Figure 1.

Schematic representation of ubiquitin evolution from an ancestral beta-grasp fold gene. (A) Beta-grasp fold is found in multiple protein families in prokaryotes and eukaryotes. Arrows show functional diversification at the levels of: beta-grasp fold ancestor, sulphur transfer precursor and ubiquitin-type families, and boxes contain groups of proteins, as reported in (3). (B) An evolutionary tree including branches of main groups is shown, and the origin of eukaryotes is hypothetically linked to the origin of ubiquitin.