move in the direction that corresponds tothe buildup of actin.
The researchers found that in addition to its chemical andstructural attraction to the receptor, Enabled also appearsto be simultaneously attracted to cell membrane moleculesknown as phospholipids. They found that a positively chargedregion of the Enabled protein is drawn to the negativecharge of the phospholipids. This dual control - throughEnabled's attraction to both the receptor protein and themembrane phospholipids -- could greatly increase thereliability of the marching orders, the researchers suggest,just as if a command to attack had to be confirmed by twodifferent officers in the field before soldiers werecommitted to advance.
"Our examination of the structure of Enabled shows that thisorganizing protein is built to interact with multiplepartners," said Wendell Lim, PhD, assistant professor ofmolecular and cellular pharmacology at UCSF and seniorauthor on the paper in Cell. "Requiring dual inputs foractivation allows more controlled and precise cellularmovements," an evolutionary advantage over a moreerror-prone one-signal system.
The researchers also discovered that Listeria is able tomimic both cellular aspects that attract Enabled. Themicrobe presents multiple copies of a protein very similarto the normal cellular receptor that binds Enabled, makingitself even more attractive to Enabled than the cell'snormal receptor is. In addition, the microbe posseses a"negatively charged tail," which attracts the positivecharged region of EVH1 more strongly than do the membranephospholipids.
Taken together, these Listeria biochemical traits sosuccessfully mimic the cell's normal strategy to bind EVH1,that the molecule has 100 times greater affinity to thebacterium than it does to its own evolved binding partners.
With this enhanced ability to recruit the organizingprotein, Listeria can efficiently hijack the human cell'sown motility protein, actin. Powered by actin, Listeriamicrobes blast through their host cells like erratictorpedoes, leaving infection in their wake.
"Listeria appears to emulate the multiple signals that arenormally used in mammalian cells to activate Enabled, butdoes so with a single, tighter binding molecule," saidKenneth E. Prehoda, PhD, postdoctoral fellow in Lim'slaboratory and lead author on the study. "So, while normalcell movements are delicate and highly regulated, Listeriaharnesses the cell's engine at full-throttle."
Understanding how proteins like Enabled are activated, bothin normal cells and by pathogens like Listeria may aid inthe development of treatments for infections, metastaticcancer, inflammation, and developmental diseases, Lim andPrehoda say.
Do J. Lee, staff research associate in the UCSF laboratory,also participated in the research and co-authored the Cellpaper.
The research is funded by the National Institutes of Health,the Howard Hughes Medical Institute Research ResourcesProgram, The Burroughs Welcome Young Investigator Program,the Searle Scholars Program, and the Packard Foundation.
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