Cells are huge proponents of recycling. They reuse everything from tiny electrons to massive molecular complexes. Despite this penchant for salvage, scientists have never been able to see one form of cellular recycling in action – until now. A group led by HHMI Investigator Axel Brunger recently captured a protein in the act of pulling apart a spent cluster of membrane fusion molecules.
“Membrane fusion is a process akin to the merging of soap bubbles into larger ones,” says Brunger, a structural biologist at Stanford University. “But that’s where the analogy ends, since biological membranes do not merge all that easily.” To facilitate the process, cells use membrane-bound SNARE proteins. During fusion, SNAREs located on opposing membranes zip together into a stable complex, linking the membranes. When the two membranes have become one, a protein called NSF recycles the SNARE components.
Brunger’s team used a technique called single-particle electron cryomicroscopy to freeze NSF molecules bound to SNARE complexes at several stages and then capture images of them. The resulting snapshots provide near-atomic or better detail of the SNARE disassembly process. Like a series of movie stills, the images show NSF latching onto a SNARE complex, then NSF bound to ATP – the molecule that powers the salvage operation. Yet another image captured NSF after it had finished working, bound to an energy-depleted form of ATP, called ADP.
|NSF uses adapter proteins called SNAPs (yellow) to grasp the SNARE complex (green) in multiple places. Posted with permission from Macmillan Publishers Ltd: Nature 518, 61-67, copyright 2015.|
The SNARE complex resembles a rope with a left-handed twist; the team’s images revealed that NSF uses adapter proteins called SNAPs to grasp the “rope” in multiple places. The SNAPs wrap around the SNARE complex with a right-handed twist, suggesting that the disassembly occurs via a simple unwinding motion that frees the zipped SNARE proteins.
The results, published February 5, 2015, in Nature, raise other questions the team is eager to pursue. “There is a lot to be done in order to understand the motions and conformational changes needed to disassemble the SNARE complex,” says Brunger. “Our electron microscope structures now enable us to design follow-up biophysical experiments to answer these questions at a very deep level.”