What we know currently about the Metalloproteins in the protozoa Tetrahymena pyriformis and thermophila.

Metals are fundamental for the correct functioning of cells, playing an integral role in metabolic pathways and processes. Although, their excess is toxic and the metal availability should be tightly controlled [1-3]. Moreover, metalloproteins are widespread in living organisms. In particular, the average constitution of zinc proteins in prokaryotic organisms is (6.0% and 4.9% in archaea and bacteria respectively) which is lower in eukaryotic organisms (8.8%) [4,5]. Furthermore, proteome level analyses of the occurrence of nonheme iron proteins have shown that they constitute on average 7.1% of archaeal, 3,9% of bacterial and only of 1.1% of eukaryotic proteomes [4]. Copper proteins are less pervasive than zinc and nonheme iron proteins and typically account for less than 1% of an organism’s proteome [6].

 Tetrahymena is a non-pathogenic unicellular, free-living mobile ciliate protozoan that responds rapidly with great sensitivity to the presence of pollutants in nature, special metal toxicity [7]. This has resulted in them being used as test systems for assessing ecological risk [8]. Recently, the concept of “whole-cell biosensor” (WCB) has been introduced by several authors [9,10], as a very useful alternative to classical biosensors. Both prokaryotic and eukaryotic microorganisms have been used to design WCBs for metals [11,12]. Among eukaryotic microorganisms, ciliates offer specific advantages as environmental sensors: they do not have a cell wall in their vegetative stage, minimizing the sensitivity to environmental pollutants as well as delay the cell response [13]. Recently, Tetrahymena pyriformis and thermophila have been used to design WCBs to detect heavy metals in aquatic or soil samples [14-18]. In these WCB modules, a quantifiable molecular reporter is fused to specific gene promoter of metalloprotein, known to be activated by metals inducing their overexpression. 

This article attempt to summarize the current state of knowledge of identified metalloproteins (binders of essential and not essential metals) in Tetrahymena pyriformis and thermophila, that are reviewed and manually annotated in the protein database of UniProtKB/Swiss-Prot. Furthermore, the new role of Tetrahymena as a potential whole-cell biosensor for monitoring heavy metal pollution, through the overexpression of metalloprotein targets, is discussed.