SCM-1/SCAMP Maintains Microdomain Boundaries and Cargo Sorting within the Endosomal System

After endocytosis, transmembrane cargo reaches sorting endosomes where it is partitioned into physically distinct recycling or degradative microdomains. While the J-domain protein RME-8/DNAJC13 is known to maintain these boundaries by actively removing degradative machinery from the recycling microdomain, other factors that contribute to this spatial organization remain poorly defined. Here, we identify the conserved tetraspan protein SCM-1/SCAMP as a key microdomain organizer, discovered through RME-8 proximity-dependent biotinylation screens in C. elegans and human cells. Leveraging the large endosomes of C. elegans coelomocytes, we show that SCM-1 is selectively enriched within the recycling microdomain. In scm-1 mutants, recycling and degradative microdomains still assemble but fail to remain spatially distinct, resulting in inappropriate microdomain overlap. This loss of boundary integrity occurs without increasing the recruitment of sorting machineries, indicating a mechanism distinct from the RME-8-mediated uncoating pathway. scm-1 mutants exhibit significant sorting defects, including misrouting of recycling cargo MIG-14/Wls and v-SNARE SNB-2/VAMP3 to late endosomes and lysosomes. We find that snb-2 mutants themselves missort MIG-14 to late endosomes and lysosomes, suggesting that SNB-2 sorting is key for recycling function. Our data suggest that both microdomains lose efficiency in scm-1 mutants, as cargo missorted into late endosomes and lysosomes is not depleted overall, and degradation of an independent ESCRT-dependent cargo is delayed. We conclude that SCM-1 ensures endosomal sorting fidelity by stabilizing microdomain boundary integrity, a process required for efficient recycling and degradation of transmembrane cargo.