Resumen de Characterization of URI1 from Arabidopsis thaliana and its role in stress responses
It is of fundamental importance for the plant to trigger the corresponding signaling cascades in response to environmental stress and to keep proteins and protein complexes active despite the cellular stress. The chaperone Hsp90 plays an important role in coordinating these two processes, although the mechanisms that regulate its activity in response to the environment are not fully understood. Recent studies in animals show that the Hsp90 co-chaperones prefoldin-like (PFDLs) play a role in environmental signaling. Therefore, they have the potential to carry information about the environment to modulate both the assembly of protein complexes as part of the Hsp90-R2TP/PFDL and the signaling pathways in which they are involved. Currently, there is little information on PFDLs in plant species. We have now accumulated evidence that PFDLs, particularly URI1, may exert a similar, general role in Arabidopsis, coordinating protein homeostasis with growth pathways in response to stress, e.g. low energy stress. Here we show that the R2TP/PFDL complex is formed in Arabidopsis and that URI1 is one of its subunits. The activity of URI1 is essential for certain processes, such as embryonic development, as evidenced by the early arrest caused by the knock-out mutation of URI1. With a hypomorphic uri1 allele, URI1 was shown to have a strong influence on the transcriptome. Consistent with this, the URI1 interactome shows that URI1 interacts with a relatively large number of partners, many of which are involved in fundamental processes related to mRNA metabolism. Thus, Arabidopsis URI1, like its orthologs in yeast and humans, appears to be involved in diverse cellular functions, including protein homeostasis, mRNA metabolism and signal transduction. URI is a highly versatile protein, although the molecular basis of this versatility is still unknown. Here we show that Arabidopsis URI1 possesses a large intrinsically disordered region spanning most of the C-terminal portion of the protein, a feature that is conserved in yeast and human orthologs. Our results reveal two main features of disordered proteins in URI1: promiscuity in interactions with partners and protein instability. We hypothesize that these two features contribute to endowing URI1 with functional versatility. Interestingly, the instability of URI1 is counteracted by sugar, and our genetic analysis places URI1 in the signaling pathway that controls growth in response to sugar deprivation-induced energy stress by acting as a negative upstream regulator of the master kinase TOR. We hypothesize that URI1 plays a role in preventing excessive seedling growth when energy conditions are favorable.
