Role of the Cytosolic Heat Shock Protein 70 Ssa5 in the Ciliate Protozoan Tetrahymena thermophila

Abstract

Heat shock protein 70 (Hsp70) is a member of a family of conserved chaperone proteins whose function is well investigated in many model organisms. Here we focus on an Hsp70 called Ssa5 in the ciliate protozoan Tetrahymena thermophila, and reveal that its translation is heat inducible as for general Hsps. Moreover, the protein is abundantly expressed in the cytoplasm during sexual reproduction (conjugation) as well as in response to heat-stress. Knocking out of SSA5 (ΔSSA5) does not affect the survival of the cell under heat-stress, likely due to other Hsp70 paralogs compensating for the defect. During conjugation, ΔSSA5 leads to a fertilization defect in which the two pronuclei are in close proximity but never fuse. The unfertilized pronuclei differentiate, resulting in a heterokaryon with developed haploid germline and somatic nuclei. In addition, degeneration of the parental somatic nucleus is not affected. These results suggest a specific involvement of Ssa5 in pronuclear fusion and fertilization.

Sex enhances survival inParamecium

“…For one, HSP with reproductive functions (HSP R ) might help activate HSP with stress-related functions (HSP S ) upon exposure to environmental stress, benefitting either the individuals that undergo sexual reproduction, or their resulting offspring, or both. This hypothesis is consistent with observations in the ciliate Tetrahymena thermophila, where the cytosolic HSP70 SSA5 chaperone has a crucial role in pronuclear fusion during fertilization and is also expressed in response to heat stress-without directly contributing to cell survival however [43,44]. In a non-mutually exclusive fashion, the expression of HSP R might also enhance survival by reducing sensitivity to environmental alterations.…”

“…Among these genes, Hsp70Pt04 has alongside Hsp70Pt01 and Hsp70Pt03 the highest level of sequence similarity to the ciliate Tetrahymena thermophila's HSP70 SSA5 (Figure S3). SSA5 is a key protein for pronuclear fusion during fertilization and is also expressed during development and in response to heat-stress [43]. Similarly, SSA5 homologs in P. tetraurelia are expressed throughout development ( Figure S3) and are inducible after heat shock exposure (Hsp70Pt03 and Hsp70Pt04; [73]) or up-regulated after a 3week long period of starvation (Hsp70Pt01; [74]).…”

“…Members of the HSP70 family are required for the process of meiosis: HSP70 is key to the formation of the synaptonemal complex in vertebrates [38][39][40] and in protozoans [41]. Furthermore, HSP70 is required for fertilization in vertebrates [42] and protozoans [43]. Because of the strong molecular ties that link HSP to sexual reproduction, if sex-related intracellular processes can truly enhance fitness in the context of environmental alterations, then HSP with reproductive functions may contribute to the advantage of sex.…”

Post-meiotic DNA double-strand breaks occur in Tetrahymena, and require Topoisomerase II and Spo11

“…Interestingly, when BER is compromised, γH2AX foci are formed, suggesting that DSBs are induced, which are repaired after G 1 (Wossidlo et al, 2010; Ladstätter and Tachibana-Konwalski, 2016). Thus, while mammals require karyogamy to induce the epigenetic modifications necessary for embryonic developmental programming (Zhou and Dean, 2015), Tetrahymena gametic nuclei can differentiate into MACs and MICs without karyogamy (Fukuda et al, 2015). Therefore, chromatin remodeling in Tetrahymena pronuclei might resemble zygote reprogramming rather than mammalian male pronuclear chromatin remodeling, in that the differentiated germline pronucleus is re-set to a dedifferentiated progenitor of new somatic and germline nuclei.…”

Effect of heat shock on the cell cycle duration of algae-containing and algae-free ciliates Climacostomum virens

“…It was also shown that in the ciliate Tetrahymena thermophila the heat shock protein Ssa5 is abundantly expressed in response to heat stress as well as during sexual reproduction. A specific involvement of a heat shock protein in pronuclei fusion during conjugation was suggested (Fukuda et al, 2015).…”