Like humans, bacteria make thrift a habit

WASHINGTON - Just as humans sometimes prefer to spend on recyclable materials over use-and-throw items, bacteria too ’spend’ more on proteins that will be used and recycled internally than on proteins that are secreted from the cell and lost to the environment, says a new study.

The bacterium Escherichia coli prefers recyclable proteins over those that cost more (in terms of energy) for the cell to produce, says the new study.

Differences in size, complexity and chemical properties make some amino acids cheaper for cells to produce than others.

Daniel R. Smith at the University of Michigan and co-author Matthew Chapman looked up previously tallied synthetic costs of all amino acids found in proteins. They used those figures to calculate the total cost of each E. coli protein; then for each protein, they divided total cost by the number of amino acid subunits in the protein to arrive at an average cost.

“When we compared all the proteins in E. coli, we found that CsgA was an outlier,” Smith said.

“It was one of the cheapest to produce,” he added.

CsgA is rich in glycine, a cheaply produced amino acid, so CsgA also should be inexpensive for bacteria to produce.

“Bacteria are secretion machines,” he said. “They’re very good at getting proteins out of the cell. But they have no import system. They’re very bad at getting proteins back in. So when they secrete proteins, they lose resources.”

The conclusion was: extracellular proteins are more economical. In fact, a protein’s location is a better predictor of its economy than its abundance, function or size.

The researchers performed similar protein economy calculations for a wide range of bacteria, as well as for yeast, and found the same trend: proteins secreted to the extracellular environment are made up of cheaper-to-produce parts than are proteins found inside the cell.

“Evolution has to balance function and cost,” Chapman said. “Function is most important-if an organism makes a cheap protein that doesn’t function efficiently, that’s a waste. But if an amino acid substitution reduces metabolic cost without affecting function, that will improve the organism’s evolutionary fitness.”

The study is published in the open access journal mBio. (ANI)