There are approximately 42 million protein molecules in a simple cell, according to University of Toronto’s Professor Grant Brown and co-authors.
Yeast cells expressing proteins that carry green and red fluorescent tags to make them visible. Image credit: Brendan Ho.
Proteins make up our cells and do most of the work in them. This way, they bring genetic code to life because the recipes for building proteins are stored within genes.
“Given that the cell is the functional unit of biology, it’s just a natural curiosity to want to know what’s in there and how much of each kind,” Professor Brown said.
Although biologists have studied protein abundance for years, the findings were reported in arbitrary units, sowing confusion in the field and making it hard to compare data between different labs.
“It was hard to conceptualize how many proteins there are in the cell because the data was reported on drastically different scales,” said Brandon Ho, a graduate student at the University of Toronto.
To convert arbitrary measures into the number of molecules per cell, the team turned to a species of yeast called Saccharomyces cerevisiae.
Yeasts are also the only organism for which there was enough data available to calculate molecule number for each of the 6,000 proteins encoded by the yeast genome thanks to 21 separate studies that measured abundance of all yeast proteins.
The team’s analysis reveals for the first time how many molecules of each protein there are in the cell, with a total number of molecules estimated to be around 42 million.
The majority of proteins exist within a narrow range — between 1,000 and 10,000 molecules.
Some are outstandingly plentiful at more than half a million copies, while others exist in fewer than 10 molecules in a cell.
Analyzing the data, the authors were able to glean insights into the mechanisms by which cells control abundance of distinct proteins, paving the way for similar studies in human cells that could help reveal molecular roots of disease.
They also showed that a protein’s supply correlates with its role in the cell, which means that it may be possible to use the abundance data to predict what proteins are doing.
Finally, in a finding that will rejoice cell biologists everywhere, the team showed that the common practice of stitching glowing tags onto proteins has little effect on their abundance.
“This study will be of great value to the entire yeast community and beyond,” said Dr. Robert Nash, senior biocurator of the Saccharomyces Genome Database that will make the data available to researchers worldwide.
The results appear in the journal Cell Systems (bioRxiv.org preprint).
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Brandon Ho et al. 2018. Comparative analysis of protein abundance studies to quantify the Saccharomyces cerevisiae proteome. Cell Systems, in press;
