Expression and Purification of an Intrinsically Disordered Protein

Functional analyses of Toxoplasma gondii dihydroorotase reveal a promising anti-parasitic target

Toxoplasma gondii relies heavily on the de novo pyrimidine biosynthesis pathway for fueling the high uridine-5'-monophosphate (UMP) demand during parasite growth. The third step of de novo pyrimidine biosynthesis is catalyzed by dihydroorotase (DHO), a metalloenzyme that catalyzes the reversible condensation of carbamoyl aspartate to dihydroorotate. Here, functional analyses of TgDHO reveal that tachyzoites lacking DHO are impaired in overall growth due to decreased levels of UMP, and the noticeably growth restriction could be partially rescued after supplementation with uracil or high concentrations of L-dihydroorotate in vitro. When pyrimidine salvage pathway is disrupted, both DHOH35A and DHOD284E mutant strains proliferated much slower than DHO-expressing parasites, suggesting an essential role of both TgDHO His35 and Asp284 residues in parasite growth. Additionally, DHO deletion causes the limitation of bradyzoite growth under the condition of uracil supplementation or uracil deprivation. During the infection in mice, the DHO-deficient parasites are avirulent, despite the generation of smaller tissue cysts. The results reveal that TgDHO contributes to parasite growth both in vitro and in vivo. The significantly differences between TgDHO and mammalian DHO reflect that DHO can be exploited to produce specific inhibitors targeting apicomplexan parasites. Moreover, potential DHO inhibitors exert beneficial effects on enzymatic activity of TgDHO and T. gondii growth in vitro. In conclusion, these data highlight the important role of TgDHO in parasite growth and reveal that it is a promising anti-parasitic target for future control of toxoplasmosis.

SETDB1-like MET-2 promotes transcriptional silencing and development independently of its H3K9me-associated catalytic activity

Transcriptionally silenced heterochromatin bearing methylation of histone H3 on lysine 9 (H3K9me) is critical for maintaining organismal viability and tissue integrity. Here we show that in addition to ensuring H3K9me, MET-2, the Caenorhabditis elegans homolog of the SETDB1 histone methyltransferase, has a noncatalytic function that contributes to gene repression. Subnuclear foci of MET-2 coincide with H3K9me deposition, yet these foci also form when MET-2 is catalytically deficient and H3K9me is compromised. Whereas met-2 deletion triggers a loss of silencing and increased histone acetylation, foci of catalytically deficient MET-2 maintain silencing of a subset of genes, blocking acetylation on H3K9 and H3K27. In normal development, this noncatalytic MET-2 activity helps to maintain fertility. Under heat stress MET-2 foci disperse, coinciding with increased acetylation and transcriptional derepression. Our study suggests that the noncatalytic, focus-forming function of this SETDB1-like protein and its intrinsically disordered cofactor LIN-65 is physiologically relevant.

Genetic and genome-wide analysis of a catalytically deficient SETDB1-like enzyme, MET-2, in Caenorhabditiselegans reveals that MET-2 promotes transcriptional silencing and fertility through both H3K9 methylation and focus formation, which blocks histone acetylation.

Insight into Calcium-Binding Motifs of Intrinsically Disordered Proteins

Motifs within proteins help us categorize their functions. Intrinsically disordered proteins (IDPs) are rich in short linear motifs, conferring them many different roles. IDPs are also frequently highly charged and, therefore, likely to interact with ions. Canonical calcium-binding motifs, such as the EF-hand, often rely on the formation of stabilizing flanking helices, which are a key characteristic of folded proteins, but are absent in IDPs. In this study, we probe the existence of a calcium-binding motif relevant to IDPs. Upon screening several carefully selected IDPs using NMR spectroscopy supplemented with affinity quantification by colorimetric assays, we found calcium-binding motifs in IDPs which could be categorized into at least two groups-an Excalibur-like motif, sequentially similar to the EF-hand loop, and a condensed-charge motif carrying repetitive negative charges. The motifs show an affinity for calcium typically in the ~100 μM range relevant to regulatory functions and, while calcium binding to the condensed-charge motif had little effect on the overall compaction of the IDP chain, calcium binding to Excalibur-like motifs resulted in changes in compaction. Thus, calcium binding to IDPs may serve various structural and functional roles that have previously been underreported.

Polyelectrolyte interactions enable rapid association and dissociation in high-affinity disordered protein complexes

Highly charged intrinsically disordered proteins can form complexes with very high affinity in which both binding partners fully retain their disorder and dynamics, exemplified by the positively charged linker histone H1.0 and its chaperone, the negatively charged prothymosin α. Their interaction exhibits another surprising feature: The association/dissociation kinetics switch from slow two-state-like exchange at low protein concentrations to fast exchange at higher, physiologically relevant concentrations. Here we show that this change in mechanism can be explained by the formation of transient ternary complexes favored at high protein concentrations that accelerate the exchange between bound and unbound populations by orders of magnitude. Molecular simulations show how the extreme disorder in such polyelectrolyte complexes facilitates (i) diffusion-limited binding, (ii) transient ternary complex formation, and (iii) fast exchange of monomers by competitive substitution, which together enable rapid kinetics. Biological polyelectrolytes thus have the potential to keep regulatory networks highly responsive even for interactions with extremely high affinities.