This is just in case you spotted the headline in January 2018: ‘Scientists Counted All The Protein Molecules in a Cell And The Answer Really Is 42. This is so perfect.’
Them scientists eh! The things they get up to!! The scallywags in this case were Brandon Ho & chums from the University of Toronto and Signe Dean, the journalist who came up with the headline, was referring, of course, to Douglas Adams’s “Answer to the Ultimate Question of Life …” in The Hitchhiker’s Guide to the Galaxy — though it may be noted that Ho’s paper includes neither the number 42 nor mention of Douglas Adams.
The cult that has evolved around this number is both amusing and bizarre, not least because Adams himself explained that he dreamed 42 up out of the blue. In a different context a while ago (talking about how the way you get to work might affect your life expectancy) I recounted happy evenings spent carousing in The Baron (well, having a quiet jar or two) with Douglas Adams and friends from which it was clear that he was not into abstruse mathematics, astrology or the occult. He just had a vivid imagination.
Anything for a catchy headline but
Aside from the whimsy, is there anything interesting in this paper? Well, yes. Ho & Co studied a type of yeast (Saccharomyces cerevisiae) that is mighty important because it’s been a foundation for brewing and baking since ancient times. So no merry sessions in The Baron of Beef without it! Its cells are about the same size as red blood cells (5–10 microns in diameter) but you can actually see them sometimes as films on the skin of fruit. It’s played a huge role in biology as a ‘model organism’ for studying how we work because the proteins it makes that are essential for life are pretty well identical to those in human cells — so much so that you can swap those that control cell growth and division between the two. Yeast proteins work just fine in human cells and vice versa.
Yeast on the skin of a grape. Photo: Barbara W. Beacham
The question Ho & Co asked was ‘how many protein molecules are there in one cell?’ In the age when you can sequence the DNA of practically anything at the drop of a hat, you might think we’d know the answer already but in fact it’s not been at all clear. Accordingly, what these authors did was to pull together all the relevant studies that have been done to come up with an absolute figure. The answer that emerged was that the number of protein molecules per yeast cell is 4.2 x 107 — which, of course, can also be written as 42 million. Eureka! We have our headline!! Albeit, as the authors noted, with a two-fold error range.
Does anyone care?
Now you’re just being awkward. You should be grateful to be made to picture for a moment tens of millions of proteins jiggling around in little sacs so small you could get tens of thousands of these cells on the head of a pin. And somehow, in that heaving molecular city, each protein manages to carry out its own task so that the cell works. It is quite staggering.
Mention of tasks leads to the other question Ho et al looked at: how many copies are there of the different types of protein? We know from its DNA sequence that this yeast has about 6,000 genes (Saccharomyces Genome Database). So that’s at least 6,000 different proteins. Not surprisingly, it turns out that about two thirds of them are in the middle in terms of abundance — i.e. there’s between 1,000 and 10,000 molecules of each sort per cell. The rest are either low abundance (up to about 800 molecules per cell) or at the high end — 140,000 to 750,000, i.e. somewhere in the region of half a million copies of each type of protein.
Does this distribution make sense in terms of what these proteins do?
You know the answer because if it didn’t the Toronto team wouldn’t have got their work published but, indeed, proteins present in large numbers are, for example, part of the machinery that makes new proteins (so they’re slaving away all the time) whereas, those present in small numbers do things like repair and replicate DNA and drive cells to divide — important jobs but ones that are only intermittently needed.
These results aren’t going to turn science on its head but it is awe-inspiring when a piece of work really brings us face-to-face with stunning complexity of biology. And if it takes a bonkers headline to catch our eye, so be it!
Ho, B. et al. (2018). Unification of Protein Abundance Datasets Yields a Quantitative Saccharomyces cerevisiae Proteome. Cell Systems. Published online: January 23, 2018.