Focus: When Membranes Collide

Published November 16, 2004  |  Phys. Rev. Focus 14, 20 (2004)  |  DOI: 10.1103/PhysRevFocus.14.20
Figure 1
Phys. Rev. Lett. 93, 198301 (2004)

Close encounters. The surfactant CPCl can exist as separate sheets (top) or can transform into a sponge of connected tubes. Researchers measured the energy of this transition to learn about fusion of biological cell membranes.

A team of researchers has measured the energy required for membrane fusion, a process that occurs when a virus fuses its outer membrane with that of a cell and when cells secrete packages of hormones. The experiment, reported in the 5 November PRL, used artificial membranes. It is the first experiment to show a direct connection between the energy of fusion and membranes’ molecular structure. The technique should help refine researchers’ understanding of the fusion process that occurs in every living cell.

Most researchers agree that when a pair of cell membranes touch, they can form a “stalk,” or narrow tube, connecting them. The stalk then grows in diameter until the membranes fuse and materials such as DNA and hormones can move freely through. To get to the stalk phase, the membranes must curl tightly into a tube, which can only be done if the surrounding environment offers them sufficient energy.

Now a team including Bill Hamilton of the Oak Ridge National Laboratory in Tennessee has found a way to measure the activation energy of membrane fusion. They used chemical membranes, made of the surfactant CPCl mixed with hexanol, which are somewhat softer than biological membranes but are the same in their basic structure. When sheets of this material touch, a labyrinth of tubes grows between the sheets, connecting them into one large sponge. This fusion process is similar to the fusion of biological membranes, according to the researchers.

Hamilton and his colleagues needed two values to calculate the activation energy for the formation of the sponge state–the frequency with which the membranes bump into each other and the time it takes for the membranes to fuse. These processes are usually too quick to observe, but the team was able to slow them down by adding sugar to the brine surrounding their membranes. In experiments at the neutron beam facility at the National Institute of Standards and Technology in Gaithersburg, Maryland, they placed a membrane sponge between two cylinders spinning in opposite directions fast enough to rip it into sheets. The researchers then stopped the cylinders and aimed a laser and a neutron beam at the membranes as the sponge reformed over a period of several seconds. The scattering patterns revealed the collision and fusion rates.

The team came up with an activation energy of 170 milli-electron volts, which is “a hard number in the right ballpark” of the 10-year-old stalk model and helps to validate its assumptions, says Hamilton. Most other membrane systems–including biological membranes–should have higher activation energies because they are less flexible and harder to deform, so this number also establishes the lower limit. “Which is just as well,” he says. “It would be inconvenient if living cells accidentally formed passages or fused every time they bumped in to one another.”

The team looked at the effects of membrane shape by varying the amount of hexanol. Hexanol molecules can wedge themselves between CPCl molecules and increase membrane curvature, which the researchers found decreased the activation energy for fusion. This makes sense, says Hamilton, because curved membranes are already part way to the highly curved stalk state. This is “the first really solid evidence showing how the activation energies for these processes scale–how they depend on molecular properties,” says chemical engineer Bob Prud’homme of Princeton University.

“I think this will be a classic that people in the membrane modeling community will point to,” says Prud’homme. The work is an important step toward creating human-made systems that act like biological ones, says Hamilton, and could lead to a new method of stopping viruses by interrupting their fusion with healthy cells.

–Chelsea Wald