Yeast's family tree reveals secrets of 2 million years

A Melbourne research team's identification of an ancient family of transport proteins in yeast cells has revealed details of the symbiotic pact that led to the evolution of the first eukaryotic life forms some 2 billion years ago.

Assoc Prof Trevor Lithgow, of Melbourne University's Russell Grimwade School of Biochemistry and Molecular Biology, believes the Omp85 (outer membrane protein) family of proteins literally played an integral role in endowing the first eukaryotic cells with a reliable internal energy source.

Eukaryotic cells, which generate their own biochemical energy in tiny organelles called mitochondria, are the basis of all complex life on Earth, from single-celled yeasts and algae, to multicellular higher plants and animals.

The compact mitochondrial genome appears to be a pared-down version of the genome of an early alpha-proteobacteria. Biologists believe mitochondria evolved from an ancestral microbe that formed a symbiotic relationship with a primitive single-celled organism that may already have possessed a membrane-enclosed nucleus.

Many of the original protoebacterial genes have been relocated to the eukaryotic cell's nucleus, where they are better insulated against mutation -- among them, the genes of the Omp85 family, which Lithgow at his colleagues discovered last year.

All bacteria are enclosed by a membrane, but many species -- including the proteobacterial ancestor of mitochondria -- possess a second, outer membrane. Lithgow says that in these dual-membrane species, Omp85 proteins appear to have a vital role in assembling and inserting pore proteins into the external membrane.

The pore proteins form portals through which free-living bacteria import essential materials like proteins and lipids to fuel their growth, and export their own proteins and toxins.

Lithgow and two postgraduate students, Ian Gentle and Kip Gabriel, working with Dr Peter Beech from Deakin University and Dr Ross Waller from the University of British Columbia in Canada, have shown that even though the Omp85 genes now reside in the nucleus, they are still fulfilling their original role of installing pore proteins into the outer mitochondrial membrane.

Housing a cell within a cell posed problems, according to Lithgow -- not the least being how the host cell could control the orderly growth growth and replication of its new tenants.

"The trick, early on in evolution, was for the host cell to deliver some of its own protein molecules back into the bacterium to force its growth and division and form new bacteria, or, mitochondria," he said. "That way the host cell has enough power, and has enough mitochondria for every time the cell needs to divide. But for a long time we've been in the dark about how proteins and other molecules could be delivered across the membranes that encase the bacterium/mitochondria."

Lithgow and his colleagues sifted through the now-extensive genomic databases on bacteria and discovered that all free-living dual-membrane bacteria possess Omp85 genes. They include some human and animal pathogens, and 'extremophiles' like radiation-resistant Deinococcus, and heat-tolerant Thermotoga, which thrives at 75oC.

Some Omp85-endowed pathogenic bacteria, including species that cause bacterial meningitis in humans, have subverted the eukaryotic cell's outside-in pore-forming system to parasitise and kill their hosts' cells.

They secrete their own pore-forming proteins into the cytosol of the infected host cell, which link up with the host cell's own Omp85 proteins. The system causes the alien proteins to be inserted into mitochondria, where Lithgow believes they form large pores in the mitochondrial membrane.

The mitochondria's contents leak into the cytosol, and with their 'batteries' flattened, the infected cells run out of energy. They collapse and die by apoptosis, programmed cell suicide.

Lithgow et al have published their new findings in the current issue of the Journal of Cell Biology.

"Understanding how Omp85 transports protein and lipid molecules, including the bacterial 'pore proteins' into membranes will provide the knowledge we need to design therapies to block bacterial infection," Lithgow said.