TSSC1 is novel component of the endosomal retrieval machinery.

Endosomes function as a hub for multiple protein-sorting events, including retrograde transport to the trans-Golgi network (TGN) and recycling to the plasma membrane. These processes are mediated by tubular-vesicular carriers that bud from early endosomes and fuse with a corresponding acceptor compartment. Two tethering complexes named (composed of ANG2, VPS52, VPS53, and VPS54 subunits) and (composed of ANG2, VPS52, VPS53, and Syndetin subunits) were previously shown to participate in SNARE-dependent fusion of endosome-derived carriers with the TGN and recycling endosomes, respectively. Little is known, however, about other proteins that function with Gduanyu37 and Eduanyu37 in these processes. Here we identify a protein named TSSC1 as a specific interactor of both Gduanyu37 and Eduanyu37 and as a novel component of the endosomal retrieval machinery. TSSC1 is a predicted WD40/β-propeller protein that coisolates with both Gduanyu37 and Eduanyu37 in affinity purification, immunoprecipitation, and gel filtration analyses. Confocal fluorescence microscopy shows colocalization of TSSC1 with both Gduanyu37 and Silencing of TSSC1 impairs transport of internalized Shiga toxin B subunit to the TGN, as well as recycling of internalized transferrin to the plasma membrane. Fluorescence recovery after photobleaching shows that TSSC1 is required for efficient recruitment of Gduanyu37 to the TGN. These studies thus demonstrate that TSSC1 plays a critical role in endosomal retrieval pathways as a regulator of both Gduanyu37 and Eduanyu37 function.

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