Princeton scientists construct synthetic proteins that sustain life

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Princeton scientists construct synthetic proteins that sustain life

In a groundbreaking achievement that could help scientists 'build' new

biological systems, Princeton University scientists have constructed for the

first time artificial proteins that enable the growth of living cells.

The team of researchers created genetic sequences never before seen in nature,

and the scientists showed that they can produce substances that sustain life in

cells almost as readily as proteins produced by nature's own toolkit.

'What we have here are molecular machines that function quite well within a

living organism even though they were designed from scratch and expressed from

artificial genes,' said Hecht, a professor of chemistry at Princeton,

who led the research. 'This tells us that the molecular parts kit for life need

not be limited to parts - genes and proteins - that already exist in nature.'

The work, Hecht said, represents a significant advance in synthetic biology, an

emerging area of research in which scientists work to design and fabricate

biological components and systems that do not already exist in the natural

world. One of the field's goals is to develop an entirely artificial genome

composed of unique patterns of chemicals.

'Our work suggests,' Hecht said, 'that the construction of artificial genomes

capable of sustaining cell life may be within reach.'

Nearly all previous work in synthetic biology has focused on reorganising parts

drawn from natural organisms. In contrast, Hecht said, the results described by

the team show that biological functions can be provided by macromolecules that

were not borrowed from nature, but designed in the laboratory.

Although scientists have shown previously that proteins can be designed to fold

and, in some cases, catalyse reactions, the Princeton team's work represents a

new frontier in creating these synthetic proteins.

The research, which Hecht conducted with three former Princeton students and a

former postdoctoral fellow, is described in a report published online Jan. 4 in

the journal Public Library of Science ONE.

Hecht and the students in his lab study the relationship between biological

processes on the molecular scale and processes at work on a larger magnitude.

For example, he is studying how the errant folding of proteins in the brain can

lead to Alzheimer's disease, and is involved in a search for compounds to thwart

that process. In work that relates to the new paper, Hecht and his students also

are interested in learning what processes drive the routine folding of proteins

on a basic level - as proteins need to fold in order to function - and why

certain key sequences have evolved to be central to existence. Proteins are the

workhorses of organisms, produced from instructions encoded into cellular DNA.

The identity of any given protein is dictated by a unique sequence of 20

chemicals known as amino acids. If the different amino acids can be viewed as

letters of an alphabet, each protein sequence constitutes its own unique

'sentence.'

And, if a protein is 100 amino acids long (most proteins are even longer), there

are an astronomically large number of possibilities of different protein

sequences, Hecht said. At the heart of his team's research was to question how

there are only about 100,000 different proteins produced in the human body, when

there is a potential for so many more. They wondered, are these particular

proteins somehow special? Or might others work equally well, even though

evolution has not yet had a chance to sample them?

Hecht and his research group set about to create artificial proteins encoded by

genetic sequences not seen in nature. They produced about 1 million amino acid

sequences that were designed to fold into stable three-dimensional structures.

'What I believe is most intriguing about our work is that the information

encoded in these artificial genes is completely novel - it does not come from,

nor is it significantly related to, information encoded by natural genes, and

yet the end result is a living, functional microbe,' said Fisher, a

co-author of the paper who earned his Ph.D. at Princeton in 2010 and is now a

postdoctoral fellow at the University of California-Berkeley. 'It is perhaps

analogous to taking a sentence, coming up with brand new words, testing if any

of our new words can take the place of any of the original words in the

sentence, and finding that in some cases, the sentence retains virtually the

same meaning while incorporating brand new words.'

Once the team had created this new library of artificial proteins, they inserted

those proteins into various mutant strains of bacteria in which certain natural

genes previously had been deleted. The deleted natural genes are required for

survival under a given set of conditions, including a limited food supply. Under

these harsh conditions, the mutant strains of bacteria died - unless they

acquired a life-sustaining novel protein from Hecht's collection. This was

significant because formation of a bacterial colony under these selective

conditions could occur only if a protein in the collection had the capacity to

sustain the growth of living cells.

In a series of experiments exploring the role of differing proteins, the

scientists showed that several different strains of bacteria that should have

died were rescued by novel proteins designed in the laboratory. 'These

artificial proteins bear no relation to any known biological sequences, yet they

sustained life,' Hecht said.

Added Kara McKinley, also a co-author and a 2010 Princeton graduate who is now a

Ph.D. student at the Massachusetts Institute of Technology: 'This is an exciting

result, because it shows that unnatural proteins can sustain a natural system,

and that such proteins can be found at relatively high frequency in a library

designed only for structure.'