PA28

PF3D7_0907700 (PA28) is a proteasome activator that protects Plasmodium falciparum against proteotoxic stress by stimulating proteasomal peptide degradation (PMID:31384003). It assembles into a bell-shaped heptamer whose inner pore shows strong charge segregation, with disordered loops extending from the heptamer to surround the pore (PMID:31384003). This heptamer caps one or both ends of the Pf20S proteasome core, engaging the core asymmetrically to form singly and doubly capped complexes (PMID:31384003), and it associates specifically with the 20S assembly rather than the 19S regulatory particle or the 26S/30S proteasome (PMID:34155201). Functionally, PF3D7_0907700 stimulates Pf20S peptidase activity in a ubiquitin- and ATP-independent manner (PMID:31384003), and molecular dynamics based on the cryo-EM structure indicates a conformationally flexible, leaky PA28–Pf20S interface that could permit lateral egress of short degradation products (PMID:31384003). Genetic deletion of PF3D7_0907700 sensitizes parasites to dihydroartemisinin, linking this activity to defense against proteotoxic stress in vivo (PMID:31384003).

1. 2019 High

   The architecture of PF3D7_0907700 was unknown; solving its structure established that it forms a bell-shaped heptameric activator with a charge-segregated pore flanked by disordered loops.

   Evidence X-ray crystallography combined with SAXS on purified PfPA28

   PMID:31384003

   Open questions at the time

   • Function of the disordered loops surrounding the pore not resolved structurally
   • Stoichiometry of subunit assembly in vivo not directly addressed
2. 2019 High

   How PF3D7_0907700 engages the proteasome was unknown; cryo-EM of the PA28–Pf20S complex showed it caps one or both ends of the 20S core and binds asymmetrically, defining the mode of proteasome engagement.

   Evidence Single-particle cryo-EM of singly (3.9 Å) and doubly (3.8 Å) capped complexes

   PMID:31384003

   Open questions at the time

   • Structural basis of asymmetric engagement not mechanistically explained
   • Regulation of singly vs doubly capped states in vivo unknown
3. 2019 High

   The functional consequence of PA28 binding was untested; in vitro reconstitution showed PF3D7_0907700 stimulates Pf20S peptidase activity without ubiquitin or ATP, establishing it as a bona fide proteasome activator.

   Evidence In vitro peptidase activity assay with purified PfPA28 and Pf20S

   PMID:31384003

   Open questions at the time

   • Physiological substrates degraded via PA28-Pf20S not identified
   • Whether activation is substrate-selective not established
4. 2019 Medium

   The mechanism of product release was unknown; MD simulations from the cryo-EM structure proposed a flexible, leaky interface allowing lateral egress of short peptides.

   Evidence Molecular dynamics simulation informed by cryo-EM structure

   PMID:31384003

   Open questions at the time

   • Simulation-based proposal not experimentally validated independently
   • Direct demonstration of lateral peptide transfer lacking
5. 2019 High

   The biological role of PA28 was unknown; genetic knockout sensitized parasites to dihydroartemisinin, linking PF3D7_0907700 to defense against proteotoxic stress in vivo.

   Evidence Genetic knockout with dihydroartemisinin IC50 drug sensitivity assay

   PMID:31384003

   Open questions at the time

   • Molecular pathway connecting PA28 loss to DHA hypersensitivity not defined
   • Role across developmental stages beyond the assayed condition unclear
6. 2021 Medium

   It was unclear whether PA28 associates with the 19S/26S machinery; complexome profiling placed PF3D7_0907700 exclusively with the 20S assembly, confirming it is a dedicated 20S cap rather than a 26S component.

   Evidence Complexome profiling (native PAGE with mass spectrometry) of asexual blood-stage lysates

   PMID:34155201

   Open questions at the time

   • Whether PA28 and 19S can co-occupy the same 20S core (hybrid particles) not addressed
   • Stage- or condition-dependent complex composition not resolved

• The endogenous substrate repertoire and the signals governing PA28 recruitment to Pf20S under proteotoxic stress remain undefined.
• No physiological substrates identified
• Regulatory triggers for PA28-Pf20S assembly unknown
• Lateral egress mechanism awaits experimental validation