THIS week sees the completion of a new atlas. The cartography in question, though, is biological, not terrestrial. The “Human Protein Atlas” consists of photographic maps, 13m of them, of tissue from all bodily organs. It shows which proteins are found where.
The maps were made by creating antibodies to 17,000 individual proteins, attaching staining molecules to those antibodies, and then applying the combination to thin slices of preserved tissue to see, by what colour the tissue went, which antibodies had stuck to it.
The result will be of great value to researchers trying to understand how tissues differ at the molecular level. Most biological functions depend on proteins, so it is the mix of proteins within a cell which defines what that cell is. The atlas will also (because 20 types of malignant tumour are included in it) help to explain how cancerous tissues differ from their healthy progenitors.
There are around 20,000 protein-coding genes in the human genome, so even though the atlas covers all parts of the anatomy, it is not yet complete. Indeed, the cellular machinery that translates the information contained in genes and uses it to construct proteins often applies tweaks to those proteins as it goes. This means there are more sorts of proteins than genes, expanding the task of mapping what is happening still further.
The researchers who created the atlas, led by Mathias Uhlen of the Royal Institute of Technology, in Stockholm, have nevertheless discovered that 3,500 human genes encode proteins specific to just one or two tissues. These are, presumably, crucial to what those tissues do. Many of these proteins are peculiar to the cerebral cortex—which is not surprising, as the brain is usually regarded as the body’s most complex component. But, proteinwise, it turns out that it is only the second most complex. The winning organ (a slice of which is pictured), with a third of those 3,500 genes, is the testis.
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