Modularity in Protein Evolution: Modular Organization and De Novo Domain Evolution in Mollusk Metallothioneins

Abstract

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 Molluscs. We have shown that most molluscan MTs are bi-modular proteins that combine six domains –α, β1, β2, β3, γ and δ– in a lineage specific manner. We have functionally characterized the Neritimorpha β3β1 and the Patellogastropoda γβ1 MTs, demonstrating the metal-binding capacity of the new γ domain. Our results have revealed a modular organization of mollusc 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.

The Enigmatic Metallothioneins: A Case of Upward-Looking Research

“…The high negative charge of the unique metal-sulfur clusters, such as the Zn 4 Cys 11 -α-clusters and Zn 3 Cys 9 -β-clusters in MT-1 and MT-2, is balanced by seven lysine residues, resulting in the equally unique dumbbell shape of the MT proteins which sets them apart from many other biomolecules found in nature [ 38 , 39 ]. Other primary structures of MTs comprising diverse Cys content and coordination, hence varying in metal load and affinities, have been documented [ 40 , 41 , 42 ].…”

“…Still, the widespread prevalence of MTs across organisms and their unique biochemical properties suggest that MTs serve, or at least have served, as we shall elaborate below, some important biological role(s). Recent studies have reported a divergence in the function(s) of MTs based on the type of cysteine motifs in invertebrates and vertebrates [ 40 , 47 ]. The new γ domain identified in Patellogastropoda, for instance, favors cadmium over zinc and is more resilient against demetallation [ 40 ].…”

“…Recent studies have reported a divergence in the function(s) of MTs based on the type of cysteine motifs in invertebrates and vertebrates [ 40 , 47 ]. The new γ domain identified in Patellogastropoda, for instance, favors cadmium over zinc and is more resilient against demetallation [ 40 ]. Thus, the γ domain may be alluding to a specialized metal detoxification function in this natural group.…”

“…Thus, the γ domain may be alluding to a specialized metal detoxification function in this natural group. It should also be noted that a role for the noncoordinating amino acids is also being recognized, such as in the case of Nerita peloronta MT1 and MT2 (NpeMT1 and NpeMT2) [ 40 , 48 ]. Yet, while the evolutionary forces and developments may be understood better in evolutionarily older and less complex organisms, even here the jury is still out with respect to both the precise function(s) of MTs and their evolutionary history (e.g., [ 41 , 49 ]).…”

“…Maybe these functions are somehow linked to metabolizing metals and that is what makes the sulfur-rich metalloproteins such a good candidate molecule for the jobs in question. Contemporary research on invertebrate MTs actually lends support to these evolutionary considerations [ 40 , 42 , 49 ]. Genetic and biochemical data from mollusks suggests that MTs might have played crucial roles in metal detoxification throughout early evolution.…”