Ogunribido, D. | bioRxiv Science

bioRxiv2025

Stage-specific regulation of the Plasmodium falciparum proteasome activity reveals adaptive rewiring in artemisinin resistance

The ubiquitin-proteasome system (UPS) is essential for Plasmodium falciparum to maintain protein homeostasis, adapt to proteotoxic stress, and regulate parasite growth and stage transitions. The proteolytic 20S proteasome core is the central component of the UPS, where unfolded protein substrates are degraded into oligopeptides. Mechanisms regulating malaria parasite proteasome activity are poorly understood and have not been thoroughly studied. This knowledge gap is especially critical in the context of artemisinin (ART) resistance, where parasite survival depends on an enhanced stress response, including a greater reliance on the UPS. Here, we profiled proteasome activity and abundance across the parasite intraerythrocytic developmental cycle (IDC) in both ART-sensitive (ART-S) Dd2 and ART-resistant (ART-R) Dd2K13R539T parasites. We uncovered striking stage-specific regulation: proteasome activity was abundant in the ring stage, decreased in trophozoites, and then peaked in schizonts. Furthermore, ART-R Dd2K13R539T parasites exhibited higher ring-stage proteasome activity than ART-S Dd2, despite reduced proteasome abundance, suggesting a unique adaptive rewiring of proteasome function. To study proteasome regulation in the parasite, we manipulated proteasome abundance in Dd2 and Dd2K13R539T by creating a conditional knockdown of PfUMP1, a conserved proteasome maturation factor. PfUMP1 depletion disrupted 20S assembly, decreased proteasome activity, and led to parasite death. These experiments uncovered two key features of proteasome regulation in P. falciparum: (1) the absence of canonical transcriptional regulation of proteasome genes in response to downregulation of proteasome activity, and (2) ART-R parasites exhibit a ring-stage specific increased sensitivity to proteasome downregulation. Together, our findings reveal a previously unrecognized layer of proteasome regulation in malaria parasites and how, as part of their survival adaptations to decreased hemoglobin uptake associated with ART resistance, these parasites alter proteasome function to survive. This work reinforces the therapeutic potential of the proteasome as a stage- and resistance-specific antimalarial target.

microbiology↗

bioRxiv2024

Defining the conformational states that enable transglutaminase 2 to promote cancer cell survival versus cell death

Transglutaminase 2 (TG2) is a GTP-binding/protein-crosslinking enzyme that has been investigated as a therapeutic target for Celiac disease, neurological disorders, and aggressive cancers. TG2 has been suggested to adopt two conformational states that regulate its functions: a GTP-bound, closed conformation, and a calcium-bound, crosslinking-active open conformation. TG2 mutants that constitutively adopt an open conformation are cytotoxic to cancer cells. Thus, small molecules that maintain the open conformation of TG2 could offer a new therapeutic strategy. Here, we investigate TG2, using static and time-resolved small-angle X-ray scattering (SAXS) and single-particle cryoelectron microscopy (cryo-EM), to determine the conformational states responsible for conferring its biological effects. We also describe a newly developed TG2 inhibitor, LM11, that potently kills glioblastoma cells and use SAXS to investigate how LM11 affects the conformational states of TG2. Using SAXS and cryo-EM, we show that guanine nucleotide-bound TG2 adopts a monomeric closed conformation while calcium-bound TG2 assumes an open conformational state that can form higher order oligomers. SAXS analysis also suggests how a TG2 mutant that constitutively adopts the open state binds nucleotides through an alternative mechanism to wildtype TG2. Furthermore, we use time-resolved SAXS to show that LM11 increases the ability of calcium to drive TG2 to an open conformation, which is not reversible by guanine nucleotides and is cytotoxic to cancer cells. Taken together, our findings demonstrate that the conformational dynamics of TG2 are more complex than previously suggested and highlight how conformational stabilization of TG2 by LM11 maintains TG2 in a cytotoxic conformational state. Significance StatementThe multi-functional protein transglutaminase 2 (TG2) undergoes large conformational changes in response to nucleotide and calcium binding, resulting in diverse cellular effects that can differentially promote either cancer cell survival or cell death. Previous biochemical and structural characterizations have revealed that TG2 primarily adopts two conformational states, a closed nucleotide-bound conformation, and an open calcium-bound conformation. In this study, we use advanced structural methods to describe the conformational changes associated with TG2 activation and inhibition and define the mechanism by which small molecule inhibitors maintain TG2 in a structural state that kill cancer cells.

biophysics↗