Modularity In Protein Evolution: Modular Organization And De Novo Domain Evolution In Mollusk Metallothioneins

Metallothioneins (MTs) are proteins devoted to the control of metal homeostasis and detoxification, and therefore, MTs have been crucial for the adaptation of the living beings to variable situations of metal bioavailability. The evolution of MTs is, however, not yet fully understood, and to provide new insights into it, we have investigated the MTs in the diverse classes of Mollusks. We have shown that most molluskan MTs are bimodular proteins that combine six domains-alpha, beta 1, beta 2, beta 3, gamma, and delta-in a lineage-specific manner. We have functionally characterized the Neritimorpha beta(3)beta(1) and the Patellogastropoda gamma beta(1) MTs, demonstrating the metal-binding capacity of the new gamma domain. Our results have revealed a modular organization of mollusk MT, whose evolution has been impacted by duplication, loss, and de novo emergence of domains. MTs represent a paradigmatic example of modular evolution probably driven by the structural and functional requirements of metal binding.