First person – Bin Yu

ABSTRACT First Person is a series of interviews with the first authors of a selection of papers published in Journal of Cell Science, helping early-career researchers promote themselves alongside their papers. Bin Yu is first author on ‘ SUMO proteases SENP3 and SENP5 spatiotemporally regulate the kinase activity of Aurora A’, published in JCS. Bin is a PhD graduate in the lab of Jing Yi at Shanghai Jiao Tong University School of Medicine, Shanghai, China, investigating protein post-translational modifications in cell cycle control.

SUMO proteases SENP3 and SENP5 spatiotemporally regulate the kinase activity of Aurora A

Precise chromosome segregation is mediated by a well-assembled mitotic spindle, which requires balance of the kinase activity of Aurora A (AurA). However, how this kinase activity is regulated remains largely unclear. Here, using in vivo and in vitro assays, we report that conjugation of SUMO2 with AurA at K258 in early mitosis promotes the kinase activity of AurA and facilitates the binding with its activator, Bora. Knockdown of the SUMO proteases SENP3 and SENP5 disrupted the deSUMOylation of AurA, leading to an increased kinase activity and abnormalities in spindle assembly and chromosomes segregation which could be rescued by suppressing the kinase activity of AurA. Collectively, these results demonstrate that SENP3 and SENP5 deSUMOylate AurA to render a spatiotemporal control on its kinase activity in mitosis.

SUMO-Targeted Ubiquitin Ligases and Their Functions in Maintaining Genome Stability

Small ubiquitin-like modifier (SUMO)-targeted E3 ubiquitin ligases (STUbLs) are specialized enzymes that recognize SUMOylated proteins and attach ubiquitin to them. They therefore connect the cellular SUMOylation and ubiquitination circuits. STUbLs participate in diverse molecular processes that span cell cycle regulated events, including DNA repair, replication, mitosis, and transcription. They operate during unperturbed conditions and in response to challenges, such as genotoxic stress. These E3 ubiquitin ligases modify their target substrates by catalyzing ubiquitin chains that form different linkages, resulting in proteolytic or non-proteolytic outcomes. Often, STUbLs function in compartmentalized environments, such as the nuclear envelope or kinetochore, and actively aid in nuclear relocalization of damaged DNA and stalled replication forks to promote DNA repair or fork restart. Furthermore, STUbLs reside in the same vicinity as SUMO proteases and deubiquitinases (DUBs), providing spatiotemporal control of their targets. In this review, we focus on the molecular mechanisms by which STUbLs help to maintain genome stability across different species.

SUMO enables substrate selectivity by mitogen-activated protein kinases to regulate immunity in plants

The versatility of mitogen-activated protein kinases (MAPKs) in translating exogenous and endogenous stimuli into appropriate cellular responses depends on its substrate specificity. In animals, several mechanisms have been proposed about how MAPKs maintain specificity to regulate distinct functional pathways. However, little is known of mechanisms that enable substrate selectivity in plant MAPKs. Small ubiquitin-like modifier (SUMO), a posttranslational modification system, plays an important role in plant development and defense by rapid reprogramming of cellular events. In this study we identified a functional SUMO interaction motif (SIM) in Arabidopsis MPK3 and MPK6 that reveals a mechanism for selective interaction of MPK3/6 with SUMO-conjugated WRKY33, during defense. We show that WRKY33 is rapidly SUMOylated in response to Botrytis cinerea infection and flg22 elicitor treatment. SUMOylation mediates WRKY33 phosphorylation by MPKs and consequent transcription factor activity. Disruption of either WRKY33 SUMO or MPK3/6 SIM sites attenuates their interaction and inactivates WRKY33-mediated defense. However, MPK3/6 SIM mutants show normal interaction with a non-SUMOylated form of another transcription factor, SPEECHLESS, unraveling a role for SUMOylation in differential substrate selectivity by MPKs. We reveal that the SUMO proteases, SUMO PROTEASE RELATED TO FERTILITY1 (SPF1) and SPF2 control WRKY33 SUMOylation and demonstrate a role for these SUMO proteases in defense. Our data reveal a mechanism by which MPK3/6 prioritize molecular pathways by differentially selecting substrates using the SUMO–SIM module during defense responses.

The protein modifier SUMO is critical for Arabidopsis shoot meristem maintenance at warmer ambient temperatures

Short heat waves (>37°C) are extremely damaging to non-acclimated plants and their capacity to recover from heat stress is key for their survival. To acclimate, the HEAT SHOCK TRANSCRIPTION FACTOR A1 (HSFA1) subfamily activates a transcriptional response that resolves incurred damages. In contrast, little is known how plants acclimate to sustained non-detrimental warm periods at 27-28°C. Plants respond to this condition with a thermomorphogenesis response. In addition, HSFA1 is critical for plant survival during these warm periods. We find that SUMO, a protein modification whose conjugate levels rise sharply during acute heat stress in eukaryotes, is critical too for plant longevity during warm periods, in particular for normal shoot meristem development. The known SUMO ligases were not essential to endure these warm periods, alone or in combination. Thermo-lethality was also not seen when plants lacked certain SUMO proteases or when SUMO chain formation was blocked. The SUMO-dependent thermo-resilience was as well independent of the autoimmune phenotype of the SUMO mutants. As acquired thermotolerance was normal in the sumo1/2 knockdown mutant, our data thus reveal a role for SUMO in heat acclimation that differs from HSFA1 and SIZ1. We conclude that SUMO is critical for shoot meristem integrity during warm periods.HighlightThe protein modifier SUMO governs shoot meristem maintenance in Arabidopsis allowing sustained rosette development when plants endure a sustained warm non-detrimental period of 28 degrees Celsius.

An Insight into the Factors Influencing Specificity of the SUMO System in Plants

Due to their sessile nature, plants are constantly subjected to various environmental stresses such as drought, salinity, and pathogen infections. Post-translational modifications (PTMs), like SUMOylation, play a vital role in the regulation of plant responses to their environment. The process of SUMOylation typically involves an enzymatic cascade containing the activation, (E1), conjugation (E2), and ligation (E3) of SUMO to a target protein. Additionally, it also requires a class of SUMO proteases that generate mature SUMO from its precursor and cleave it off the target protein, a process termed deSUMOylation. It is now clear that SUMOylation in plants is key to a plethora of adaptive responses. How this is achieved with an extremely limited set of machinery components is still unclear. One possibility is that novel SUMO components are yet to be discovered. However, current knowledge indicates that only a small set of enzymes seem to be responsible for the modification of a large number of SUMO substrates. It is yet unknown where the specificity lies within the SUMO system. Although this seems to be a crucial question in the field of SUMOylation studies, not much is known about the factors that provide specificity. In this review, we highlight the role of the localisation of SUMO components as an important factor that can play a vital role in contributing to the specificity within the process. This will introduce a new facet to our understanding of the mechanisms underlying such a dynamic process.

Phylogenetic distribution and evolutionary dynamics of nod and T3SS genes in the genus Bradyrhizobium

Bradyrhizobium are abundant soil bacteria and the major symbiont of legumes. The recent availability of Bradyrhizobium genome sequences provides a large source of information for analysis of symbiotic traits. In this study, we investigated the evolutionary dynamics of the nodulation genes (nod) and their relationship with the genes encoding type III secretion systems (T3SS) and their effectors among bradyrhizobia. Based on the comparative analysis of 146 Bradyrhizobium genome sequences, we identified six different types of T3SS gene clusters. The two predominant cluster types are designated RhcIa and RhcIb and both belong to the RhcI-T3SS family previously described in other rhizobia. They are found in 92/146 strains, most of them also containing nod genes. RhcIa and RhcIb gene clusters differ in the genes they carry: while the translocon-encoding gene nopX is systematically found in strains containing RhcIb, the nopE and nopH genes are specifically conserved in strains containing RhcIa, suggesting that these last two genes might functionally substitute nopX and play a role related to effector translocation. Phylogenetic analysis suggests that bradyrhizobia simultaneously gained nod and RhcI-T3SS gene clusters via horizontal transfer or subsequent vertical inheritance of a symbiotic island containing both. Sequence similarity searches for known Nop effector proteins in bradyrhizobial proteomes revealed the absence of a so-called core effectome, i.e. that no effector is conserved among all Bradyrhizobium strains. However, NopM and SUMO proteases were found to be the main effector families, being represented in the majority of the genus. This study indicates that bradyrhizobial T3SSs might play a more significant symbiotic role than previously thought and provides new candidates among T3SS structural proteins and effectors for future functional investigations.

Targeted mutagenesis of the SUMO protease, Overly Tolerant to Salt1 in rice through CRISPR/Cas9-mediated genome editing reveals a major role of this SUMO protease in salt tolerance

SUMO proteases are encoded by a large gene family in rice and are a potential source of specificity within the SUMO system that is responding to different environmental cues. We previously demonstrated a critical role of OsOTS class of SUMO proteases in salt and drought stress in rice by silencing several family members collectively via RNAi methods. However, to date it has not been possible to assign a role to specific family members due to limitations of RNAi mediated off target silencing across several members of the gene family. The clustered regularly interspaced short palindromic repeats (CRISPR)-associated endonuclease 9 (CRISPR/Cas9) system has emerged as a promising technology for specific gene editing in crop plants. Here, we demonstrate targeted mutagenesis of OsOTS1 in rice using the CRISPR/Cas9 gene editing system in the rice cultivar Kitaake. Guide RNA mediated mutations in OsOTS1 was highly efficient with almost 95% of T0 transgenics showing the desired effect with no off-target mutations. The OsOTS1 mutations observed in T0 generation were heritable in subsequent generations. OsOTS1 CRISPR lines show enhanced sensitivity to salt with reduced root and shoot biomass indicating that OsOTS1 has a major role in salt stress tolerance in rice. This unexpected finding indicates that precise and effective genome editing can be used to characterise specificity within the SUMO system in rice.