HEUL

PF3D7_0826100 (HEUL) is an active HECT E3 ubiquitin ligase in Plasmodium falciparum that participates in ubiquitin-dependent regulation of the intraerythrocytic developmental cycle (PMID:40249735, PMID:31797898). Its enzymatic activity was established directly: HEUL is captured by a ubiquitin activity-based (Ub-Dha) probe and catalyzes ubiquitination in vitro, and it selectively engages a subset of both human and P. falciparum E2 ubiquitin-conjugating enzymes (PMID:40249735). HEUL is a functional target of the phosphatase PfPP1 in the context of parasite egress from erythrocytes, linking its regulation to phospho-dependent control of the erythrocytic cycle (PMID:32669539). Perturbing HEUL expression has cellular consequences: overexpression lengthens the S/M phase of the intraerythrocytic cycle and reduces merozoite invasion efficiency, and it modulates quinine/quinidine sensitivity in a genetic-background-dependent manner (PMID:31797898, PMID:24830312). In P. yoelii, the ortholog Pyheul controls parasite growth and host mortality through allelic variation and expression level (PMID:28790316). Beyond these findings, the physiological substrates of HEUL and the mechanistic basis of its PfPP1 regulation have not been characterized in the available corpus.

1. 2014 Medium

   Before any biochemical characterization, it was unknown whether this HECT ligase had a functional role in the parasite; QTL-guided overexpression established a direct, if moderate, contribution to antimalarial drug response.

   Evidence Episomal overexpression with quinine/quinidine IC50 assays and QTL analysis across genetic backgrounds in P. falciparum

   PMID:24830312

   Open questions at the time

   • Effect was background-dependent and modest
   • No mechanistic link between ubiquitin ligase activity and drug sensitivity established
   • No substrate identified
2. 2017 Medium

   To test whether the ligase influences parasite fitness in vivo, the P. yoelii ortholog was mapped and manipulated, showing Pyheul regulates parasite growth and host mortality.

   Evidence Genetic cross, genome-wide QTL mapping (chromosome 7, LOD=8.1), allelic exchange and expression modification in P. yoelii

   PMID:28790316

   Open questions at the time

   • Molecular mechanism connecting allelic variation to virulence unknown
   • Substrates of Pyheul not identified
   • Findings are on the ortholog, not P. falciparum HEUL directly
3. 2019 Medium

   It was unclear which cellular process HEUL affects; conditional overexpression pinned its dosage to S/M-phase cell-cycle progression and merozoite invasion efficiency.

   Evidence glmS-ribozyme conditional expression system (unexpectedly ~2.5-fold overexpression), cell-cycle staging, and invasion assays with glucosamine rescue in P. falciparum

   PMID:31797898

   Open questions at the time

   • Phenotype from overexpression, not loss-of-function
   • Catalytic mechanism not dissected
   • Substrates driving the cell-cycle/invasion phenotype unknown
4. 2020 Medium

   To place HEUL in a regulatory pathway, phosphoproteomic and chemical-genetic analysis identified it as a functional target of the phosphatase PfPP1 relevant to erythrocyte egress.

   Evidence Phosphoproteomics combined with chemical-genetic analysis of PfPP1 in P. falciparum

   PMID:32669539

   Open questions at the time

   • Direct HEUL-PfPP1 interaction not validated beyond identification
   • Phosphosites and their functional consequences on ligase activity not defined
   • Role of dephosphorylation in egress not mechanistically resolved
5. 2025 High

   Whether HEUL is a catalytically active ligase and which E2 enzymes it uses was unresolved; activity-based profiling confirmed active HECT ligase function and defined selective E2 usage.

   Evidence Ubiquitin-Dha activity-based probe capture, in vitro ubiquitination assay, and biochemical E2 interaction analysis in P. falciparum

   PMID:40249735

   Open questions at the time

   • Physiological substrates not identified
   • Structural basis of E2 selectivity unknown
   • Link between in vitro activity and cell-cycle/egress phenotypes not established

• The endogenous substrates of HEUL and the mechanistic connection between its PfPP1-regulated phosphorylation, its ligase activity, and its roles in cell-cycle progression, egress, and invasion remain undefined.
• No ubiquitination substrate identified
• No structural model of HEUL or its E2 complex
• Loss-of-function/essentiality in P. falciparum not established