Figure 4.

Protein age. The age of a protein is here estimated from the age of its domains. Domains may be found in: eukaryotes and prokaryotes (Ancient); eukaryotes (Euk); or yeast. Domains and proteins that lack homologs are called orphan domains (ODs) and orphan proteins (OPs). The age of a single domain protein is equal to the age of its composing domain, whereas each domain family represented in a multi-domain protein contributes equally to its age classification. Furthermore, each protein contributes equally to the age of its connectivity group. Hence, a two-domain protein may be half ancient and half eukaryotic. The figure shows fractions of proteins, that is, party hubs (PHs), date hubs (DHs) and non-hubs (NHs) in each age class in DIP and FYI.