Balogh, V. | bioRxiv Science

bioRxiv2026

The Uvrag-containing PI3K complex promotes Hsc70-4 dependent endosomal clathrin removal and lysosomal maturation in Drosophila nephrocytes

The class III phosphatidylinositol 3-kinase complex (PI3K(III)) generates phosphatidylinositol-3-phosphate (PI(3)P), a lipid that defines endosomal membrane identity. Two PI3K(III) complexes share core subunits but differ in their fourth component: the Atg14-containing complex I functions in autophagy, whereas the Uvrag-containing complex II is required for endosomal maturation. Despite this, the mechanism by which complex II promotes lysosomal function remains unclear. Using Drosophila nephrocytes, we show that PI(3)P is enriched on Rab7-positive late endosomes and that the Hsp70 chaperone Hsc70-4 binds phosphoinositides. Loss of PI3K complex II disrupts endolysosomal organization and phenocopies Hsc70-4 inhibition. In both cases, clathrin accumulates on intracellular, often endosomal membranes, Rab7 compartments are disorganized, and abnormal endolysosomal structures form. These defects are accompanied by impaired HOPS recruitment, lysosomal dysfunction, and secretion of endolysosomal content. Importantly, clathrin depletion partially rescues these defects. Together, our findings identify a role for PI3K complex II in promoting clathrin removal from endosomal membranes and link PI(3)P and Hsc70-4 activity to lysosomal maturation.

cell biology↗

bioRxiv2026

Multiple overlapping SNARE complexes drive endosome maturation in Drosophila nephrocytes

Endosomal maturation determines whether internalized cargo is recycled or degraded, yet the molecular logic governing early endosomal fusion remains poorly defined. This process is often depicted as a linear Rab5-to-Rab7 transition mediated by a single, ordered SNARE pathway, but extensive redundancy in mammalian systems has obscured pathway architecture. Here, using Drosophila nephrocytes as a genetically tractable in vivo model with minimal SNARE redundancy, we show that early endosome maturation is driven by multiple parallel, non-interchangeable SNARE-dependent pathways. We first resolve a long-standing discrepancy in Syntaxin 7 family orthology, demonstrating that the Drosophila protein previously termed Syx7/Avl is functionally analogous to mammalian STX12 rather than STX7, while late endosomal and lysosomal fusion is mediated by a distinct Syntaxin 7 homolog (Syx13). Based on this reclassification, we define a Syx12L-Snap29-Ykt6 complex that drives canonical homotypic early endosomal fusion. In addition, we identify two related SNARE assemblies - Syx7L-Snap29-Ykt6 and Syx7L-Snap29-Vamp7-that promote later stages of endosomal and lysosomal fusion with distinct Rab GTPase requirements. These partially compensatory complexes remain active when the canonical pathway is disrupted, producing divergent morphological outcomes, including the formation of aberrant endolysosomal swirls. We establish Snap29 as a central Qbc-SNARE integrating all endosomal fusion routes and uncover a dual role for Ykt6 in promoting maturation while also participating in endosomal recycling. Together, our findings revise the prevailing model of endosome maturation, revealing a network of parallel, regulated fusion pathways that confer robustness and plasticity to the endolysosomal system.

cell biology↗