Molecular Insights into SNX27-Retromer Mediated Endosomal Sorting in Health and Disease

Abstract

The dynamic composition of the cell surface integral membrane proteome is controlled by protein delivery from the biosynthetic pathway balanced with removal through endocytosis. Endocytosed integral membrane proteins (termed ‘cargo’) are subsequently sorted at endosomes between lysosomal degradation or entry into recycling pathways for repopulation of the plasma membrane. Recycling is governed by peripherally localised sorting complexes that are recruited to the endosomal membrane by compartment identity cues and sequence motifs in the cytoplasmic tail of cargoes. A key cargo recycling complex is Retromer, a heterotrimer of VPS35, VPS26A/B, and VPS29. In collaboration with the cargo adaptor sorting nexin 27 (SNX27), Retromer is responsible for recycling of PDZ binding motif containing cargoes from endosomes to the plasma membrane. Aberrant SNX27-Retromer function is associated with disease, most notably neurodegeneration, yet there is a paucity of molecular and mechanistic understanding of how SNX27-Retromer orchestrates cargo recycling. In this thesis, I employ AlphaFold-based modelling, validatory structure-guided mutagenesis, quantitative proteomics, and single-cell microscopy to establish new molecular and mechanistic insight into the function of SNX27-Retromer. Initial experiments explore the putative association of SNX27-Retromer with Alzheimer’s disease related protein SORLA. Through detailed quantitative analysis of the SNX27 interactome, I establish the association of SNX27Retromer with regulators of endosomal phosphoinositide identity. Specifically, I dissect the molecular basis of SNX27’s association with FIG4, a constituent of the PIKfyve lipid kinase complex (PLKC). Phenotypic analysis of SNX27Retromer:PLKC holo-complex formation suggest that coupling may contribute to establishing distinct phosphoinositide identities at the endosomal retrieval subdomain. Overall, by providing molecular and phenotypic insight into the coupling of SNX27-Retromer to phosphoinositide regulatory enzymes, my work has expanded our understanding of this essential endosomal recycling complex. Conceptually, these data provide initial evidence suggestive of a mechanistic link VIII between SNX27-Retromer mediated sequence-dependent cargo sorting and the controlled switching of endosomal phosphoinositide compartment identity.

Date of Award	20 Jan 2026
Original language	English
Awarding Institution	University of Bristol
Supervisor	Pete J Cullen (Supervisor) & Michael Ashby (Supervisor)