Stress granules (SGs) are ribonucleoprotein condensates assembled in all eukaryotic cells in response to stress. SGs are pivotal during the adaptive cellular response, post-stress recovery, signaling, and metabolic rewire. When stress conditions abate, SGs disassemble and release their components to the cytoplasm to reactivate translation and reassume plant growth and development. SG formation mostly depends on the liquid-liquid phase separation (LLPS) properties of protein-containing low-complexity and RNA-binding domains. In Arabidopsis, the OLIGOURIDYLATE BINDING PROTEIN 1C has been reported as an SG component; however, understanding the function of the UBP1C protein needs to be clarified. The UBP1C protein comprises an N-terminus low-complexity polyglutamine [poly glutamine (Q)] repeat and three RNA recognition motives (RRMs). To uncover the molecular basis of the UBP1C SG formation, we used quantitative microscopy. First, we analyzed the dynamics of SGs of the UBP1C wild-type version compared to numerous mutant variants using both in vitro and in vivo approaches. We observed that the UBP1C protein undergoes LLPS in vitro. For in vivo studies, we complemented an ubp1c-1 mutant background with the different UBP1C-derived versions fused to mCherry under the control of the UBQ10 promoter. We observed that the UBP1C protein formed SG after 30 min of heat shock (42ºC). Deleting the polyQ repeats reduced condensate size; in contrast, extended polyQ length correlated with increased UBP1C SGs. Interestingly, changing glutamine to glycine (QtoG) in the low-complexity domain did not affect the size but affected the disassembly properties of SGs. FRAP measurements revealed a liquid-like dynamic for wild-type UBP1C granules. In contrast, versions of SGs where polyQ repeat of UBP1C was manipulated showed more solid-like properties. Lastly, the QtoG variant was slightly less liquid-like than the wild type. These results indicate that the polyQ repeat influences the dynamics of UBP1C SGs and probably heat resistance in Arabidopsis.
