How many protein molecules in a single cell? Go on, guess

The question “How many angels can dance on the head of a pin?” has been a popular example of an absurd, unanswerable question since – some scholars suggest – the time of the Emperor Constantine.

As a symbol of a pointless debating point it has been bettered – other scholars would aver – only once, in 1992, when Australian folk singer Paul Kelly answered it in kind with “How many tears in a bottle of gin?”.

Other questions, however, that seem at first blush to fall within the same category sometimes turn out to be enquiries of an altogether different nature. Consider, for instance, this one: How many protein molecules in a single cell?

The answer, it turns out, is a remarkably specific 42 million.

At least, it is if the cell in question belongs to yeast.

To make the finding, a team led by Grant Brown from the University of Toronto in Canada combined the data from just under two dozen large studies looking at protein abundance in yeast cells.

Yeast is the only organism that has been studied often and thoroughly enough to produce sufficient information to permit large scale analysis of the molecular make-up of cell proteins – but that didn’t mean the job of Brown’s team was simply one of mathematics.

Protein abundance has been studied in many different contexts. In some cases, it has been reported piecemeal, focussed only one a subset of targets. In others – especially when the protein count has been estimated by means of fluorescent tagging – differences in recording equipment quality and tolerances means there is little uniformity between studies.

“It was hard to conceptualise how many proteins there are in the cell because the data was reported on drastically different scales,” says co-author Brandon Ho.

Every yeast cell contains 6000 proteins. Brown and colleagues faced the daunting task of taking each one and finding out how many molecules they contained.

In the end, they discovered that most of the proteins fell into a fairly narrow range, containing between 1000 and 10,000 molecules. A few were enormous – topping half a million molecules – while at the other end of the scale some contained fewer than 10.

The study, published in the journal Cell Systems, isn’t merely a work of curiosity – a question perhaps for a molecular biology round at a pub trivia quiz – but potentially the key to some very serious research.

Techniques and insight from this study will likely lead to new attempts to quantify protein molecules in human cells – and thus to insights into the mechanisms involved in protein-implicated diseases. 

Try doing that with a pin and bottle of Tanqueray.

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