Abstract
BackgroundHeat-shock proteins of the 70 kDa family (Hsp70s) are essential chaperones required for key cellular functions. In eukaryotes, four subfamilies can be distinguished according to their function and localisation in different cellular compartments: cytosol, endoplasmic reticulum, mitochondria and chloroplasts. Generally, multiple cytosol-type Hsp70s can be found in metazoans that show either constitutive expression and/or stress-inducibility, arguing for the evolution of different tasks and functions. Information about the hsp70 copy number and diversity in microbial eukaryotes is, however, scarce, and detailed knowledge about the differential gene expression in most protists is lacking. Therefore, we have characterised the Hsp70 gene family of Paramecium caudatum to gain insight into the evolution and differential heat stress response of the distinct family members in protists and to investigate the diversification of eukaryotic hsp70s focusing on the evolution of heat-inducibility.ResultsEleven putative hsp70 genes could be detected in P. caudatum comprising homologs of three major Hsp70-subfamilies. Phylogenetic analyses revealed five evolutionarily distinct Hsp70-groups, each with a closer relationship to orthologous sequences of Paramecium tetraurelia than to another P. caudatum Hsp70-group. These highly diverse, paralogous groups resulted from duplications preceding Paramecium speciation, underwent divergent evolution and were subject to purifying selection. Heat-shock treatments were performed to test for differential expression patterns among the five Hsp70-groups as well as for a functional conservation within Paramecium. These treatments induced exceptionally high mRNA up-regulations in one cytosolic group with a low basal expression, indicative for the major heat inducible hsp70s. All other groups showed comparatively high basal expression levels and moderate heat-inducibility, signifying constitutively expressed genes. Comparative EST analyses for P. tetraurelia hsp70s unveiled a corresponding expression pattern, which supports a functionally conserved evolution of the Hsp70 gene family in Paramecium.ConclusionsOur analyses suggest an independent evolution of the heat-inducible cytosol-type hsp70s in Paramecium and in its close relative Tetrahymena, as well as within higher eukaryotes. This result indicates convergent evolution during hsp70 subfunctionalization and implies that heat-inducibility evolved several times during the course of eukaryotic evolution.
Highlights
Heat-shock proteins of the 70 kDa family (Hsp70s) are essential chaperones required for key cellular functions
These analyses further revealed the expression of one MT, five CY and five endoplasmic reticulum (ER) related Hsp70 proteins in P. caudatum
Even though this study showed a significant up-regulation in mRNA levels of the CY-A group members after heat shock (~3.0-fold), this does not necessarily cause the rejection of their Hsc70 affiliation, since many constitutively expressed hsp70s can be induced under specific conditions (e.g. [41,42,43,44])
Summary
Heat-shock proteins of the 70 kDa family (Hsp70s) are essential chaperones required for key cellular functions. The environmental stress response in all organisms as diverse as pro- and eukaryotes is generally coupled with a remarkable change in gene expression patterns and an enhanced synthesis of several ‘stress proteins’ [1] Because they were first described in Drosophila melanogaster larvae that were accidentally exposed to elevated temperatures [2], these stress-related proteins were called heat-shock proteins (Hsps). Extensive research on Hsps revealed a constitutive expression of some members of these proteins, suggesting that they are essential in maintaining the cellular functions under normal physiological conditions. These members are designated as heat-shock cognate proteins (Hscs) [3,4]. Hsp genes and proteins have been used for phylogenetic studies of different protozoan parasites such as Trypanosoma or Leishmania [14,15], as well as of non-parasitic protozoans such as Euplotes or Paramecium [16,17]
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