Discovery illuminates elusive proton channel gene in dinoflagellates

The gene, discovered in a tiny marine organism called a dinoflagellate (Karlodinium veneficum), controls voltage-gated proton channels, which, in addition to triggering luminescence in certain single-cell sea creatures, activate many important biological mechanisms in other species, including humans.

Results of the study by DeCoursey, Susan M. E. Smith and co-researchers were published in the Oct. 17, 2011 issue of the Proceedings of the National Academy of Sciences. The study was funded in part by grants from the National Science Foundation and the National Institutes of Health.

The existence of a voltage-gated proton channel in bioluminescent dinoflagellates was proposed in 1972 by J. Woodland Hastings, a co-author on the current study, and his colleague Margaret Fogel. They hypothesized that proton channels helped trigger the flash by activating luciferase, an enzyme that helps produce luminescence. But until now, the genetic code responsible for the proton channels in dinoflagellates had not been identified, although it had been decrypted in humans, mice, algae and sea squirts.

Voltage-gated proton channels are extremely versatile. In humans, they are involved in several basic biological processes, including release of histamine in basophils, a type of white blood cell. Proton channels also play a role in the production of reactive oxygen species such as hydrogen peroxide that kill bacteria in phagocytes, another kind of white blood cell, and in maturation of sperm immediately before fertilization.

In the current study, DeCoursey and co-researchers mined the gene sequence library of a K veneficum dinoflagellate and found a gene named kHv1 that is similar to those already known to code for proton channels in other species. Not surprisingly, there were many differences in the make-up of the proton channel molecules in humans and tiny sea creatures, but the most important parts of the molecules turned out to be almost identical. Electrophysiologic tests confirmed that the genetically coded protein was indeed a proton channel -- but one with an unprecedented quality.

Proton currents in K veneficum differ from all known proton currents in having large inward currents -- a result of the channels opening at membrane potentials about 60 mV more negative than in other species, the researchers found.

"Vertebrate proton channels open to allow acid extrusion, while dinoflagellate proton channels open to allow proton influx into a cell's cytoplasm, making the channel ideally suited to trigger bioluminescence," DeCoursey explained.

When dinoflagellates floating in water are mechanically stimulated by movement, an impulse (action potential) is sent along the membrane of an internal compartment called a vacuole. Clustered along the inside of this membrane are tiny pockets called scintillons, containing a combination of luciferin and luciferase -- proteins that are able to produce a light flash under the right circumstances. The inside of the vacuole compartment is very acidic and has an abundance of protons.

As the electric impulse travels along the membrane, it causes the voltage-sensitive proton channels to open. Protons then flow from the vacuole into the scintillon, where they react with the luciferase and a flash of light results.

In nonbioluminescent mixotrophic species like K veneficum, proton influx might be involved in prey digestion (e.g., signaling prey capture) or prey capture (e.g., extrusion of stinging trichocysts).

Co-investigator Susan Smith carried out a phylogenetic analysis of known Hv1 sequences, finding high sequence diversity among the single-celled species and among invertebrates. She interpreted this finding to suggest the possibility of other novel functions of Hv1 in these species.

"As in multicellular organisms, ion channels in dinoflagellates play various roles in regulating basic life functions, which make them targets for controlling dinoflagellate populations and behavior," the authors suggested.

Future research will show whether targeting proton channels might give us a handle on controlling dinoflagellate blooms that cause deadly red tides and are responsible for massive fresh kills. Certain dinoflagellate species produce some of the most deadly poisons known, such as saxitoxin, a neurotoxin 100,000 times more potent than cocaine. Paralytic shellfish poisoning occurs in humans who eat shellfish that have consumed toxic dinoflagellates.

In addition to DeCoursey, Smith (Emory School of Medicine) and Hastings (Harvard University), the authors of this paper include Deri Morgan, Boris Musset and Vladimir V. Cherny of Rush University Medical School, and Allen R. Place of the University of Maryland Center for Environmental Sciences.