Discovery of Subcellular Pouch Could Alter Approach to Disease Treatment
Excerpted from a story by Jim Barlow, UI News Burea

Acidocalcisomes (the black spheres) as viewed in a trypanosome, a family of parasites that cause African sleeping sickness, Chagas disease and leishmaniasis. The cell is approximately 10 µm long and 4 µm wide.

Researchers looking inside a pathogenic soil bacterium have found an organelle, a subcellular pouch, existing independently from the plasma membrane. The discovery within a prokaryotic organism challenges the theory on the origin of eukaryotic organelles and suggests a targeted approach to killing many disease-causing organisms.

“The organelle we found in the bacterium Agrobacterium tumefaciens is practically identical to the organelle called acidocalcisome in unicellular eukaryotes,” says Dr. Roberto Docampo, professor of veterinary pathobiology and scientific director of the Center for Zoonoses Research.

Dr. Docampo began researching these organelles in 1994. He soon determined that a tiny granule in yeast, fungi, and bacteria, thought to be for storage, was a fully operational organelle containing pyrophosphatase, a pump-like enzyme that allows proton transport. He named it an acidocalcisome for its acidic and calcium components. In 2000, he reported its existence in Plasmodium berghei, a malaria-causing eukaryotic parasite.

The newest discovery was published by the Journal of Biological Chemistry in a paper by Dr. Docampo and colleagues at the Center for Zoonoses Research and Laboratory of Molecular Parasitology at the College, including Drs. Manfredo Seufferheld, Mauricio C.F. Vieira, Felix A. Ruiz, Claudia O. Rodrigues, and Silvia N.J. Moreno. The National Institutes of Health funded the research.

Agrobacterium tumefaciens is a prokaryote, a unicellular organism lacking membrane-bound nuclei. It causes crown gall disease in many broad-leaved plants but also is a favored tool for plant breeding because of its model system of DNA transfer into the hosts it invades.

Bacteria and other prokaryotes generally lack an endomembrane system. Thus bacteria are presumed to lack compartments such as organelles not somehow linked to the plasma membrane ringing the organisms.

“What we describe is a discrete organelle independent of the plasma membrane,” Dr. Docampo says. “It has a proton pump in its membrane, which is used to maintain its interior acidic content. This has never been described before in a bacterium.”

The existence of discrete organelles is a defining component of unicellular eukaryotes, which have membrane-bound nuclei and specialized structures in their cell boundaries. The evolution of eukaryotic organelles “is a matter of extensive debate,” Dr. Docampo says. The principle of endosymbiosis says that as microorganisms engulfed others, then new, membrane-surrounded organelles emerged in eukaryotes.

“It appears that this organelle has been conserved in evolution from prokaryotes to eukaryotes, since it is present in both. This argues against the belief that all eukaryotic organelles were formed when early eukaryotes swallowed prokaryotes,” he says.

Using transmission electron and immunoelectron microscopy and X-ray microanalysis on the bacterium, researchers got a highly magnified and illuminated view.

Many parasites such as those that cause malaria, African sleeping sickness and toxoplasmosis and bacteria that contain these acidocalcisome organelles are pathogens.

Some pharmaceutical approaches have targeted pyrophosphate-related enzymes, Dr. Docampo notes. “Our suggestion is that if drugs specifically targeted these organelles, you may be able to kill the entire organisms.”