Abstract
SummarySARA and endofin are endosomal adaptor proteins that drive Smad phosphorylation by ligand-activated transforming growth factor β/bone morphogenetic protein (TGFβ/BMP) receptors. We show in this study that SARA and endofin also recruit the tumor supressor HD-PTP, a master regulator of endosomal sorting and ESCRT-dependent receptor downregulation. High-affinity interactions occur between the SARA/endofin N termini, and the conserved hydrophobic region in the HD-PTP Bro1 domain that binds CHMP4/ESCRT-III. CHMP4 engagement is a universal feature of Bro1 proteins, but SARA/endofin binding is specific to HD-PTP. Crystallographic structures of HD-PTPBro1 in complex with SARA, endofin, and three CHMP4 isoforms revealed that all ligands bind similarly to the conserved site but, critically, only SARA/endofin interact at a neighboring pocket unique to HD-PTP. The structures, together with mutagenesis and binding analysis, explain the high affinity and specific binding of SARA/endofin, and why they compete so effectively with CHMP4. Our data invoke models for how endocytic regulation of TGFβ/BMP signaling is controlled.
Highlights
Endosomes are hubs for regulating cell surface receptor-dependent signaling pathways (Mellman and Yarden, 2013; von Zastrow and Sorkin, 2007)
Lysosomal delivery first requires sorting of receptors to intralumenal vesicles (ILVs) within the multivesicular body (MVB), a process that is orchestrated by ESCRT (Endosomal Sorting Complex Required for Transport) complexes (ESCRT-0 to -III) (Hurley, 2015; Olmos and Carlton, 2016; Schuh and Audhya, 2014)
An HD-PTPBro1-coiled coil (CC) construct was used as bait in a yeast two-hybrid (Y2H) screen to identify new interacting partners of HD-PTP that could be relevant to ESCRT function on the endocytic pathway
Summary
Endosomes are hubs for regulating cell surface receptor-dependent signaling pathways (Mellman and Yarden, 2013; von Zastrow and Sorkin, 2007). Lysosomal delivery first requires sorting of receptors to intralumenal vesicles (ILVs) within the multivesicular body (MVB), a process that is orchestrated by ESCRT (Endosomal Sorting Complex Required for Transport) complexes (ESCRT-0 to -III) (Hurley, 2015; Olmos and Carlton, 2016; Schuh and Audhya, 2014). ESCRT-mediated degradation of E-cadherin (Palacios et al, 2005) and a5b1 integrin (Kharitidi et al, 2015; Lobert et al, 2010) controls cell adhesion and migration. Crucial for these processes is the assembly of ESCRT-III filaments that drive invagination of the endosomal membrane during ILV formation (Schoneberg et al, 2016)
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