Browsing by Author "Bonifacino, Juan S."

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    The Adaptor Protein-1 μ1B Subunit Expands the Repertoire of Basolateral Sorting Signal Recognition in Epithelial Cells
    (2013) Guo, Xiaoli; Mattera, Rafael; Ren, Xuefeng; Chen, Yu; Retamal, Claudio; Gonzalez, Alfonso; Bonifacino, Juan S.
    An outstanding question in protein sorting is why polarized epithelial cells express two isoforms of the mu 1 subunit of the AP-1 clathrin adaptor complex: the ubiquitous mu 1A and the epithelial-specific mu 1B. Previous studies led to the notion that mu 1A and mu 1B mediate basolateral sorting predominantly from the trans-Golgi network (TGN) and recycling endosomes, respectively. Using improved analytical tools, however, we find that mu 1A and mu 1B largely colocalize with each other. They also colocalize to similar extents with TGN and recycling endosome markers, as well as with basolateral cargoes transiting biosynthetic and endocytic-recycling routes. Instead, the two isoforms differ in their signal-recognition specificity. In particular, mu 1B preferentially binds a subset of signals from cargoes that are sorted basolaterally in a mu 1B-dependent manner. We conclude that expression of distinct mu 1 isoforms in epithelial cells expands the repertoire of signals recognized by AP-1 for sorting of a broader range of cargoes to the basolateral surface.
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    The Reelin receptor ApoER2 is a cargo for the adaptor protein complex AP-4: Implications for Hereditary Spastic Paraplegia
    (2024) Caracci, Mario O.; Pizarro, Hector; Alarcon-Godoy, Carlos; Fuentealba, Luz M.; Farfan, Pamela; De Pace, Raffaella; Santibanez, Natacha; Cavieres, Viviana A.; Pastor, Tammy P.; Bonifacino, Juan S.; Mardones, Gonzalo A.; Marzolo, Maria-Paz
    Adaptor protein complex 4 (AP-4) is a heterotetrameric complex that promotes export of selected cargo proteins from the trans-Golgi network. Mutations in each of the AP-4 subunits cause a complicated form of Hereditary Spastic Paraplegia (HSP). Herein, we report that ApoER2, a receptor in the Reelin signaling pathway, is a cargo of the AP-4 complex. We identify the motif ISSF/Y within the ApoER2 cytosolic domain as necessary for interaction with the canonical signal-binding pocket of the mu 4 (AP4M1) subunit of AP-4. AP4E1- knock-out (KO) HeLa cells and hippocampal neurons from Ap4e1-KO mice display increased co-localization of ApoER2 with Golgi markers. Furthermore, hippocampal neurons from Ap4e1-KO mice and AP4M1-KO human iPSC-derived cortical i3Neurons exhibit reduced ApoER2 protein expression. Analyses of biosynthetic transport of ApoER2 reveal differential post -Golgi trafficking of the receptor, with lower axonal distribution in KO compared to wild -type neurons, indicating a role of AP-4 and the ISSF/Y motif in the axonal localization of ApoER2. Finally, analyses of Reelin signaling in mouse hippocampal and human cortical KO neurons show that AP4 deficiency causes no changes in Reelin-dependent activation of the AKT pathway and only mild changes in Reelin-induced dendritic arborization, but reduces Reelin-induced ERK phosphorylation, CREB activation, and Golgi deployment. This work thus establishes ApoER2 as a novel cargo of the AP-4 complex, suggesting that defects in the trafficking of this receptor and in the Reelin signaling pathway could contribute to the pathogenesis of HSP caused by mutations in AP-4 subunits.