Actin remodelling is fundamental to diverse cellular processes, within endosomal systems, precise regulation of actin assembly and turnover is essential for cargo sorting, cargo capture, vesicle budding and recycling. These processes are affected in inherited neurological conditions and in cancer. How the molecular machinery coordinating endosomal transport interfaces with actin regulatory modules remains poorly understood despite being central to loss of function in disease.
We identify a central role for the Retromer complex and its cargo adaptor SNX27 in coupling cargo selection to actin nucleation through their coordinated engagement of the WASH complex. Structural, biochemical, and cell-based analyses demonstrate that the WASH subunit Fam21 directly interacts with both the SNX27 FERM domain and the Retromer subunit VPS35 via its conserved acidic-Asp-Leu-Phe (aDLF) motifs. These interactions spatially tether the WASH-Arp2/3 complex to Retromer-positive endosomes, where localized actin nucleation promotes membrane tubulation and cargo retrieval. This coordinated mechanism defines specialized recycling domains that ensure efficient endosome-to-plasma-membrane transport of numerous transmembrane cargos, including nutrient transporters and signalling receptors.
Building on this framework, we further uncover a complementary mechanism involving the small GTPase Rab13 and its effector MICAL-L2 (JRAB), which together regulate actin turnover and epithelial tight-junction remodelling. In its active GTP-bound state, Rab13 recruits MICAL-L2 to recycling endosomes and cell-cell junctions, where MICAL-L2 remodels F-actin to coordinate junctional dynamics and membrane trafficking. Cell imaging and biochemical assays validate that Rab13-MICAL-L2 assemblies are involved in balancing actin polymerization and depolymerization, therefore maintaining cargo mobility and membrane plasticity.
Together, our findings establish a unified model in which SNX27-Retromer-WASH-driven actin nucleation and Rab13-MICAL-L2-mediated actin turnover act sequentially and spatially in concert to coordinate cargo recycling and junctional remodelling. This integrated actin regulatory circuit underscores how trafficking machinery and small-GTPase signalling dynamically interface to preserve endosomal homeostasis and epithelial barrier integrity.