While we commonly think of cells dividing and multiplying in our bodies, it is also possible for two cells to join together. In fact, invading viruses commonly fuse with healthy cells in order to inject foreign genes, and cellular fusion is the basic process by which sperm and egg share genetic information. Since most cells in our bodies touch one another without fusing, scientists are keen to understand what starts the fusion process and how it occurs.
One reason cell fusion is little understood is because bonding begins at the membrane, the ultra-thin envelope of molecules around each cell. All biological membranes consist of two layers of lipid molecules, called a bilayer, that have a large population of proteins embedded in them. As two-dimensional liquid films, membranes remain one of the least understood components in cells because the most powerful techniques in biochemistry -- X-ray crystallography and high-resolution nuclear magnetic resonance -- are difficult to apply.
"Membrane fusion is governed by a group of large, complex proteins, but scientists have no idea how these proteins work," said Huey Huang, Sam and Helen Worden Professor of physics and astronomy at Rice University and the senior author of the Science paper. "Our research will help scientists who are studying these proteins.
Scientists know cellular fusion begins when cell membranes form an initial junction, a tiny hole between the two cells. This junction widens over time until one single, continuous membrane envelopes the contents of both cells. Huang and co-author Lin Yang, a former graduate student of Huang at Rice and now a postdoctoral physicist at Brookhaven's National Synchrotron Light Source (NSLS), used a variant of X-ray crystallography called X-ray diffraction to experimentally confirm a longstanding theory about the initial bridge that forms between membranes. Understanding the basic structure of this initial molecular connection is critical for isolating the energy barriers that fusion proteins need to overcome in order to initiate cell fusion.
To reveal the structure of the fused cell membranes, Huang and Yang first produced small crystals composed of stacks of membranes made of phospholipids. Then, they projected x-rays produced by the NSLS toward the crystals. By looking at how the x-rays scattered off the crystals, Huang and Yang were able to create diffraction patterns, maps of the atomic structure of the phospholipid layers in the membranes.
By dehydrating a stack of membranes, the scientists were able to induce membrane fusion. Analysis of the diffraction patterns of these samples confirmed that the two membranes were caught in the act of fusing. These diffraction patterns showed that membrane fusion begins with the formation of an hourglass-shaped structure called a stalk, which theorists had predicted.
"Now that we know what the structure is, we can calculate the free energy pathway -- which is a sort of map that will show where the energy barrier is for membrane fusion," said Huang. "Ultimately, medical researchers working on gene therapy and drug delivery would like to find out how two membranes fuse, because they'd like to be able to activate the fusion process in order to deliver new genes and drugs to patients. Currently, fusion is the bottleneck to non-viral gene delivery."
The research was funded by the National Institutes of Health, the Welch Foundation, and the Department of Energy.
Journal
Science