RESUMO
Mast cell (MC) secretory granules are Lysosome-Related Organelles (LROs) whose biogenesis is associated with the post-Golgi secretory and endocytic pathways in which the sorting of proteins destined for a specific organelle relies on the recognition of sorting signals by adaptor proteins that direct their incorporation into transport vesicles. The adaptor protein 3 (AP-3) complex mediates protein trafficking between the trans-Golgi network (TGN) and late endosomes, lysosomes, and LROs. AP-3 has a recognized role in LROs biogenesis and regulated secretion in several cell types, including many immune cells such as neutrophils, natural killer cells, and cytotoxic T lymphocytes. However, the relevance of AP-3 for these processes in MCs has not been previously investigated. AP-3 was found to be expressed and distributed in a punctate fashion in rat peritoneal mast cells ex vivo. The rat MC line RBL-2H3 was used as a model system to investigate the role of AP-3 in mast cell secretory granule biogenesis and mediator release. By immunofluorescence and immunoelectron microscopy, AP-3 was localized both to the TGN and early endosomes indicating that AP-3 dependent sorting of proteins to MC secretory granules originates in these organelles. ShRNA mediated depletion of the AP-3 δ subunit was shown to destabilize the AP-3 complex in RBL-2H3 MCs. AP-3 knockdown significantly affected MC regulated secretion of ß-hexosaminidase without affecting total cellular enzyme levels. Morphometric evaluation of MC secretory granules by electron microscopy revealed that the area of MC secretory granules in AP-3 knockdown MCs was significantly increased, indicating that AP-3 is involved in MC secretory granule biogenesis. Furthermore, AP-3 knockdown had a selective impact on the secretion of newly formed and newly synthesized mediators. These results show for the first time that AP-3 plays a critical role in secretory granule biogenesis and mediator release in MCs.
Assuntos
Complexo 3 de Proteínas Adaptadoras/metabolismo , Degranulação Celular , Mastócitos/metabolismo , Complexo 3 de Proteínas Adaptadoras/genética , Animais , Biomarcadores , Degranulação Celular/genética , Linhagem Celular , Citocinas/metabolismo , Endocitose , Feminino , Expressão Gênica , Técnicas de Silenciamento de Genes , Mediadores da Inflamação/metabolismo , Masculino , Mastócitos/imunologia , Estabilidade Proteica , Transporte Proteico , RNA Interferente Pequeno/genética , Ratos , Receptores de IgE/genética , Receptores de IgE/metabolismoRESUMO
Tyrosine-based signals fitting the YXXØ motif mediate sorting of transmembrane proteins to endosomes, lysosomes, the basolateral plasma membrane of polarized epithelial cells, and the somatodendritic domain of neurons through interactions with the homologous µ1, µ2, µ3, and µ4 subunits of the corresponding AP-1, AP-2, AP-3, and AP-4 complexes. Previous x-ray crystallographic analyses identified distinct binding sites for YXXØ signals on µ2 and µ4, which were located on opposite faces of the proteins. To elucidate the mode of recognition of YXXØ signals by other members of the µ family, we solved the crystal structure at 1.85 Å resolution of the C-terminal domain of the µ3 subunit of AP-3 (isoform A) in complex with a peptide encoding a YXXØ signal (SDYQRL) from the trans-Golgi network protein TGN38. The µ3A C-terminal domain consists of an immunoglobulin-like ß-sandwich organized into two subdomains, A and B. The YXXØ signal binds in an extended conformation to a site on µ3A subdomain A, at a location similar to the YXXØ-binding site on µ2 but not µ4. The binding sites on µ3A and µ2 exhibit similarities and differences that account for the ability of both proteins to bind distinct sets of YXXØ signals. Biochemical analyses confirm the identification of the µ3A site and show that this protein binds YXXØ signals with 14-19 µm affinity. The surface electrostatic potential of µ3A is less basic than that of µ2, in part explaining the association of AP-3 with intracellular membranes having less acidic phosphoinositides.