The freeze-frame image of a molecular relay race, in which one enzyme passes off a protein like a baton to another enzyme, has solved a key mystery to how cells control some vital functions, according to investigators at St. Jude Children’s Research Hospital. A report on this work appears in the January 14 advanced online publication issue of Nature.
The St. Jude discovery explains how a simple chemical link between molecules called a thioester bond acts like a switch to control the handoff of a protein called NEDD8 like a baton from one enzyme called E1 to a second enzyme called E2. When attached to E2, this thioester bond allows E2 to bind to a third enzyme called E3, which then helps E2 hand NEDD8 off to the ultimate target molecule at the end of the race. In the cell, this NEDD8 relay race triggers a number of biochemical reactions, one of which takes the brakes off cell division, allowing cells to multiply, according to the researchers. These thioester bonds are chemical links between two biological molecules that form when a sulfur atom on one of the molecules binds to an atom that is part of the other molecule.
Understanding how the thioester bond switch works is important not only because it explains a critical step in the NEDD8 hand-off from E1 to E2, but also because enzymes related to E1 and E2 run similar relays with other important protein batons, said Brenda Schulman, Ph.D., associate member of the St. Jude Structural Biology and Genetics and Tumor Cell Biology departments. “Our study shows that this simple switch could control comparable relays in charge of several different biochemical activities that keep cells alive and functioning normally.” Schulman, a Howard Hughes Medical Institute (HHMI) investigator, is senior author of the Nature report.
Scientists already knew that E1 momentarily juggles two NEDD8 molecules at once, holding them in two different “hands.” They also knew that E2 takes the NEDD8 that is in E1’s left hand; and that the other NEDD8 then hops over from the right hand to E1’s empty left hand. That leaves the right hand free to grab yet another NEDD8. By continually passing the NEDD8 proteins from the right hand to the left hand and on to E2, E1 keeps a relay of these batons flowing. In turn, that keeps the specific biochemical cascade triggered by NEDD8 in action.
But in order for the handoff to work, E2 must first insert itself into a docking site on E1, next to E1’s left hand, so it can grab NEDD8, Schulman explained. However, the E2 docking site is initially turned away from E1: so if E2 hopped into the docking site at that point, it would be too far away from NEDD8 to grab it, she noted. And that was the puzzle the St. Jude researchers solved.
Specifically, the St. Jude study showed that when NEDD8 forms a thioester bond with E1’s left hand, it squeezes itself next to the E2 docking site, which is facing away from NEDD8, according to Danny Huang, Ph.D., HHMI postdoctoral research associate and Harold Hunt, HHMI research technologist,. These researchers in Schulman’s laboratory did most of the work on this project.