The structure and function of the beta 2-adaptin appendage domain.
The heterotetrameric AP2 adaptor (alpha, beta 2, mu 2 and sigma 2 subunits) plays a central role in clathrin-mediated endocytosis. We present the protein recruitment function and 1.7 A resolution structure of its beta 2-appendage domain to complement those previously determined for the mu 2 subunit and alpha appendage. Using structure-directed mutagenesis, we demonstrate the ability of the beta 2 appendage alone to bind directly to clathrin and the accessory proteins AP180, epsin and eps15 at the same site. Clathrin polymerization is promoted by binding of clathrin simultaneously to the beta 2-appendage site and to a second site on the adjacent beta 2 hinge. This results in the displacement of the other ligands from the beta 2 appendage. Thus clathrin binding to an AP2-accessory protein complex would cause the controlled release of accessory proteins at sites of vesicle formation. (+info)
The highly conserved C-terminal dileucine motif in the cytosolic domain of the human immunodeficiency virus type 1 envelope glycoprotein is critical for its association with the AP-1 clathrin adaptor [correction of adapter].
Short amino acid sequences in the cytosolic domains of transmembrane proteins are recognized by specialized adaptor [corrected] proteins which are part of coated vesicles utilized to transport membrane proteins between the trans-Golgi network (TGN) and the plasma membrane (forward and backward). Previously, we and others reported that the membrane-proximal tyrosine residues Y712 (human immunodeficiency virus [HIV]) and Y721 (simian immunodeficiency virus [SIV]) in the envelope glycoprotein (Env) of the primate lentiviruses are crucial for the association of Env with clathrin-associated adaptor [corrected] complex AP-2. The same tyrosine-based endocytosis motifs in the cytosolic domains (EnvCD) of transmembrane gp41 of HIV type 1 (HIV-1) and SIV, respectively, were also shown to modulate the interaction with TGN- and endosome-based clathrin-associated complex AP-1. Our findings suggested that EnvCD binding to AP-1, unlike the association of EnvCD with AP-2, is dependent largely on residues other than Y712 and Y721. Here, we tested if motifs downstream of Y712 affect HIV-1 EnvCD-AP-1 binding and Env trafficking. Mutational analysis revealed that the C-terminal leucine-based motif in Env was crucial for the recruitment of AP-1 in vitro and in Env-expressing cells. In addition to affecting Env-AP-1 association, mutations at the C terminus of Env also altered the subcellular localization of Env, suggesting that proper post-Golgi routing of Env depends on its recruitment of AP-1. Finally, the C-terminal dileucine was shown to assist the membrane-proximal Y712 motif in restricting the cell surface expression of Env. (+info)
PACS-1 binding to adaptors is required for acidic cluster motif-mediated protein traffic.
PACS-1 is a cytosolic protein involved in controlling the correct subcellular localization of integral membrane proteins that contain acidic cluster sorting motifs, such as furin and human immunodeficiency virus type 1 (HIV-1) NEF: We have now investigated the interaction of PACS-1 with heterotetrameric adaptor complexes. PACS-1 associates with both AP-1 and AP-3, but not AP-2, and forms a ternary complex between furin and AP-1. A short sequence within PACS-1 that is essential for binding to AP-1 has been identified. Mutation of this motif yielded a dominant-negative PACS-1 molecule that can still bind to acidic cluster motifs on cargo proteins but not to adaptor complexes. Expression of dominant-negative PACS-1 causes a mislocalization of both furin and mannose 6-phosphate receptor from the trans-Golgi network, but has no effect on the localization of proteins that do not contain acidic cluster sorting motifs. Furthermore, expression of dominant-negative PACS-1 inhibits the ability of HIV-1 Nef to downregulate MHC-I. These studies demonstrate the requirement for PACS-1 interactions with adaptor proteins in multiple processes, including secretory granule biogenesis and HIV-1 pathogenesis. (+info)
The beta-appendages of the four adaptor-protein (AP) complexes: structure and binding properties, and identification of sorting nexin 9 as an accessory protein to AP-2.
Adaptor protein (AP) complexes are essential components for the formation of coated vesicles and the recognition of cargo proteins for intracellular transport. Each AP complex exposes two appendage domains with that function to bind regulatory accessory proteins in the cytosol. Secondary structure predictions, sequence alignments and CD spectroscopy were used to relate the beta-appendages of all human AP complexes to the previously published crystal structure of AP-2. The results suggested that the beta-appendages of AP-1, AP-2 and AP-3 have similar structures, consisting of two subdomains, whereas that of AP-4 lacks the inner subdomain. Pull-down and overlay assays showed partial overlap in the binding specificities of the beta-appendages of AP-1 and AP-2, whereas the corresponding domain of AP-3 displayed a unique binding pattern. That AP-4 may have a truncated, non-functional domain was indicated by its apparent inability to bind any proteins from cytosol. Of several novel beta-appendage-binding proteins detected, one that had affinity exclusively for AP-2 was identified as sorting nexin 9 (SNX9). SNX9, which contains a phox and an Src homology 3 domain, was found in large complexes and was at least partially associated with AP-2 in the cytosol. SNX9 may function to assist AP-2 in its role at the plasma membrane. (+info)
The delta subunit of AP-3 is required for efficient transport of VSV-G from the trans-Golgi network to the cell surface.
Vesicular stomatitis virus glycoprotein (VSV-G) is a transmembrane protein that functions as the surface coat of enveloped viral particles. We report the surprising result that VSV-G uses the tyrosine-based di-acidic motif (-YTDIE-) found in its cytoplasmic tail to recruit adaptor protein complex 3 for export from the trans-Golgi network. The same sorting code is used to recruit coat complex II to direct efficient transport from the endoplasmic reticulum to the Golgi apparatus. These results demonstrate that a single sorting sequence can interact with sequential coat machineries to direct transport through the secretory pathway. We propose that use of this compact sorting domain reflects a need for both efficient endoplasmic reticulum export and concentration of VSV-G into specialized post-trans-Golgi network secretory-lysosome type transport containers to facilitate formation of viral coats at the cell surface. (+info)
Subunit H of the V-ATPase binds to the medium chain of adaptor protein complex 2 and connects Nef to the endocytic machinery.
Nef is an accessory protein of human and simian immunodeficiency viruses (HIV and SIV) that is required for efficient viral infectivity and pathogenicity. It decreases the expression of CD4 on the surface of infected cells. V1H is the regulatory subunit H of the vacuolar membrane ATPase (V-ATPase). Previously, the interaction between Nef and V1H has been found to facilitate the internalization of CD4, suggesting that V1H could connect Nef to the endocytic machinery. In this study, we demonstrate that V1H binds to the C-terminal flexible loop in Nef from HIV-1 and to the medium chain (mu2) of the adaptor protein complex 2 (AP-2) in vitro and in vivo. The interaction sites of V1H and mu2 were mapped to a central region in V1H from positions 133 to 363, which contains 4 armadillo repeats, and to the N-terminal adaptin-binding domain in mu2 from positions 1 to 145. Fusing Nef to V1H reproduced the appropriate trafficking of Nef. This chimera internalized CD4 even in the absence of the C-terminal flexible loop in Nef. Finally, blocking the expression of V1H decreased the enhancement of virion infectivity by Nef. Thus, V1H can function as an adaptor for interactions between Nef and AP-2. (+info)
Phosphoinositide 3-kinase regulates beta2-adrenergic receptor endocytosis by AP-2 recruitment to the receptor/beta-arrestin complex.
Internalization of beta-adrenergic receptors (betaARs) occurs by the sequential binding of beta-arrestin, the clathrin adaptor AP-2, and clathrin. D-3 phosphoinositides, generated by the action of phosphoinositide 3-kinase (PI3K) may regulate the endocytic process; however, the precise molecular mechanism is unknown. Here we demonstrate that betaARKinase1 directly interacts with the PIK domain of PI3K to form a cytosolic complex. Overexpression of the PIK domain displaces endogenous PI3K from betaARK1 and prevents betaARK1-mediated translocation of PI3K to activated beta2ARs. Furthermore, disruption of the betaARK1/PI3K interaction inhibits agonist-stimulated AP-2 adaptor protein recruitment to the beta2AR and receptor endocytosis without affecting the internalization of other clathrin dependent processes such as internalization of the transferrin receptor. In contrast, AP-2 recruitment is enhanced in the presence of D-3 phospholipids, and receptor internalization is blocked in presence of the specific phosphatidylinositol-3,4,5-trisphosphate lipid phosphatase PTEN. These findings provide a molecular mechanism for the agonist-dependent recruitment of PI3K to betaARs, and support a role for the localized generation of D-3 phosphoinositides in regulating the recruitment of the receptor/cargo to clathrin-coated pits. (+info)
AP-3 directs the intracellular trafficking of HIV-1 Gag and plays a key role in particle assembly.
Gag proteins direct the process of retroviral particle assembly and form the major protein constituents of the viral core. The matrix region of the HIV-1 Gag polyprotein plays a critical role in the transport of Gag to the plasma membrane assembly site. Recent evidence indicates that Gag trafficking to late endosomal compartments, including multivesicular bodies, occurs prior to viral particle budding from the plasma membrane. Here we demonstrate that the matrix region of HIV-1 Gag interacts directly with the delta subunit of the AP-3 complex, and that this interaction plays an important functional role in particle assembly. Disruption of this interaction eliminated Gag trafficking to multivesicular bodies and diminished HIV particle formation. These studies illuminate an early step in retroviral particle assembly and provide evidence that the trafficking of Gag to late endosomes is part of a productive particle assembly pathway. (+info)