Endosome biogenesis is controlled by ER and the cytoskeleton at tripartite junctions.

The plasma membrane (PM) and its associated cargo are internalized into small vesicles via endocytosis funneling cargo into endosomes. The endosomal system must efficiently deliver cargos, as well as recycle cargo receptors and membrane to maintain homeostasis. In animal cells, endosome trafficking, maturation, and cargo recycling rely on the actin and microtubule cytoskeleton. Microtubules and their associated motor proteins provide the roads on which endosomes move and fuse during cargo sorting and delivery. In addition, highly dynamic assemblies of actin adjust the shape of the endosomal membrane to promote cargo segregation into budding domains allowing for receptor recycling. Recent work has revealed that the endoplasmic reticulum (ER) frequently acts as an intermediary between endosomes and their cytoskeletal regulators via membrane contact sites (MCSs). This review will discuss the factors which form these tripartite junction between the ER, endosomes, and the cytoskeleton as well as their function.

Coronin 1C restricts endosomal branched actin to organize ER contact and endosome fission.

ER contact sites define the position of endosome bud fission during actin-dependent cargo sorting. Disrupting endosomal actin structures prevents retrograde cargo movement; however, how actin affects ER contact site formation and endosome fission is not known. Here we show that in contrast with the WASH complex, actin, its nucleator ARP2/3, and COR1C form a contained structure at the bud neck that defines the site of bud fission. We found that actin confinement is facilitated by type I coronins. Depletion of type I coronins allows actin to extend along the length of the bud in an ARP2/3-dependent manner. We demonstrate that extension of branched actin prevents ER recruitment and stalls buds before fission. Finally, our structure-function studies show that the COR1C's coiled-coil domain is sufficient to restore actin confinement, ER recruitment, and endosome fission. Together, our data reveal how the dynamics of endosomal actin and activity of actin regulators organize ER-associated bud fission.

A Novel Class of ER Membrane Proteins Regulates ER-Associated Endosome Fission.

Endoplasmic reticulum (ER) membrane contact sites (MCSs) mark positions where endosomes undergo fission for cargo sorting. To define the role of ER at this unique MCS, we targeted a promiscuous biotin ligase to cargo-sorting domains on endosome buds. This strategy identified the ER membrane protein TMCC1, a member of a conserved protein family. TMCC1 concentrates at the ER-endosome MCSs that are spatially and temporally linked to endosome fission. When TMCC1 is depleted, endosome morphology is normal, buds still form, but ER-associated bud fission and subsequent cargo sorting to the Golgi are impaired. We find that the endosome-localized actin regulator Coronin 1C is required for ER-associated fission of actin-dependent cargo-sorting domains. Coronin 1C is recruited to endosome buds independently of TMCC1, while TMCC1/ER recruitment requires Coronin 1C. This link between TMCC1 and Coronin 1C suggests that the timing of TMCC1-dependent ER recruitment is tightly regulated to occur after cargo has been properly sequestered into the bud.