Regulation of inositol 5-phosphatase activity by the C2 domain of SHIP1 and SHIP2

SHIP1, an inositol 5-phosphatase, plays a central role in cellular signaling. As such, it has been implicated in many conditions. Exploiting SHIP1 as a drug target will require structural knowledge and the design of selective small molecules. We have determined apo, and magnesium and phosphate-bound structures of the phosphatase and C2 domains of SHIP1. The C2 domains of SHIP1 and the related SHIP2 modulate the activity of the phosphatase domain. To understand the mechanism, we performed activity assays, hydrogen-deuterium exchange mass spectrometry, and molecular dynamics on SHIP1 and SHIP2. Our findings demonstrate that the influence of the C2 domain is more pronounced for SHIP2 than SHIP1. We determined 91 structures of SHIP1 with fragments bound, with some near the interface between the two domains. We performed a mass spectrometry screen and determined four structures with covalent fragments. These structures could act as starting points for the development of potent, selective probes.

Characterization of covalent inhibitors that disrupt the interaction between the tandem SH2 domains of SYK and FCER1G phospho-ITAM

RNA sequencing and genetic data support spleen tyrosine kinase (SYK) and high affinity immunoglobulin epsilon receptor subunit gamma (FCER1G) as putative targets to be modulated for Alzheimer's disease (AD) therapy. FCER1G is a component of Fc receptor complexes that contain an immunoreceptor tyrosine-based activation motif (ITAM). SYK interacts with the Fc receptor by binding to doubly phosphorylated ITAM (p-ITAM) via its two tandem SH2 domains (SYK-tSH2). Interaction of the FCER1G p-ITAM with SYK-tSH2 enables SYK activation via phosphorylation. Since SYK activation is reported to exacerbate AD pathology, we hypothesized that disruption of this interaction would be beneficial for AD patients. Herein, we developed biochemical and biophysical assays to enable the discovery of small molecules that perturb the interaction between the FCER1G p-ITAM and SYK-tSH2. We identified two distinct chemotypes using a high-throughput screen (HTS) and orthogonally assessed their binding. Both chemotypes covalently modify SYK-tSH2 and inhibit its interaction with FCER1G p-ITAM, however, these compounds lack selectivity and this limits their utility as chemical tools.

Discovery of FERM domain protein-protein interaction inhibitors for MSN and CD44 as a potential therapeutic approach for Alzheimer's disease

Proteomic studies have identified moesin (MSN), a protein containing a four-point-one, ezrin, radixin, moesin (FERM) domain, and the receptor CD44 as hub proteins found within a coexpression module strongly linked to Alzheimer's disease (AD) traits and microglia. These proteins are more abundant in Alzheimer's patient brains, and their levels are positively correlated with cognitive decline, amyloid plaque deposition, and neurofibrillary tangle burden. The MSN FERM domain interacts with the phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2) and the cytoplasmic tail of CD44. Inhibiting the MSN-CD44 interaction may help limit AD-associated neuronal damage. Here, we investigated the feasibility of developing inhibitors that target this protein-protein interaction. We have employed structural, mutational, and phage-display studies to examine how CD44 binds to the FERM domain of MSN. Interestingly, we have identified an allosteric site located close to the PIP2 binding pocket that influences CD44 binding. These findings suggest a mechanism in which PIP2 binding to the FERM domain stimulates CD44 binding through an allosteric effect, leading to the formation of a neighboring pocket capable of accommodating a receptor tail. Furthermore, high-throughput screening of a chemical library identified two compounds that disrupt the MSN-CD44 interaction. One compound series was further optimized for biochemical activity, specificity, and solubility. Our results suggest that the FERM domain holds potential as a drug development target. Small molecule preliminary leads generated from this study could serve as a foundation for additional medicinal chemistry efforts with the goal of controlling microglial activity in AD by modifying the MSN-CD44 interaction.

Characterization of covalent inhibitors that disrupt the interaction between the tandem SH2 domains of SYK and FCER1G phospho-ITAM

RNA sequencing and genetic data support spleen tyrosine kinase (SYK) and high affinity immunoglobulin epsilon receptor subunit gamma (FCER1G) as putative targets to be modulated for Alzheimer's disease (AD) therapy. FCER1G is a component of Fc receptor complexes that contain an immunoreceptor tyrosine-based activation motif (ITAM). SYK interacts with the Fc receptor by binding to doubly phosphorylated ITAM (p-ITAM) via its two tandem SH2 domains (SYK-tSH2). Interaction of the FCER1G p-ITAM with SYK-tSH2 enables SYK activation via phosphorylation. Since SYK activation is reported to exacerbate AD pathology, we hypothesized that disruption of this interaction would be beneficial for AD patients. Herein, we developed biochemical and biophysical assays to enable the discovery of small molecules that perturb the interaction between the FCER1G p-ITAM and SYK-tSH2. We identified two distinct chemotypes using a high-throughput screen (HTS) and orthogonally assessed their binding. Both chemotypes covalently modify SYK-tSH2 and inhibit its interaction with FCER1G p-ITAM.

Development of FERM domain protein-protein interaction inhibitors for MSN and CD44 as a potential therapeutic strategy for Alzheimer's disease

Recent genome-wide association studies have revealed genetic risk factors for Alzheimer's disease (AD) that are exclusively expressed in microglia within the brain. A proteomics approach identified moesin (MSN), a FERM (four-point-one ezrin radixin moesin) domain protein, and the receptor CD44 as hub proteins found within a co-expression module strongly linked to AD clinical and pathological traits as well as microglia. The FERM domain of MSN interacts with the phospholipid PIP2 and the cytoplasmic tails of receptors such as CD44. This study explored the feasibility of developing protein-protein interaction inhibitors that target the MSN-CD44 interaction. Structural and mutational analyses revealed that the FERM domain of MSN binds to CD44 by incorporating a beta strand within the F3 lobe. Phage-display studies identified an allosteric site located close to the PIP2 binding site in the FERM domain that affects CD44 binding within the F3 lobe. These findings support a model in which PIP2 binding to the FERM domain stimulates receptor tail binding through an allosteric mechanism that causes the F3 lobe to adopt an open conformation permissive for binding. High-throughput screening of a chemical library identified two compounds that disrupt the MSN-CD44 interaction, and one compound series was further optimized for biochemical activity, specificity, and solubility. The results suggest that the FERM domain holds potential as a drug development target. The small molecule preliminary leads generated from the study could serve as a foundation for additional medicinal chemistry effort with the goal of controlling microglial activity in AD by modifying the MSN-CD44 interaction.

FAS2FURIOUS: Moderate-Throughput Secreted Expression of Difficult Recombinant Proteins in Drosophila S2 Cells

Recombinant protein expression in eukaryotic insect cells is a powerful approach for producing challenging targets. However, due to incompatibility with standard baculoviral platforms and existing low-throughput methodology, the use of the Drosophila melanogaster “S2” cell line lags behind more common insect cell lines such as Sf9 or High-Five™. Due to the advantages of S2 cells, particularly for secreted and secretable proteins, the lack of a simple and parallelizable S2-based platform represents a bottleneck, particularly for biochemical and biophysical laboratories. Therefore, we developed FAS2FURIOUS, a simple and rapid S2 expression pipeline built upon an existing low-throughput commercial platform. FAS2FURIOUS is comparable in effort to simple E. coli systems and allows users to clone and test up to 46 constructs in just 2 weeks. Given the ability of S2 cells to express challenging targets, including receptor ectodomains, secreted glycoproteins, and viral antigens, FAS2FURIOUS represents an attractive orthogonal approach for protein expression in eukaryotic cells.

Rec8 phosphorylation by casein kinase 1 and Cdc7-Dbf4 kinase regulates cohesin cleavage by separase during meiosis

During meiosis, two rounds of chromosome segregation after a single round of DNA replication produce haploid gametes from diploid precursors. At meiosis I, maternal and paternal kinetochores are pulled toward opposite poles, and chiasmata holding bivalent chromosomes together are resolved by cleavage of cohesin's alpha-kleisin subunit (Rec8) along chromosome arms. This creates dyad chromosomes containing a pair of chromatids joined solely by cohesin at centromeres that had resisted cleavage. The discovery that centromeric Rec8 is protected from separase during meiosis I by shugoshin/MEI-S332 proteins that bind PP2A phosphatase suggests that phosphorylation either of separase or cohesin may be necessary for Rec8 cleavage. We show here that multiple phosphorylation sites within Rec8 as well as two different kinases, casein kinase 1delta/epsilon (CK1delta/epsilon) and Dbf4-dependent Cdc7 kinase (DDK), are required for Rec8 cleavage and meiosis I nuclear division. Rec8 with phosphomimetic mutations is no longer protected from separase at centromeres and is cleaved even when the two kinases are inhibited. Our data suggest that PP2A protects centromeric cohesion by opposing CK1delta/epsilon- and DDK-dependent phosphorylation of Rec8.

Building sister chromatid cohesion: smc3 acetylation counteracts an antiestablishment activity

Cohesin's Smc1, Smc3, and Scc1 subunits form a tripartite ring that entraps sister DNAs. Scc3, Pds5, and Rad61 (Wapl) are regulatory subunits that control this process. We describe here smc3, scc3, pds5, and rad61 mutations that permit yeast cell proliferation and entrapment of sister DNAs by cohesin rings in the absence of Eco1, an acetyl transferase normally essential for establishing sister chromatid cohesion. The smc3 mutations cluster around and include a highly conserved lysine (K113) close to Smc3's ATP-binding pocket, which, together with K112, is acetylated by Eco1. Lethality caused by mutating both residues to arginine is suppressed by the scc3, pds5, and rad61 mutants. Scc3, Pds5, and Rad61 form a complex and inhibit entrapment of sister DNAs by a process involving the "K112/K113" surface on Smc3's ATPase. According to this model, Eco1 promotes sister DNA entrapment partly by relieving an antiestablishment activity associated with Scc3, Pds5, and Rad61.

Thermodynamic Analysis of Catalysis by the Dihydroorotases from Hamster and Bacillus caldolyticus, As Compared with the Uncatalyzed Reaction

Dihydroorotase (DHOase, EC 3.5.2.3) from the extreme thermophile Bacillus caldolyticus has been subcloned, sequenced, expressed, and purified as a monomer. The catalytic properties of this thermophilic DHOase have been compared with another type I enzyme, the DHOase domain from hamster, to investigate how the thermophilic enzyme is adapted to higher temperatures. B. caldolyticus DHOase has higher Vmax and Ks values than hamster DHOase at the same temperature. The thermodynamic parameters for the binding of L-dihydroorotate were determined at 25 degrees C for hamster DHOase (deltaG = -6.9 kcal/mol, deltaH = -11.5 kcal/mol, TdeltaS = -4.6 kcal/mol) and B. caldolyticus DHOase (deltaG = -5.6 kcal/mol, deltaH = -4.2 kcal/mol, TdeltaS = +1.4 kcal/mol). The smaller enthalpy release and positive entropy for thermophilic DHOase are indicative of a weakly interacting Michaelis complex. Hamster DHOase has an enthalpy of activation of 12.3 kcal/mol, similar to the release of enthalpy upon substrate binding, rendering the kcat/Ks value almost temperature independent. B. caldolyticus DHOase shows a decrease in the enthalpy of activation from 12.2 kcal/mol at temperatures from 30 to 50 degrees C to 5.3 kcal/mol for temperatures of 50-70 degrees C. Vibrational energy at higher temperatures may facilitate the transition ES --> ES(double dagger), making kcat/Ks almost temperature independent. The pseudo-first-order rate constant for water attack on L-dihydroorotate, based on experiments at elevated temperature, is 3.2 x 10(-11) s(-1) at 25 degrees C, with deltaH(double dagger) = 24.7 kcal/mol and TdeltaS(double dagger) = -6.9 kcal/mol. Thus, hamster DHOase enhances the rate of substrate hydrolysis by a factor of 1.6 x 10(14), achieving this rate enhancement almost entirely by lowering the enthalpy of activation (delta deltaH(double dagger) = -19.5 kcal/mol). Both the rate enhancement and transition state affinity of hamster DHOase increase steeply with decreasing temperature, consistent with the development of H-bonds and electrostatic interactions in the transition state that were not present in the enzyme-substrate complex in the ground state.

Protein phosphatase 2A protects centromeric sister chromatid cohesion during meiosis I

Segregation of homologous maternal and paternal centromeres to opposite poles during meiosis I depends on post-replicative crossing over between homologous non-sister chromatids, which creates chiasmata and therefore bivalent chromosomes. Destruction of sister chromatid cohesion along chromosome arms due to proteolytic cleavage of cohesin's Rec8 subunit by separase resolves chiasmata and thereby triggers the first meiotic division. This produces univalent chromosomes, the chromatids of which are held together by centromeric cohesin that has been protected from separase by shugoshin (Sgo1/MEI-S332) proteins. Here we show in both fission and budding yeast that Sgo1 recruits to centromeres a specific form of protein phosphatase 2A (PP2A). Its inactivation causes loss of centromeric cohesin at anaphase I and random segregation of sister centromeres at the second meiotic division. Artificial recruitment of PP2A to chromosome arms prevents Rec8 phosphorylation and hinders resolution of chiasmata. Our data are consistent with the notion that efficient cleavage of Rec8 requires phosphorylation of cohesin and that this is blocked by PP2A at meiosis I centromeres.

Spo13 facilitates monopolin recruitment to kinetochores and regulates maintenance of centromeric cohesion during yeast meiosis

BACKGROUND

Cells undergoing meiosis perform two consecutive divisions after a single round of DNA replication. During the first meiotic division, homologous chromosomes segregate to opposite poles. This is achieved by (1) the pairing of maternal and paternal chromosomes via recombination producing chiasmata, (2) coorientation of homologous chromosomes such that sister chromatids attach to the same spindle pole, and (3) resolution of chiasmata by proteolytic cleavage by separase of the meiotic-specific cohesin Rec8 along chromosome arms. Crucially, cohesin at centromeres is retained to allow sister centromeres to biorient at the second division. Little is known about how these meiosis I-specific events are regulated.

RESULTS

Here, we show that Spo13, a centromere-associated protein produced exclusively during meiosis I, is required to prevent sister kinetochore biorientation by facilitating the recruitment of the monopolin complex to kinetochores. Spo13 is also required for the reaccumulation of securin, the persistence of centromeric cohesin during meiosis II, and the maintenance of a metaphase I arrest induced by downregulation of the APC/C activator CDC20.

CONCLUSION

Spo13 is a key regulator of several meiosis I events. The presence of Spo13 at centromere-surrounding regions is consistent with the notion that it plays a direct role in both monopolin recruitment to centromeres during meiosis I and maintenance of centromeric cohesion between the meiotic divisions. Spo13 may also limit separase activity after the first division by ensuring securin reaccumulation and, in doing so, preventing precocious removal from chromatin of centromeric cohesin.

Cdc14 phosphatase induces rDNA condensation and resolves cohesin-independent cohesion during budding yeast anaphase

At anaphase onset, the protease separase triggers chromosome segregation by cleaving the chromosomal cohesin complex. Here, we show that cohesin destruction in metaphase is sufficient for segregation of much of the budding yeast genome, but not of the long arm of chromosome XII that contains the rDNA repeats. rDNA in metaphase, unlike most other sequences, remains in an undercondensed and topologically entangled state. Separase, concomitantly with cleaving cohesin, activates the phosphatase Cdc14. We find that Cdc14 exerts two effects on rDNA, both mediated by the condensin complex. Lengthwise condensation of rDNA shortens the chromosome XII arm sufficiently for segregation. This condensation depends on the aurora B kinase complex. Independently of condensation, Cdc14 induces condensin-dependent resolution of cohesin-independent rDNA linkage. Cdc14-dependent sister chromatid resolution at the rDNA could introduce a temporal order to chromosome segregation.

Maintenance of cohesin at centromeres after meiosis I in budding yeast requires a kinetochore-associated protein related to MEI-S332

BACKGROUND

The halving of chromosome number that occurs during meiosis depends on three factors. First, homologs must pair and recombine. Second, sister centromeres must attach to microtubules that emanate from the same spindle pole, which ensures that homologous maternal and paternal pairs can be pulled in opposite directions (called homolog biorientation). Third, cohesion between sister centromeres must persist after the first meiotic division to enable their biorientation at the second.

RESULTS

A screen performed in fission yeast to identify meiotic chromosome missegregation mutants has identified a conserved protein called Sgo1 that is required to maintain sister chromatid cohesion after the first meiotic division. We describe here an orthologous protein in the budding yeast S. cerevisiae (Sc), which has not only meiotic but also mitotic chromosome segregation functions. Deletion of Sc SGO1 not only causes frequent homolog nondisjunction at meiosis I but also random segregation of sister centromeres at meiosis II. Meiotic cohesion fails to persist at centromeres after the first meiotic division, and sister centromeres frequently separate precociously. Sgo1 is a kinetochore-associated protein whose abundance declines at anaphase I but, nevertheless, persists on chromatin until anaphase II.

CONCLUSIONS

The finding that Sgo1 is localized to the centromere at the time of the first division suggests that it may play a direct role in preventing the removal of centromeric cohesin. The similarity in sequence composition, chromosomal location, and mutant phenotypes of sgo1 mutants in two distant yeasts with that of MEI-S332 in Drosophila suggests that these proteins define an orthologous family conserved in most eukaryotic lineages.

Polo-like kinase Cdc5 promotes chiasmata formation and cosegregation of sister centromeres at meiosis I

During meiosis, two rounds of chromosome segregation occur after a single round of DNA replication, producing haploid progeny from diploid progenitors. Three innovations in chromosome behaviour during meiosis I accomplish this unique division. First, crossovers between maternal and paternal sister chromatids (detected cytologically as chiasmata) bind replicated maternal and paternal chromosomes together. Second, sister kinetochores attach to microtubules from the same pole (mono-polar orientation), causing maternal and paternal centromere pairs (and not sister chromatids) to be separated. Third, sister chromatid cohesion near centromeres is preserved at anaphase I when cohesion along chromosome arms is destroyed. The finding that destruction of mitotic cohesion is regulated by Polo-like kinases prompted us to investigate the meiotic role of the yeast Polo-like kinase Cdc5. We show here that cells lacking Cdc5 synapse homologues and initiate recombination normally, but fail to efficiently resolve recombination intermediates as crossovers. They also fail to properly localize the Lrs4 (ref. 3) and Mam1 (ref. 4) monopolin proteins, resulting in bipolar orientation of sister kinetochores. Cdc5 is thus required both for the formation of chiasmata and for cosegregation of sister centromeres at meiosis I.

Septal localization of the membrane-bound division proteins of Bacillus subtilis DivIB and DivIC is codependent only at high temperatures and requires FtsZ

Using immunofluorescence microscopy, we have examined the dependency of localization among three Bacillus subtilis division proteins, FtsZ, DivIB, and DivIC, to the division site. DivIC is required for DivIB localization. However, DivIC localization is dependent on DivIB only at high growth temperatures, at which DivIB is essential for division. FtsZ localization is required for septal recruitment of DivIB and DivIC, but FtsZ can be recruited independently of DivIB. These localization studies suggest a more specific role for DivIB in division, involving interaction with DivIC.

Membrane-bound division proteins DivIB and DivIC of Bacillus subtilis function solely through their external domains in both vegetative and sporulation division

The Bacillus subtilis membrane-bound division proteins, DivIB and DivIC, each contain a single transmembrane segment flanked by a short cytoplasmic N-terminal domain and a larger external C-terminal domain. Both proteins become localized at the division site prior to septation. Mutagenesis of both divIB and divIC was performed whereby the sequences encoding the cytoplasmic domains were replaced by the corresponding sequence of the other gene. Finally, the cytoplasmic-plus-transmembrane-encoding domain of each protein was replaced by a totally foreign sequence not involved in division, that encodes the N-terminal-plus-transmembrane domains of the Escherichia coli TolR protein. B. subtilis strains expressing the divIB and divIC hybrids, in the absence of the wild-type gene, were viable when grown under conditions in which the wild-type genes were found previously to be essential. Furthermore, these strains were able to sporulate to near normal levels. Thus, the cytoplasmic and transmembrane segments of DivIB and DivIC do not appear to have any specific functions other than to anchor these proteins correctly in the membrane. The implications of these findings are discussed.

Characterization of the essential cell division gene ftsL(yIID) of Bacillus subtilis and its role in the assembly of the division apparatus

We have identified the Bacillus subtilis homologue of the essential cell division gene, ftsL, of Escherichia coli. Repression of ftsL in a strain engineered to carry a conditional promoter results in cell filamentation, with a near immediate arrest of cell division. The filaments show no sign of invagination, indicating that division is blocked at an early stage. FtsL is also shown to be required for septation during sporulation, and depletion of FtsL blocks the activation but not the synthesis of the prespore-specific sigma factor, sigmaF. Immunofluorescence microscopy shows that depletion of FtsL has little or no effect on FtsZ ring formation, but the assembly of other division proteins, DivIB and DivIC, at the site of division is prevented. Repression of FtsL also results in a rapid loss of DivIC protein, indicating that DivIC stability is dependent on the presence of FtsL, in turn suggesting that FtsL is intrinsically unstable. The instability of one or more components of the division apparatus may be important for the cyclic assembly/disassembly of the division apparatus.

The Bacillus subtilis division protein DivIC is a highly abundant membrane-bound protein that localizes to the division site

The Bacillus subtilis divIC gene is involved in the initiation of cell division. It encodes a 14.7 kDa protein, with a potential transmembrane region near the N-terminus. In this paper, we show that DivIC is associated with the cell membrane and, in conjunction with previously published sequence data, conclude that it is oriented such that its small N-terminus is within the cytoplasm and its larger C-terminus is external to the cytoplasm. DivIC is shown to be a highly abundant division protein, present at approximately 50000 molecules per cell. Using immunofluorescence microscopy, DivIC was seen to localize at the division site of rapidly dividing cells between well-segregated nucleoids. Various DivIC immunostaining patterns were observed, and these correlated with different cell lengths, suggesting that the DivIC localization takes on various forms during the cell cycle. The DivIC immunolocalization patterns are very similar to those of another membrane-bound B. subtilis division protein, DivIB.

DivIB, FtsZ and cell division in Bacillus subtilis

The Bacillus subtilis cell-division protein DivIB is shown to be present at an approximately 100-fold higher abundance (approximately 5000 molecules per cell) than its Escherichia coli FtsQ homologue. B. subtilis contains much more DivIB (at least 60-fold) than is needed to maintain the normal rate of cell division at moderate temperatures (up to 37 degrees C). However, a high level of DivIB is needed to achieve the normal rate of division at high temperature (47 degrees C). It is proposed that membrane-bound DivIB is involved in stabilizing or promoting the assembly of the division complex (which is intrinsically temperature sensitive) in a manner that requires more of the protein at higher temperatures. The (at least) 60-fold accumulation of DivIB and FtsZ from an undetectable level, following germination and outgrowth of spores up until the stage of the first cell division, was unaffected by blocking of initiation of the first round of replication. It is concluded that there is no major synthesis of either of these 'division initiation' proteins linked to initiation, progression or completion of the first round of replication accompanying spore outgrowth.