Ciliate metallothioneins: unique microbial eukaryotic heavy-metal-binder molecules

Abstract

This article represents an updated review of ciliate metallothioneins (Tetrahymena species) including a comparative analysis with regard to well-known metallothioneins (MTs) from other organisms and discussion of their exclusive features. It opens with an introduction to ciliates, summarizing the main characteristics of these eukaryotic microorganisms and their use as cellular models to study metallothioneins and metal-eukaryotic cell interactions. It has been experimentally proved that at least three different metal resistance mechanisms exist in ciliates, of which bioaccumulation is the most studied. Structural comparative analysis reveals that Tetrahymena MTs have unique characteristics, such as longer length, a considerably higher cysteine content, different metal-MT stoichiometry values, the presence of new cysteine clusters, and a strictly conserved modular-submodular structure. Gene expression analysis reveals a multistress and differential response to diverse metals and other environmental stressors, which corroborates the classification of these MTs. An in silico analysis of the promoter sequences of some MT genes reveals the presence of conserved motifs that are probably involved in gene expression regulation. We also discuss the great advantages of the first ciliate whole-cell biosensors based on MT promoters from Tetrahymena thermophila to detect heavy metal ions in environmental samples.

Functional GFP-metallothionein fusion protein from Tetrahymena thermophila: a potential whole-cell biosensor for monitoring heavy metal pollution and a cell model to study metallothionein overproduction effects

“…The ciliate T. thermophila has five MT isoforms; three CdMTs ( MTT1, MTT3 and MTT5 ) and two CuMTs ( MTT2 and MTT4 ) (Díaz et al, 2007; Gutierrez et al, 2009; 2011). The CdMT genes are expressed at very different levels in response to metal stress (Díaz et al, 2007).…”

Molecular Characterization of a Copper Metallothionein Gene From a Ciliate Tetrahymena farahensis

“…These proteins are short of aromatic amino acids, which has facilitated the detailed optical spectroscopic studies to observe metal charge transfer transition using circular dichroism, electron absorption, and emission [Stillman, ; Vasak and Bogumil, ]. Histidine (His) residue is very infrequent in MT polypeptide chain, whereas positively charged amino acids Arg and Lys also have asymmetry in MTs [Gutierrez et al, ]. Interaction between metal ions and MT is complex one and it requires a series of reactions before reaching a thermodynamically stable stage.…”

“…Presence of intron can delay regulatory responses [Jeffares et al, ]. Ciliate MTs are quite larger than vertebrate MTs so with higher molecular weight [Gutierrez et al, ]. Binz and Kagi [], on the basis of phylogenetic relationships, designated MTs as superfamily and divided it into 15 families representing different groups.…”

“…Cysteine (Cys) residues are arranged in the form of clusters within primary structure of MT. In the case of CuMT, only typical MT clusters, that is, CXC are commonly observed [Gutierrez et al, ]. Subfamily 7b (CuMT) does not have clear motifs like subfamily 7a but have CXCX 2‐5 CXC repeats [Amaro et al, ].…”

Cd‐Metallothioneins in Three Additional Tetrahymena Species: Intragenic Repeat Patterns and Induction by Metal Ions

“…A comprehensive review of Tetrahymena MTs, and their comparison with the well‐known vertebrate MTs, was recently published (Gutierrez et al. ). Unfortunately, the available information on Tetrahymena CdMT genes has been limited to a small number of species.…”