Tetrahymena, a tiny single-celled-organism, hides a surprising secret: It is respiration — using oxygen to generate cellular energy — different from other organisms such as plants, animals or yeast. Discovery, published on 31 March ScienceHighlights the power of new techniques in structural biology and reveals gaps in our knowledge of a major branch of the tree of life.
“We thought we knew about respiration from studies of other organisms, but this shows us how much we still don’t know,” said Maria Maldonado, a postdoctoral researcher in the Department of Molecular and Cellular Biology at the University of California, Davis and co. he said. – First author on the paper.
Tetrahymena is a genus of free-living, single-celled organisms commonly found swimming quietly around ponds by beating their coats, or cilia, of short hairs. Like us, they are eukaryotes, with their genetic material contained in a nucleus. They belong to a large and diverse group of organisms called the SAR supergroup. With a few exceptions, such as the malaria parasite Plasmodium, the SAR supergroup is little studied.
“It’s a big part of the biosphere, but we don’t think about them much,” Maldonado said.
Like all other eukaryotes — and some bacteria — Tetrahymena consume oxygen to generate energy through respiration, said James Letts, an assistant professor of molecular and cellular biology in the UC Davis College of Biological Sciences.
Oxygen comes at the end of the chain of chemical reactions involved in respiration. Electrons are passed through a series of proteins located in structures called cristae in the inner membrane of the mitochondrion. It forms water from oxygen and hydrogen atoms, pumping protons across the membrane, which in turn induces the formation of ATP, the storehouse of chemical energy for the cell. This electron transport chain is fundamental to oxygen-based respiration in humans and other eukaryotes.
New perspectives in structural biology
There were clues that something is different about the electron transport chain in Tetrahymena, Letts said. In the 1970s and 80s, scientists discovered that its electron-carrying protein – cytochrome C – and the oxygen-consuming enzyme at the end of the chain – terminal oxidase – acts differently than in plants and animals. Until now, it was unclear how or why these enzymes differed in Tetrahymena, when they were conserved in the other studied eukaryotes.
Maldonado, Letts and co-first author Long Zhou used new approaches in structural biology to uncover the Tetrahymena electron transport chain. These included the cryo-electron microscopy structural proteomics approach – working out the structures of a large number of proteins in a mixed sample at the same time.
Cryo-electron microscopy freezes samples at extremely low temperatures, producing images at nearly atomic resolution. Instead of imaging a single, pure protein, the team worked with mixed samples isolated from the mitochondrial membrane and then taught an algorithm to recognize the associated structures.
In this way, they were able to scan through hundreds of thousands of protein images and identify the structures of 277 proteins in three large assemblies, representing the Tetrahymena electron transport chain, at near atomic resolution. Some of these proteins have no matching genes in the known Tetrahymena genome database – indicating that there must be gaps in the available reference genomes.
Letts said that by revealing gaps in our knowledge about a common organism, the work shows our blind spots with respect to biodiversity. It also shows the potential of these new approaches in structural biology as a discovery tool, he said.
Part of the work was conducted with a cryo-electron microscope at the BioEM Core Facility at the UC Davis College of Biological Sciences. Additional authors on the paper are Abhilash Padavanil and Fei Guo, both at UC Davis. Zhou is now at the Zhejiang University School of Medicine, Hangzhou, China. The work was supported by the NIH.
material provided by University of California – Davis, Original written by Andy Fell. Note: Content can be edited for style and length.