Structural basis for the selective inhibition of the PI3KC3-C2 complex by Rubicon in endolysosome maturation and mitophagy

Technical Summary: Structural Basis for the Selective Inhibition of the PI3KC3-C2 Complex by Rubicon

Problem Statement
Rubicon (RUN domain Beclin-1-interacting and cysteine-rich containing protein) is a well-characterized negative regulator of autophagy and the endolysosomal network (ELN). It functions by antagonizing the class III phosphatidylinositol 3-kinase complex II (PI3KC3-C2), which is essential for endolysosome maturation and the production of phosphatidylinositol 3-phosphate (PI(3)P). A critical biological question remains: how does Rubicon selectively inhibit PI3KC3-C2 while sparing the structurally similar, purely autophagic complex, PI3KC3-C1? PI3KC3-C1 and C2 differ primarily in that C1 contains the subunit ATG14, whereas C2 contains UVRAG. Previous intermediate-resolution structures failed to explain the atomic basis of this selectivity, leaving a gap in understanding how to therapeutically target Rubicon to upregulate autophagy and ELN function without disrupting other pathways.

Methodology
The authors employed a multidisciplinary approach combining high-resolution structural biology, biochemistry, and cell biology:

• Cryo-Electron Microscopy (Cryo-EM): The study determined high-resolution structures of three distinct complexes: the Rubicon:PI3KC3-C2 complex (3.38 Å local resolution in the adaptor arm), unbound PI3KC3-C2 (3.83 Å), and unbound PI3KC3-C1 (3.77 Å).
• Structural Comparison: Comparative analysis of these structures was used to map conformational differences between the complexes, specifically focusing on the BECN1 subunit and its interaction with the mutually exclusive UVRAG and ATG14 subunits.
• Site-Directed Mutagenesis: Based on structural insights, the authors designed specific mutations in the BECN1 BARA domain (Mut1: E492K/S499K/E507K; Mut2: F496D/I503D; and a quintuple Mut1+2) to disrupt the Rubicon-binding interface.
• Biochemical Assays: Bead-binding assays and lipid kinase activity assays (measuring ATP consumption on small unilamellar vesicles) were performed to validate the binding interface and assess the impact of mutations on PI3KC3-C2 enzymatic activity.
• Cellular Assays:
  • Mitophagy: Using ARPE-19 cells expressing the mito-QC reporter, the authors quantified mitolysosome formation in Rubicon knockout (KO) cells rescued with wild-type (WT) or mutant Rubicon.
  • Lysosomal Function: Primary rat hippocampal neurons were transfected with WT or mutant Rubicon to assess lysosomal proteolytic activity (using Magic Red cathepsin B substrate) and lysosomal recruitment.

Key Contributions and Results

1. Atomic Structure of the Interface: The study reveals that Rubicon binds directly to the PI3KC3-C2 complex exclusively via the PI3K-binding domain (PIKBD) of Rubicon (specifically helix α1, residues 488-515) and the BARA domain of the BECN1 subunit. The interface consists of a positively charged groove flanked by two hydrophobic pockets on BECN1 that engage specific residues on Rubicon (E492, S499, E507, F496, I503).
2. Mechanism of Selectivity: Contrary to the hypothesis that Rubicon might directly contact UVRAG or be sterically blocked by ATG14, the structural data show that Rubicon does not contact UVRAG. Instead, selectivity is driven by allosteric remodeling. The presence of UVRAG in PI3KC3-C2 induces a specific conformation of the BECN1 BARA domain that creates the necessary binding groove. In contrast, the presence of ATG14 in PI3KC3-C1 remodels the BECN1 BARA domain, narrowing the "neck" of the binding groove (from 9 Å to 3 Å) and eliminating the hydrophobic pockets required for Rubicon binding. Thus, the mutually exclusive subunits (UVRAG vs. ATG14) dictate the conformation of the shared BECN1 scaffold, conferring specificity.
3. Validation of the Interface: Mutations designed to disrupt the electrostatic and hydrophobic interactions (Mut1, Mut2, and Mut1+2) abolished Rubicon binding to PI3KC3-C2 in bead-binding assays. Consequently, these mutants failed to inhibit PI3KC3-C2 lipid kinase activity in vitro, restoring ATP consumption to levels comparable to the absence of Rubicon.
4. Cellular Phenotypes:
   • In ARPE-19 cells, the Rubicon Mut1+2 mutant failed to suppress mitophagy, resulting in mitolysosome numbers comparable to Rubicon KO cells, whereas WT Rubicon significantly suppressed mitophagy.
   • In primary hippocampal neurons, expression of the mutant Rubicon restored lysosomal proteolytic activity (Magic Red signal) to wild-type levels, despite the mutant retaining normal lysosomal localization (via the Rab7-binding RH domain). This confirms that the loss of function is due to the inability to inhibit PI3KC3-C2, not a loss of localization.

Significance
The paper establishes that the negative regulatory effects of Rubicon on both autophagy and endolysosomal maturation are fully accounted for by its selective inhibition of PI3KC3-C2. The study demonstrates that disrupting the specific Rubicon:PI3KC3-C2 interface is sufficient to rescue mitophagy and lysosomal function to the same extent as complete gene deletion of Rubicon.

The authors conclude that this interaction is mediated by the differential allosteric remodeling of the shared BECN1 subunit by UVRAG and ATG14. These findings provide a mechanistic framework for understanding complex-specific regulation and suggest that targeting the Rubicon:PI3KC3-C2 interface—rather than globally inhibiting or degrading Rubicon—could represent a more selective therapeutic strategy for enhancing autophagy and endolysosome function in conditions such as Parkinson's disease.