An international team of researchers and another group from the Technische Universität München (Technical University of Munich) have independently produced a nearly-complete map of the human proteome. While the human genome project sought to decode all three billion base pairs of human DNA, the human proteome project seeks to characterize the protein products produced from the 20,300 human genes. The project thus far is considered to be 84 to 90 percent complete. The results were published on May 29th in the academic journal Nature.
The international team is a collaborative effort between researchers from Johns Hopkins University and the Institute of Bioinformatics in Bangalore, India. This team has successfully identified the proteins encoded in 17,924 genes, which is 84 percent of all the genes currently understood to code for proteins in humans. Independently of them, the German team has characterized over 18,000 proteins.
Decoding the proteome is not as simple as simply reading genes. In contrast to decoding the human genome, in which every cell (except for haploid sex cells) contains an exact copy of a person’s genetic information, cells in different human tissues will express different proteins. For example, cells in the lining of the stomach and small intestine must create proteins to break-down food molecules, but do not need to produce proteins that produce color vision. Even the ubiquitous “housekeeping” proteins, which perform routine functions throughout the majority of human tissues, can vary in their abundance, thus creating important physiological changes within tissues and organs. To produce a comprehensive picture of the human proteome, the researchers sampled 30 tissues: 17 adult tissues, seven fetal tissues, and six from hematopoietic stem cells.
In order to decode an organism’s proteome, proteins are first extracted from the various chosen tissues. They are then cut up into many smaller fragments called “peptides” using cleaving enzymes. These peptides are then analyzed using mass spectrometers, which help to characterize the proteins based on composition and peptide abundance.
The “map” that was produced is a map in the sense that it cites in what tissues certain proteins are found. However, it can also be thought of as a catalog that contains information about the abundance and variants of all of the proteins that together make up a human. The catalog itself, while a tremendous feat of scientific achievement, can be thought of as just the start to a better understanding of the human body. The current research findings can be accessed for free on the Human Proteome Map’s website.
Researchers have already been surprised by some of the findings of the human proteome project. So far, investigators have identified 193 novel proteins that apparently are produced from segments of the genome hitherto thought to be non-coding regions. Furthermore, the researchers have as of yet been unable to locate approximately 2,000 proteins that, according to the genome, should exist. Perhaps these proteins simply have yet to be discovered, but it is also possible that the genes in question are un-used relics of the human evolutionary past. In either case, these surprising findings indicate that both the coding genome and the proteome are more complex than previously estimated.
By Sarah Takushi