| Proteomes in Different Organisms |
Although scientists have sequenced dozens of genomes from organisms as diverse as viruses, bacteria, nematode, fruit fly, puffer fish, mouse, and human, we still don't know what uniquely characterizes each of these organisms. For example, both mouse and human genomes contain around 30,000 genes. How many of these genes do they share? Based on comparisons of the two genomes, ninety-nine percent of the genes are conserved in both species and are, thus, derived from a common evolutionary ancestor. The remaining one percent represents genes that evolved independently in mouse or human. If these two organisms share so many similar genes, how can they be so different? A simple example may help us to understand that the presence of a gene does not mean that the protein is expressed. Pigs produce cell surface proteins, which are modified by glycosylation to contain a sugar called galactose (GAL). Those GAL-proteins, present in pig blood vessels, are seen as foreign by the human immune system. This leads to the very rapid destruction of pig organs that have been transplanted into humans when a human organ was not available. Interestingly, humans lack GAL-proteins but still have the gene for making them; the gene is not expressed in humans. Therefore, the presence of a gene does not mean that it is expressed. In fact, every somatic cell in an organism shares the same genes; so, the differences between tissue types - say liver and heart - result from differences in gene expression. (See the Genetics of Development unit.)
Identification of proteins may provide the most useful information in determining the significant differences between species. How different are the proteins in even closely related organisms? With the development of proteomic techniques, scientists are beginning to tackle this difficult question. One answer is that very similar genes in two organisms may be expressed very differently. Dr. Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology analyzed the proteins from brains of human and chimps. (See the Genomics and Human Evolution units.) He found that many, very similar genes produced much more protein in human brain cells than in chimp brain cells. In contrast, the same type of experiment done with blood or liver cells showed much less difference between human and chimp in the amount of protein produced..
At a different level, there are some clear differences in protein composition between the cells of eukaryotes and those of the other kingdoms. One is that eukaryotes have many more long proteins, more proteins with regular secondary structure and less random globular structure, and more loop regions in their proteins. Certain conserved structural domains show up in proteins, but are used in a number of different pathways. While there are many protein homologues conserved across many different organisms, some proteins are unique to one organism. As more genomes and proteomes are characterized, comparative genomics and proteomics will allow scientists to further understand how organisms differ.