HOP co-chaperones contribute to GA signaling by promoting the accumulation of the F-box protein SNE in Arabidopsis

Gibberellins (GAs) play important roles in multiple developmental processes and in plant response to the environment. Within the GA pathway, a central regulatory step relies on GA-dependent degradation of the DELLA transcriptional regulators. Nevertheless, the relevance of the stability of other key proteins in this pathway, such as SLY1 and SNE (the F-box proteins involved in DELLA degradation), remains unknown. Here, we take advantage of mutants in the HSP70-HSP90 organizing protein (HOP) co-chaperones and reveal that these proteins contribute to the accumulation of SNE in Arabidopsis. Indeed, HOP proteins, along with HSP90 and HSP70, interact in vivo with SNE, and SNE accumulation is significantly reduced in the hop mutants. Concomitantly, greater accumulation of the DELLA protein RGA is observed in these plants. In agreement with these molecular phenotypes, hop mutants show a hypersensitive response to the GA inhibitor paclobutrazol and display a partial response to the ectopic addition of GA when GA-regulated processes are assayed. These mutants also display different phenotypes associated with alterations in the GA pathway, such as reduced germination rate, delayed bolting, and reduced hypocotyl elongation in response to warm temperatures. Remarkably, ectopic overexpression of SNE reverts the delay in germination and the thermally dependent hypocotyl elongation defect of the hop1 hop2 hop3 mutant, revealing that SNE accumulation is the key aspect of the hop mutant phenotypes. Together, these data reveal a pivotal role for HOP in SNE accumulation and GA signaling.

Native Mass Spectrometry-Guided Screening Identifies Hit Fragments for HOP-HSP90 PPI Inhibition

Contemporary medicinal chemistry considers fragment-based drug discovery (FBDD) and inhibition of protein-protein interactions (PPI) as important means of expanding the volume of druggable chemical space. However, the ability to robustly identify valid fragments and PPI inhibitors is an enormous challenge, requiring the application of sensitive biophysical methodology. Accordingly, in this study, we exploited the speed and sensitivity of nanoelectrospray (nano-ESI) native mass spectrometry to identify a small collection of fragments which bind to the TPR2AB domain of HOP. Follow-up biophysical assessment of a small selection of binding fragments confirmed binding to the single TPR2A domain, and that this binding translated into PPI inhibitory activity between TPR2A and the HSP90 C-terminal domain. An in-silico assessment of binding fragments at the PPI interfacial region, provided valuable structural insight for future fragment elaboration strategies, including the identification of losartan as a weak, albeit dose-dependent inhibitor of the target PPI.

The co-chaperone HOP participates in TIR1 stabilisation and in auxin response in plants

HOP (HSP70-HSP90 organising protein) is a conserved family of co-chaperones well known in mammals for its role in the folding of signalling proteins associated with development. In plants, HOP proteins have been involved in the response to multiple stresses, but their role in plant development remains elusive. Herein, we describe that the members of the HOP family participate in different aspects of plant development as well as in the response to warm temperatures through the regulation of auxin signalling. Arabidopsis hop1 hop2 hop3 triple mutant shows different auxin-related phenotypes and a reduced auxin sensitivity. HOP interacts with TIR1 auxin coreceptor in vivo. Furthermore, TIR1 accumulation and auxin transcriptional response are reduced in the hop1 hop2 hop3 triple mutant, suggesting that HOP's function in auxin signalling is related, at least, to TIR1 interaction and stabilisation. Interestingly, HOP proteins form part of the same complexes as SGT1b (a different HSP90 co-chaperone) and these co-chaperones synergistically cooperate in auxin signalling. This study provides relevant data about the role of HOP in auxin regulation in plants and uncovers that both co-chaperones, SGT1b and HOP, cooperate in the stabilisation of common targets involved in plant development.

HOP family plays a major role in long-term acquired thermotolerance in Arabidopsis

HSP70-HSP90 organizing protein (HOP) is a family of cytosolic cochaperones whose molecular role in thermotolerance is quite unknown in eukaryotes and unexplored in plants. In this article, we describe that the three members of the AtHOP family display a different induction pattern under heat, being HOP3 highly regulated during the challenge and the attenuation period. Despite HOP3 is the most heat-regulated member, the analysis of the hop1 hop2 hop3 triple mutant demonstrates that the three HOP proteins act redundantly to promote long-term acquired thermotolerance in Arabidopsis. HOPs interact strongly with HSP90 and part of the bulk of HOPs shuttles from the cytoplasm to the nuclei and to cytoplasmic foci during the challenge. RNAseq analyses demonstrate that, although the expression of the Hsf targets is not generally affected, the transcriptional response to heat is drastically altered during the acclimation period in the hop1 hop2 hop3 triple mutant. This mutant also displays an unusual high accumulation of insoluble and ubiquitinated proteins under heat, which highlights the additional role of HOP in protein quality control. These data reveal that HOP family is involved in different aspects of the response to heat, affecting the plant capacity to acclimate to high temperatures for long periods.

HOP3 a new regulator of the ER stress response in Arabidopsis with possible implications in plant development and response to biotic and abiotic stresses

HOPs (heat shock protein 70 (HSP70)-heat shock protein 90 (HSP90) organizing proteins) are a highly conserved family of cytosolic cochaperones. In a recent study we showed that HOP3, a member of the HOP family in Arabidopsis, plays an essential role during endoplasmic reticulum (ER) stress in plants. Interestingly, we also demonstrated that AtHOP3 interacts with binding immunoglobulin protein (BiP), a major ER-resident chaperone. All these data suggest that HOP3 could assist BiP in protein folding in the ER. These findings open the exciting possibility that HOP3, through its role in the alleviation of ER stress, could play an important function during different developmental processes and in response to different biotic and abiotic stresses.