(TKP2021-NVA-19) Támogató: Innovációs és Technológiai Minisztérium
Assemblysomes are EDTA- and RNase-resistant ribonucleoprotein (RNP) complexes of paused
ribosomes with protruding nascent polypeptide chains. They have been described in
yeast and human cells for the proteasome subunit Rpt1, and the disordered N-terminal
part of the nascent chain was found to be indispensable for the accumulation of the
Rpt1-RNP into assemblysomes. Motivated by this, to find other assemblysome-associated
RNPs we used bioinformatics to rank subunits of Saccharomyces cerevisiae protein complexes
according to their N-terminal disorder propensity. The results revealed that gene
products involved in DNA repair are enriched among the top candidates. The Sgs1 DNA
helicase was chosen for experimental validation. We found that indeed nascent chains
of Sgs1 form EDTA-resistant RNP condensates, assemblysomes by definition. Moreover,
upon exposure to UV, SGS1 mRNA shifted from assemblysomes to polysomes, suggesting
that external stimuli are regulators of assemblysome dynamics. We extended our studies
to human cell lines. The BLM helicase, ortholog of yeast Sgs1, was identified upon
sequencing assemblysome-associated RNAs from the MCF7 human breast cancer cell line,
and mRNAs encoding DNA repair proteins were overall enriched. Using the radiation-resistant
A549 cell line, we observed by transmission electron microscopy that 1,6-hexanediol,
an agent known to disrupt phase-separated condensates, depletes ring ribosome structures
compatible with assemblysomes from the cytoplasm of cells and makes the cells more
sensitive to X-ray treatment. Taken together these findings suggest that assemblysomes
may be a component of the DNA damage response from yeast to human.
