Highlight: Shining New Light on Coral Fluorescent Proteins

Scientists can now track how cancer spreads, study cells in the retina, or watch HIV infections progress, thanks to the harnessing and development of green fluorescent protein (GFP). It is fair to say GFP has transformed biomedical research. In fact, Martin Chalfie, Osamu Shimomura, and Roger Tsien received the 2008 Nobel Prize in chemistry for their discovery and development of GFP.

Despite several decades of use, GFP and fluorescent proteins (FPs) in general are surrounded by a halo of mystery. GFP was found in Aequorea victoria jellyfish, sometimes called the crystal jelly. It is unknown why jellyfish have such proteins, though they are far from alone in carrying them. Several coral species also have them.

Better understanding these proteins and how they serve marine species could push science forward in several ways. It could help biotech engineers develop new fluorescent markers, for applications where GFP cannot serve. For example, GFP does not work well in live mammals, as hemoglobin absorbs the visible light the protein emits, often masking its presence. Ecologists also would love to know why reef-building corals carry these proteins, as they look for ways to aid threatened coral reefs.

A recent paper in Genome Biology Evolution takes a step toward these goals by analyzing the FPs in two stony coral species: Acropora digitifera and Acropora tenuis (Takahashi-Kariyazono et al. 2016). In it Yohey Terai, biologist at the Graduate University for Advanced Studies in Japan, and colleagues uncover new diversity for coral FPs, along with an unexpectedly high number of gene copies. The results indicate that FPs play some yet-to-be-determined crucial biological role.

“We did not imagine we'd find this. It's surprising because we still don't know the function,” says Terai, adding that he hopes their results will lead them towards understanding the proteins' role in living corals.

How Bright is Your Light?

The researchers traced the ancestry of coral FP colors, finding that red and cyan are the oldest. GFPs have evolved twice, once from cyan and once from red. Acropora species, the researchers report, have three major types of FPs, corresponding largely to emission wavelength.

“We also found the genus Acropora persisted in keeping the high copy number of fluorescent proteins during their evolution,” says Terai. This implies these proteins are important somehow for the corals' survival, as they have retained both high copy number and the functional diversity during their evolution.

Terai found the high number of copies remarkable. In previous studies, the largest number of such genes found in a single species was just four. In the current study Terai and colleagues estimate 31–35 copies of FP genes in the genomes of A. digitifera. The range they saw they attributed to variation among individuals.

One novel aspect of the study was to measure the fluorescence of living corals (in the 470–600 nm range). How many different proteins contribute to this? Specimens of adult and larval A. digitifera were collected so the team could examine the genetic basis of its fluorescence.

“No sequence was identical between them, suggesting life stage-specific FP gene expression,” write the authors. “The sequence difference between the adult and larva may reflect these life stage-specific fluorescence patterns.” (Though they note there is a small possibility that differences between individual specimens could have caused this.) Could this be a clue to these proteins' function?

Some researchers have suggested that FPs act as a kind of sunscreen, though this seems to be falling out of favor (they are found in corals that live in the dark, for instance).

“The issue is far from being resolved, and we are still struggling with it. Clearly, these are not just sunscreens,” says researcher Mikhail Matz, of the University of Texas, Austin, who was not involved in the work.

“One of the most exciting functional leads we found recently is that FPs in coral larvae might be involved physiological priming to achieve long-range dispersal,” says Matz, referring to recent research (Kenkel 2011; Strader 2016). He hopes to test this idea in the future by genome editing with the CRISPR-Cas9 system to create FP knock-down and/or knock-out corals.

After this investigation into coral genetics, the mystery of the glowing proteins may be better defined but is clearly just beginning.