Sexual reproduction and viral infections have a lot in common. According to new research, both processes rely on a single protein for the seamless fusion of two cells—sperm and egg cells and virus and cell membrane. This protein is widespread among viruses, single-celled protozoans, and many plants and arthropods, but is not found in fungi or vertebrates such as humans.
William Snell, a senior author of the study and research professor at the University of Maryland, Department of Cell Biology and Molecular Genetics, and colleagues from the Pasteur Institute, University of Texas Southwestern Medical Center, Global Phasing, Ltd., Hannover Medical School and German Center for Infection Research, published their findings in the February 23 issue of Cell.
The international research team notes that the protein, called HAP2, acts as a common, biochemical “key” that enables two cell membranes to become one, resulting in the combination of genetic material—a necessary step for sexual reproduction. The researchers say the findings suggest that the protein could provide a promising target for the development of vaccines, therapies and other disease control methods, which could help fight parasitic diseases, such as malaria, and boost efforts to control insect pests.
“Our findings show that nature has a limited number of ways it can cause cells to fuse together into a single cell,” said Snell. “A protein that first made sex possible—and is still used for sexual reproduction in many of Earth’s organisms—is identical to the protein used by dengue and Zika viruses to enter human cells. This protein must have really put the spice in the primordial soup.”
Snell and team studied HAP2, in the single-celled green alga Chlamydomonas reinhardtii. HAP2 is common among single-celled protozoans and plants and arthropods. Prior results from Snell and collaborators, as well as other research groups, indicate that HAP2 is necessary for sex cell fusion in the organisms that possess the protein. But prior to this new study, the precise mechanism was unclear.
For the current study, Snell and his UT Southwestern colleagues used sophisticated computer analysis tools to compare the amino acid sequence of Chlamydomonas HAP2 with that of known viral fusion proteins. The results suggested a striking degree of similarity, especially in a region called the “fusion loop” that allows the viral proteins to successfully invade a cell. If HAP2 functioned like a viral fusion protein, Snell reasoned, then disrupting HAP2’s fusion loop should block its ability to fuse sex cells.
When Snell’s team changed just a single amino acid in the fusion loop of Chlamydomonas HAP2, the protein lost its function entirely. The sex cells were able to stick together—a process that depends on other proteins—but they were not able to complete the final fusion of their cell membranes. Similarly, the cells could not fuse when the researchers introduced an antibody that covered up the HAP2 fusion loop.
“We were thrilled with these results, because they supported our new model of HAP2 function,” Snell said. “But we needed to visualize the three-dimensional structure of the HAP2 protein to be sure it was similar to viral fusion proteins.”
Snell reached out to Felix Rey, a structural biologist at the Pasteur Institute in Paris who specializes in viruses. Rey and his colleagues determined the structure of Chlamydomonas HAP2 using X-ray crystallography. Rey’s results demonstrated that HAP2 was functionally identical to dengue and Zika viral fusion proteins.
“The HAP2 protein from Chlamydomonas is folded in an identical fashion to the viral proteins,” Rey said, referring to the molecular folding that creates the three-dimensional structure of all proteins from a simple chain of amino acids. “The resemblance is unmistakable.”
HAP2 appears to be necessary for cell fusion in a wide variety of organisms, including disease-causing protozoans, invasive plants and destructive insect pests. So far, every known version of HAP2 shares the one critical amino acid in the fusion loop region. As such, HAP2 could provide a promising target for vaccines, therapies and other control methods.
Snell is particularly encouraged by the possibility of controlling malaria, which is caused by the single-celled protozoan Plasmodium falciparum.
“Developing a vaccine that blocks the fusion of Plasmodium sex cells would be a huge step forward,” Snell said, noting that Plasmodium has a complex life cycle that depends on both mosquito and human hosts. “Our findings strongly suggest new strategies to target Plasmodium HAP2 that could disrupt the mosquito-borne stage of the Plasmodium life cycle.”
In addition to Snell and Rey, co-authors of study, “The ancient gamete fusogen HAP2 is a eukaryotic class II fusion protein,” include Juliette Fedry, Gerard Péhau-Arnaudet, M. Alejandra Tortorici, Francois Traincard and Annalisa Meola (Pasteur Institute); Yanjie Liu, Jimin Pei, Wenhao Li and Nick Grishin (UT Southwestern); Gerard Bricogne (Global Phasing, Ltd.) and Thomas Krey (Pasteur Institute, Hannover Medical School and German Center for Infection Research).
Research was supported by the United States National Institutes of Health (Award Nos. GM56778 and GM094575), the Welch Foundation (Award No. I-1505), the European Research Council, the Pasteur Institute and the French National Center for Scientific Research. The content of this article does not necessarily reflect the views of these organizations.