Intrathecal AAV9/AP4M1 gene therapy for hereditary spastic paraplegia 50 shows safety and efficacy in preclinical studies

Spastic paraplegia 50 (SPG50) is an ultrarare childhood-onset neurological disorder caused by biallelic loss-of-function variants in the AP4M1 gene. SPG50 is characterized by progressive spastic paraplegia, global developmental delay, and subsequent intellectual disability, secondary microcephaly, and epilepsy. We preformed preclinical studies evaluating an adeno-associated virus (AAV)/AP4M1 gene therapy for SPG50 and describe in vitro studies that demonstrate transduction of patient-derived fibroblasts with AAV2/AP4M1, resulting in phenotypic rescue. To evaluate efficacy in vivo, Ap4m1-KO mice were intrathecally (i.t.) injected with 5 × 1011, 2.5 × 1011, or 1.25 × 1011 vector genome (vg) doses of AAV9/AP4M1 at P7-P10 or P90. Age- and dose-dependent effects were observed, with early intervention and higher doses achieving the best therapeutic benefits. In parallel, three toxicology studies in WT mice, rats, and nonhuman primates (NHPs) demonstrated that AAV9/AP4M1 had an acceptable safety profile up to a target human dose of 1 × 1015 vg. Of note, similar degrees of minimal-to-mild dorsal root ganglia (DRG) toxicity were observed in both rats and NHPs, supporting the use of rats to monitor DRG toxicity in future i.t. AAV studies. These preclinical results identify an acceptably safe and efficacious dose of i.t.-administered AAV9/AP4M1, supporting an investigational gene transfer clinical trial to treat SPG50.

The adaptor protein chaperone AAGAB stabilizes AP-4 complex subunits

Adaptor protein 4 (AP-4) is a heterotetrameric complex composed of ε, β4, μ4 and σ4 subunits that mediates export of a subset of transmembrane cargos, including autophagy protein 9A (ATG9A), from the trans-Golgi network (TGN). AP-4 has received particular attention in recent years because mutations in any of its subunits cause a complicated form of hereditary spastic paraplegia (HSP or SPG) referred to as "AP-4-deficiency syndrome." The identification of proteins that interact with AP-4 has shed light on the mechanisms of AP-4-dependent cargo sorting and distribution within the cell. However, the mechanisms by which the AP-4 complex itself is assembled have remained unknown. Herein, we report that the alpha- and gamma-adaptin-binding protein (AAGAB, also known as p34) binds to and stabilizes the AP-4 ε-and σ4 subunits, thus promoting complex assembly. The importance of this binding is underscored by the observation that AAGAB-knockout cells exhibit reduced levels of AP-4 subunits and accumulation of ATG9A at the TGN like those in cells, mice, or patients with mutations in AP-4-subunit genes. These findings demonstrate that AP-4 assembly is not spontaneous but AAGAB-assisted, thus contributing to the understanding of an adaptor protein complex that is critically involved in development of the central nervous system.

RUSC2 and WDR47 oppositely regulate kinesin-1-dependent distribution of ATG9A to the cell periphery

Autophagy-related protein 9 (ATG9) is a transmembrane protein component of the autophagy machinery that cycles between the trans-Golgi network (TGN) in the perinuclear area and other compartments in the peripheral area of the cell. In mammalian cells, export of the ATG9A isoform from the TGN into ATG9A-containing vesicles is mediated by the adaptor protein 4 (AP-4) complex. However, the mechanisms responsible for the subsequent distribution of these vesicles to the cell periphery are unclear. Herein we show that the AP-4-accessory protein RUSC2 couples ATG9A-containing vesicles to the plus-end-directed microtubule motor kinesin-1 via an interaction between a disordered region of RUSC2 and the kinesin-1 light chain. This interaction is counteracted by the microtubule-associated protein WDR47. These findings uncover a mechanism for the peripheral distribution of ATG9A-containing vesicles involving the function of RUSC2 as a kinesin-1 adaptor and WDR47 as a negative regulator of this function.

The FTS-Hook-FHIP (FHF) complex interacts with AP-4 to mediate perinuclear distribution of AP-4 and its cargo ATG9A

The heterotetrameric adaptor protein complex 4 (AP-4) is a component of a protein coat associated with the trans-Golgi network (TGN). Mutations in AP-4 subunits cause a complicated form of autosomal-recessive hereditary spastic paraplegia termed AP-4-deficiency syndrome. Recent studies showed that AP-4 mediates export of the transmembrane autophagy protein ATG9A from the TGN to preautophagosomal structures. To identify additional proteins that cooperate with AP-4 in ATG9A trafficking, we performed affinity purification-mass spectrometry followed by validation of the hits by biochemical and functional analyses. This approach resulted in the identification of the fused toes homolog-Hook-FHIP (FHF) complex as a novel AP-4 accessory factor. We found that the AP-4-FHF interaction is mediated by direct binding of the AP-4 μ4 subunit to coiled-coil domains in the Hook1 and Hook2 subunits of FHF. Knockdown of FHF subunits resulted in dispersal of AP-4 and ATG9A from the perinuclear region of the cell, consistent with the previously demonstrated role of the FHF complex in coupling organelles to the microtubule (MT) retrograde motor dynein-dynactin. These findings thus uncover an additional mechanism for the distribution of ATG9A within cells and provide further evidence for a role of protein coats in coupling transport vesicles to MT motors.

Neuronal functions of adaptor complexes involved in protein sorting

Selective transport of transmembrane proteins to different intracellular compartments often involves the recognition of sorting signals in the cytosolic domains of the proteins by components of membrane coats. Some of these coats have as their key components a family of heterotetrameric adaptor protein (AP) complexes named AP-1 through AP-5. AP complexes play important roles in all cells, but their functions are most critical in neurons because of the extreme compartmental complexity of these cells. Accordingly, various diseases caused by mutations in AP subunit genes exhibit a range of neurological abnormalities as their most salient features. In this article, we discuss the properties of the different AP complexes, with a focus on their roles in neuronal physiology and pathology.

AP-4 mediates export of ATG9A from the trans-Golgi network to promote autophagosome formation

AP-4 is a member of the heterotetrameric adaptor protein (AP) complex family involved in protein sorting in the endomembrane system of eukaryotic cells. Interest in AP-4 has recently risen with the discovery that mutations in any of its four subunits cause a form of hereditary spastic paraplegia (HSP) with intellectual disability. The critical sorting events mediated by AP-4 and the pathogenesis of AP-4 deficiency, however, remain poorly understood. Here we report the identification of ATG9A, the only multispanning membrane component of the core autophagy machinery, as a specific AP-4 cargo. AP-4 promotes signal-mediated export of ATG9A from the trans-Golgi network to the peripheral cytoplasm, contributing to lipidation of the autophagy protein LC3B and maturation of preautophagosomal structures. These findings implicate AP-4 as a regulator of autophagy and altered autophagy as a possible defect in AP-4-deficient HSP.

Chloroquine supplementation increases the cytotoxic effect of curcumin against Her2/neu overexpressing breast cancer cells in vitro and in vivo in nude mice while counteracts it in immune competent mice

Autophagy is usually a pro-survival mechanism in cancer cells, especially in the course of chemotherapy, thus autophagy inhibition may enhance the chemotherapy-mediated anti-cancer effect. However, since autophagy is strongly involved in the immunogenicity of cell death by promoting ATP release, its inhibition may reduce the immune response against tumors, negatively influencing the overall outcome of chemotherapy. In this study, we evaluated the in vitro and in vivo anti-cancer effect of curcumin (CUR) against Her2/neu overexpressing breast cancer cells (TUBO) in the presence or in the absence of the autophagy inhibitor chloroquine (CQ). We found that TUBO cell death induced by CUR was increased in vitro by CQ and slightly in vivo in nude mice. Conversely, CQ counteracted the Cur cytotoxic effect in immune competent mice, as demonstrated by the lack of in vivo tumor regression and the reduction of overall mice survival as compared with CUR-treated mice. Immunohistochemistry analysis revealed the presence of a remarkable FoxP3 T cell infiltrate within the tumors in CUR/CQ treated mice and a reduction of T cytotoxic cells, as compared with single CUR treatment. These findings suggest that autophagy is important to elicit anti-tumor immune response and that autophagy inhibition by CQ reduces such response also by recruiting T regulatory (Treg) cells in the tumor microenvironment that may be pro-tumorigenic and might counteract CUR-mediated anti-cancer effects.

Association between Rare Variants in AP4E1, a Component of Intracellular Trafficking, and Persistent Stuttering

Stuttering is a common, highly heritable neurodevelopmental disorder characterized by deficits in the volitional control of speech. Whole-exome sequencing identified two heterozygous AP4E1 coding variants, c.1549G>A (p.Val517Ile) and c.2401G>A (p.Glu801Lys), that co-segregate with persistent developmental stuttering in a large Cameroonian family, and we observed the same two variants in unrelated Cameroonians with persistent stuttering. We found 23 other rare variants, including predicted loss-of-function variants, in AP4E1 in unrelated stuttering individuals in Cameroon, Pakistan, and North America. The rate of rare variants in AP4E1 was significantly higher in unrelated Pakistani and Cameroonian stuttering individuals than in population-matched control individuals, and coding variants in this gene are exceptionally rare in the general sub-Saharan West African, South Asian, and North American populations. Clinical examination of the Cameroonian family members failed to identify any symptoms previously reported in rare individuals carrying homozygous loss-of-function mutations in this gene. AP4E1 encodes the ε subunit of the heterotetrameric (ε-β4-μ4-σ4) AP-4 complex, involved in protein sorting at the trans-Golgi network. We found that the μ4 subunit of AP-4 interacts with NAGPA, an enzyme involved in the synthesis of the mannose 6-phosphate signal that targets acid hydrolases to the lysosome and the product of a gene previously associated with stuttering. These findings implicate deficits in intracellular trafficking in persistent stuttering.

Bivalent Motif-Ear Interactions Mediate the Association of the Accessory Protein Tepsin with the AP-4 Adaptor Complex

The heterotetrameric (ϵ-β4-μ4-σ4) complex adaptor protein 4 (AP-4) is a component of a non-clathrin coat involved in protein sorting at the trans-Golgi network (TGN). Considerable interest in this complex has arisen from the recent discovery that mutations in each of its four subunits are the cause of a congenital intellectual disability and movement disorder in humans. Despite its physiological importance, the structure and function of this coat remain poorly understood. To investigate the assembly of the AP-4 coat, we dissected the determinants of interaction of AP-4 with its only known accessory protein, the ENTH/VHS-domain-containing protein tepsin. Using a variety of protein interaction assays, we found that tepsin comprises two phylogenetically conserved peptide motifs, [GS]LFXG[ML]X[LV] and S[AV]F[SA]FLN, within its C-terminal unstructured region, which interact with the C-terminal ear (or appendage) domains of the β4 and ϵ subunits of AP-4, respectively. Structure-based mutational analyses mapped the binding site for the [GS]LFXG[ML]X[LV] motif to a conserved, hydrophobic surface on the β4-ear platform fold. Both peptide-ear interactions are required for efficient association of tepsin with AP-4, and for recruitment of tepsin to the TGN. The bivalency of the interactions increases the avidity of tepsin for AP-4 and may enable cross-linking of multiple AP-4 heterotetramers, thus contributing to the assembly of the AP-4 coat. In addition to revealing critical aspects of this coat, our findings extend the paradigm of peptide-ear interactions, previously established for clathrin-AP-1/AP-2 coats, to a non-clathrin coat.

Co-assembly of viral envelope glycoproteins regulates their polarized sorting in neurons

Newly synthesized envelope glycoproteins of neuroinvasive viruses can be sorted in a polarized manner to the somatodendritic and/or axonal domains of neurons. Although critical for transneuronal spread of viruses, the molecular determinants and interregulation of this process are largely unknown. We studied the polarized sorting of the attachment (NiV-G) and fusion (NiV-F) glycoproteins of Nipah virus (NiV), a paramyxovirus that causes fatal human encephalitis, in rat hippocampal neurons. When expressed individually, NiV-G exhibited a non-polarized distribution, whereas NiV-F was specifically sorted to the somatodendritic domain. Polarized sorting of NiV-F was dependent on interaction of tyrosine-based signals in its cytosolic tail with the clathrin adaptor complex AP-1. Co-expression of NiV-G with NiV-F abolished somatodendritic sorting of NiV-F due to incorporation of NiV-G•NiV-F complexes into axonal transport carriers. We propose that faster biosynthetic transport of unassembled NiV-F allows for its proteolytic activation in the somatodendritic domain prior to association with NiV-G and axonal delivery of NiV-G•NiV-F complexes. Our study reveals how interactions of viral glycoproteins with the host's transport machinery and between themselves regulate their polarized sorting in neurons.

Neurons are highly polarized cells exhibiting somatodendritic and axonal domains with distinct protein and lipid compositions. Some enveloped viruses target neurons by binding of the viral envelope glycoproteins to neuronal surface receptors. The ensuing fusion of the viral and neuronal membranes delivers the genetic material of the virus into the neurons. During viral replication in neurons, newly synthesized envelope glycoproteins are sorted to the somatodendritic and/or axonal domains. Although critical for viral propagation, the mechanisms responsible for this sorting are largely unknown. We studied the neuronal sorting of the attachment (NiV-G) and fusion (NiV-F) glycoproteins of Nipah virus, a pathogen that causes fatal human encephalitis. When analyzed individually, NiV-G was delivered to both the axonal and somatodendritic domains. In contrast, NiV-F was exclusively targeted to the somatodendritic domain by virtue of interaction of specific signals in this protein with AP-1, a component of the neuronal protein transport machinery. Assembly with NiV-G, however, abolished somatodendritic sorting of NiV-F due to incorporation of complexes into axon-bound vesicles. Thus, coordinated interactions of viral glycoproteins with the host's sorting machinery and between themselves allow temporal and spatial regulation of their distribution in neurons. We propose that this coordination facilitates viral spread among neurons.

The adaptor protein-1 μ1B subunit expands the repertoire of basolateral sorting signal recognition in epithelial cells

An outstanding question in protein sorting is why polarized epithelial cells express two isoforms of the μ1 subunit of the AP-1 clathrin adaptor complex: the ubiquitous μ1A and the epithelial-specific μ1B. Previous studies led to the notion that μ1A and μ1B mediate basolateral sorting predominantly from the trans-Golgi network (TGN) and recycling endosomes, respectively. Using improved analytical tools, however, we find that μ1A and μ1B 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, μ1B preferentially binds a subset of signals from cargoes that are sorted basolaterally in a μ1B-dependent manner. We conclude that expression of distinct μ1 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.

Signal-mediated, AP-1/clathrin-dependent sorting of transmembrane receptors to the somatodendritic domain of hippocampal neurons

Plasma membranes of the somatodendritic and axonal domains of neurons are known to have different protein compositions, but the molecular mechanisms that determine this polarized protein distribution remain poorly understood. Herein we show that somatodendritic sorting of various transmembrane receptors in rat hippocampal neurons is mediated by recognition of signals within the cytosolic domains of the proteins by the μ1A subunit of the adaptor protein-1 (AP-1) complex. This complex, in conjunction with clathrin, functions in the neuronal soma to exclude somatodendritic proteins from axonal transport carriers. Perturbation of this process affects dendritic spine morphology and decreases the number of synapses. These findings highlight the primary recognition event that underlies somatodendritic sorting and contribute to the evolving view of AP-1 as a global regulator of cell polarity.

The clathrin adaptor AP-1A mediates basolateral polarity

Clathrin and the epithelial-specific clathrin adaptor AP-1B mediate basolateral trafficking in epithelia. However, several epithelia lack AP-1B, and mice knocked out for AP-1B are viable, suggesting the existence of additional mechanisms that control basolateral polarity. Here, we demonstrate a distinct role of the ubiquitous clathrin adaptor AP-1A in basolateral protein sorting. Knockdown of AP-1A causes missorting of basolateral proteins in MDCK cells, but only after knockdown of AP-1B, suggesting that AP-1B can compensate for lack of AP-1A. AP-1A localizes predominantly to the TGN, and its knockdown promotes spillover of basolateral proteins into common recycling endosomes, the site of function of AP-1B, suggesting complementary roles of both adaptors in basolateral sorting. Yeast two-hybrid assays detect interactions between the basolateral signal of transferrin receptor and the medium subunits of both AP-1A and AP-1B. The basolateral sorting function of AP-1A reported here establishes AP-1 as a major regulator of epithelial polarity.

Basolateral sorting of the coxsackie and adenovirus receptor through interaction of a canonical YXXPhi motif with the clathrin adaptors AP-1A and AP-1B

The coxsackie and adenovirus receptor (CAR) plays key roles in epithelial barrier function at the tight junction, a localization guided in part by a tyrosine-based basolateral sorting signal, (318)YNQV(321). Sorting motifs of this type are known to route surface receptors into clathrin-mediated endocytosis through interaction with the medium subunit (μ2) of the clathrin adaptor AP-2, but how they guide new and recycling membrane proteins basolaterally is unknown. Here, we show that YNQV functions as a canonical YxxΦ motif, with both Y318 and V321 required for the correct basolateral localization and biosynthetic sorting of CAR, and for interaction with a highly conserved pocket in the medium subunits (μ1A and μ1B) of the clathrin adaptors AP-1A and AP-1B. Knock-down experiments demonstrate that AP-1A plays a role in the biosynthetic sorting of CAR, complementary to the role of AP-1B in basolateral recycling of this receptor. Our study illustrates how two clathrin adaptors direct basolateral trafficking of a plasma membrane protein through interaction with a canonical YxxΦ motif.

Conservation and diversification of dileucine signal recognition by adaptor protein (AP) complex variants

The clathrin-associated, heterotetrameric adaptor protein (AP) complexes, AP-1, AP-2, and AP-3, recognize signals in the cytosolic domains of transmembrane proteins, leading to their sorting to endosomes, lysosomes, lysosome-related organelles, and/or the basolateral membrane of polarized epithelial cells. One type of signal, referred to as "dileucine-based," fits the consensus motif (D/E)XXXL(L/I). Previous biochemical analyses showed that (D/E)XXXL(L/I) signals bind to a combination of two subunits of each AP complex, namely the AP-1 γ-σ1, AP-2 α-σ2, and AP-3 δ-σ3 hemicomplexes, and structural studies revealed that an imperfect variant of this motif lacking the (D/E) residue binds to a site straddling the interface of α and σ2. Herein, we report mutational and binding analyses showing that canonical (D/E)XXXL(L/I) signals bind to this same site on AP-2, and to similar sites on AP-1 and AP-3. The strength and amino acid requirements of different interactions depend on the specific signals and AP complexes involved. We also demonstrate the occurrence of diverse AP-1 heterotetramers by combinatorial assembly of various γ and σ1 subunit isoforms encoded by different genes. These AP-1 variants bind (D/E)XXXL(L/I) signals with marked preferences for certain sequences, implying that they are not functionally equivalent. Our results thus demonstrate that different AP complexes share a conserved binding site for (D/E)XXXL(L/I) signals. However, the characteristics of the binding site on each complex vary, providing for the specific recognition of a diverse repertoire of (D/E)XXXL(L/I) signals.

Human immunodeficiency virus type 1 Nef protein targets CD4 to the multivesicular body pathway

The Nef protein of human immunodeficiency virus type 1 downregulates the CD4 coreceptor from the surface of host cells by accelerating the rate of CD4 endocytosis through a clathrin/AP-2 pathway. Herein, we report that Nef has the additional function of targeting CD4 to the multivesicular body (MVB) pathway for eventual delivery to lysosomes. This targeting involves the endosomal sorting complex required for transport (ESCRT) machinery. Perturbation of this machinery does not prevent removal of CD4 from the cell surface but precludes its lysosomal degradation, indicating that accelerated endocytosis and targeting to the MVB pathway are separate functions of Nef. We also show that both CD4 and Nef are ubiquitinated on lysine residues, but this modification is dispensable for Nef-induced targeting of CD4 to the MVB pathway.

Ubiquitin binding and conjugation regulate the recruitment of Rabex-5 to early endosomes

Rab GTPases and ubiquitination are critical regulators of transmembrane cargo sorting in endocytic and lysosomal targeting pathways. The endosomal protein Rabex-5 intersects these two layers of regulation by being both a guanine nucleotide exchange factor (GEF) for Rab5 and a substrate for ubiquitin (Ub) binding and conjugation. The ability of trafficking machinery components to bind ubiquitinated proteins is known to have a function in cargo sorting. Here, we demonstrate that Ub binding is essential for the recruitment of Rabex-5 from the cytosol to endosomes, independently of its GEF activity and of Rab5. We also show that monoubiquitinated Rabex-5 is enriched in the cytosol. These observations are consistent with a model whereby a cycle of Ub binding and monoubiquitination regulates the association of Rabex-5 with endosomes.

The trans-Golgi network accessory protein p56 promotes long-range movement of GGA/clathrin-containing transport carriers and lysosomal enzyme sorting

The sorting of acid hydrolase precursors at the trans-Golgi network (TGN) is mediated by binding to mannose 6-phosphate receptors (MPRs) and subsequent capture of the hydrolase-MPR complexes into clathrin-coated vesicles or transport carriers (TCs) destined for delivery to endosomes. This capture depends on the function of three monomeric clathrin adaptors named GGAs. The GGAs comprise a C-terminal "ear" domain that binds a specific set of accessory proteins. Herein we show that one of these accessory proteins, p56, colocalizes and physically interacts with the three GGAs at the TGN. Moreover, overexpression of the GGAs enhances the association of p56 with the TGN, and RNA interference (RNAi)-mediated depletion of the GGAs decreases the TGN association and total levels of p56. RNAi-mediated depletion of p56 or the GGAs causes various degrees of missorting of the precursor of the acid hydrolase, cathepsin D. In the case of p56 depletion, this missorting correlates with decreased mobility of GGA-containing TCs. Transfection with an RNAi-resistant p56 construct, but not with a p56 construct lacking the GGA-ear-interacting motif, restores the mobility of the TCs. We conclude that p56 tightly cooperates with the GGAs in the sorting of cathepsin D to lysosomes, probably by enabling the movement of GGA-containing TCs.

Canonical interaction of cyclin G associated kinase with adaptor protein 1 regulates lysosomal enzyme sorting

The adaptor protein 1 (AP1) complex is a heterotetramer that participates in cargo sorting into clathrin-coated vesicles at the trans-Golgi network (TGN) and endosomes. The gamma subunit of AP1 possesses a C-terminal "ear" domain that recruits a cohort of accessory proteins through recognition of a shared canonical motif, PsiG[PDE][PsiLM] (where Psi is an aromatic residue). The physiological relevance of these ear-motif interactions, however, remains to be demonstrated. Here we report that the cyclin G-associated kinase (GAK) has two sequences fitting this motif, FGPL and FGEF, which mediate binding to the AP1-gamma-ear domain in vitro. Mutation of both gamma-ear-binding sequences or depletion of AP1-gamma by RNA interference (RNAi) decreases the association of GAK with the TGN in vivo. Depletion of GAK by RNAi impairs the sorting of the acid hydrolase, cathepsin D, to lysosomes. Importantly, expression of RNAi-resistant GAK restores the lysosomal sorting of cathepsin D in cells depleted of endogenous GAK, whereas expression of a similar construct bearing mutations in both gamma-ear-binding sequences fails to correct the sorting defect. Thus, interactions between the PsiG[PDE][PsiLM]-motif sequences in GAK and the AP1-gamma-ear domain are critical for the recruitment of GAK to the TGN and the function of GAK in lysosomal enzyme sorting.

The Rab5 Guanine Nucleotide Exchange Factor Rabex-5 Binds Ubiquitin (Ub) and Functions as a Ub Ligase through an Atypical Ub-interacting Motif and a Zinc Finger Domain

Rabex-5, the mammalian orthologue of yeast Vps9p, is a guanine nucleotide exchange factor for Rab5. Rabex-5 forms a tight complex with Rabaptin-5, a multivalent adaptor protein that also binds to Rab4, Rab5, and to domains present in gamma-adaptins and the Golgi-localized, gamma-ear-containing, ARF-binding proteins (GGAs). Rabaptin-5 augments the Rabex-5 exchange activity, thus generating GTP-bound, membrane-associated Rab5 that, in turn, binds Rabaptin-5 and stabilizes the Rabex-5.Rabaptin-5 complex on endosomes. Although the Rabex-5.Rabaptin-5 complex is critical to the regulation of endosomal fusion, the structural determinants of this interaction are unknown. Likewise, the possible binding and covalent attachment of ubiquitin to Rabex-5, two modifications that are critical to the function of yeast Vps9p in endosomal transport, have not been studied. In this study, we identify the 401-462 and 551-661 coiled-coils as the regions in Rabex-5 and Rabaptin-5, respectively, that interact with one another. We also demonstrate that Rabex-5 undergoes ubiquitination and binds ubiquitin, though not via its proposed C-terminal CUE-like domain. Instead, the N-terminal region of Rabex-5 (residues 1-76), comprising an A20-like Cys2/Cys2 zinc finger and an adjacent alpha-helix, is important for ubiquitin binding and ubiquitination. Importantly, we demonstrate that the Rabex-5 zinc finger displays ubiquitin ligase (E3) activity. These observations extend our understanding of the regulation of Rabex-5 by Rabaptin-5. Moreover, the demonstration that Rabex-5 is a ubiquitin ligase that binds ubiquitin and undergoes ubiquitination indicates that its role in endosome fusion may be subject to additional regulation by ubiquitin-dependent modifications.

Structural basis for ubiquitin recognition and autoubiquitination by Rabex-5

Rabex-5 is an exchange factor for Rab5, a master regulator of endosomal trafficking. Rabex-5 binds monoubiquitin, undergoes covalent ubiquitination and contains an intrinsic ubiquitin ligase activity, all of which require an N-terminal A20 zinc finger followed immediately by a helix. The structure of the N-terminal portion of Rabex-5 bound to ubiquitin at 2.5-A resolution shows that Rabex-5-ubiquitin interactions occur at two sites. The first site is a new type of ubiquitin-binding domain, an inverted ubiquitin-interacting motif, which binds with approximately 29-microM affinity to the canonical Ile44 hydrophobic patch on ubiquitin. The second is a diaromatic patch on the A20 zinc finger, which binds with approximately 22-microM affinity to a polar region centered on Asp58 of ubiquitin. The A20 zinc-finger diaromatic patch mediates ubiquitin-ligase activity by directly recruiting a ubiquitin-loaded ubiquitin-conjugating enzyme.

Epidermal growth factor-dependent phosphorylation of the GGA3 adaptor protein regulates its recruitment to membranes

The Golgi-localized, Gamma-ear-containing, Arf-binding (GGA) proteins are monomeric clathrin adaptors that mediate the sorting of transmembrane cargo at the trans-Golgi network and endosomes. Here we report that one of these proteins, GGA3, becomes transiently phosphorylated upon activation of the epidermal growth factor (EGF) receptor. This phosphorylation takes place on a previously unrecognized site in the "hinge" segment of the protein, S368, and is strictly dependent on the constitutive phosphorylation of another site, S372. The EGF-induced phosphorylation of S368 does not require internalization of the EGF receptor or association of GGA3 with membranes. This phosphorylation can be blocked by inhibitors of both the mitogen-activated protein kinase and phosphatidylinositol 3-kinase pathways that function downstream of the activated EGF receptor. Phosphorylation of GGA3 on S368 causes an increase in the hydrodynamic radius of the protein, indicating a transition to a more asymmetric shape. Mutation of S368 and S372 to a phosphomimic aspartate residue decreases the association of GGA3 with membranes. These observations indicate that EGF signaling elicits phosphorylation events that regulate the association of GGA3 with organellar membranes.

Structural mechanism for ubiquitinated-cargo recognition by the Golgi-localized, gamma-ear-containing, ADP-ribosylation-factor-binding proteins

The Golgi-localized, gamma-ear-containing, Arf (ADP-ribosylation factor)-binding (GGA) proteins are clathrin adaptors that mediate the sorting of transmembrane-cargo molecules at the trans-Golgi network and endosomes. Cargo proteins can be directed into the GGA pathway by at least two different types of sorting signals: acidic cluster-dileucine motifs and covalent modification by ubiquitin. The latter modification is recognized by the GGAs through binding to their GAT [GGA and TOM (target of Myb)] domain. Here we report the crystal structure of the GAT domain of human GGA3 in a 1:1 complex with ubiquitin at 2.8-A resolution. Ubiquitin binds to a hydrophobic and acidic patch on helices alpha1 and alpha2 of the GAT three-helix bundle that includes Asn-223, Leu-227, Glu-230, Met-231, Asp-244, Glu-246, Leu-247, Glu-250, and Leu-251. The GAT-binding surface on ubiquitin is a hydrophobic patch centered on Ile-44 that is also responsible for binding most other ubiquitin effectors. The ubiquitin-binding site observed in the crystal is distinct from the Rabaptin-5-binding site on helices alpha2 and alpha3 of the GAT domain. Mutational analysis and modeling of the ubiquitin-Rabaptin-5-GAT ternary complex indicates that ubiquitin and Rabaptin-5 can bind to the GAT domain at two different sites without any steric conflict. This ability highlights the GAT domain as a hub for interactions with multiple partners in trafficking.

The trihelical bundle subdomain of the GGA proteins interacts with multiple partners through overlapping but distinct sites

The Golgi-localized, gamma-adaptin ear-containing, ARF-binding (GGA) proteins are monomeric clathrin adaptors that mediate the sorting of cargo at the trans-Golgi network and endosomes. The GGAs contain four different domains named Vps27, Hrs, Stam (VHS); GGAs and TOM1 (GAT); hinge; and gamma-adaptin ear (GAE). The VHS domain recognizes transmembrane cargo, whereas the hinge and GAE regions bind clathrin and accessory proteins, respectively. The GAT domain is a polyfunctional module that interacts with various partners including the small GTPase ARF, the endosomal fusion regulator Rabaptin-5, ubiquitin, and the product of the tumor susceptibility gene 101 (TSG101). Previous x-ray crystallographic analyses showed that the GAT region is composed of two subdomains, an N-terminal helix-loop-helix containing the ARF binding site, and a C-terminal triple alpha-helical (trihelical) bundle. In this study, we define the Rabaptin-5 binding site on the GGA1-GAT domain and its relationship to the binding sites for ubiquitin and TSG101. Our observations show that Rabaptin-5, ubiquitin, and TSG101 bind to overlapping but distinct binding sites on the trihelical bundle. The different GAT binding partners engage in both competitive and cooperative interactions that may be important for the function of the GGAs in protein sorting.

Definition of the consensus motif recognized by gamma-adaptin ear domains

The heterotetrameric adaptor complex 1 (AP-1) and the monomeric Golgi-localized, gamma ear-containing, Arf-binding (GGA) proteins are components of clathrin coats associated with the trans-Golgi network and endosomes. The carboxyl-terminal ear domains (or gamma-adaptin ear (GAE) domains) of two gamma-adaptin subunit isoforms of AP-1 and of the GGAs are structurally similar and bind to a common set of accessory proteins. In this study, we have systematically defined a core tetrapeptide motif PsiG(P/D/E)(Psi/L/M) (where Psi is an aromatic residue), which is responsible for the interactions of accessory proteins with GAE domains. The definition of this motif has allowed us to identify novel GAE-binding partners named NECAP and aftiphilin, which also contain clathrin-binding motifs. These findings shed light on the mechanism of accessory protein recruitment to trans-Golgi network and endosomal clathrin coats.

Recognition of accessory protein motifs by the gamma-adaptin ear domain of GGA3

Adaptor proteins load transmembrane protein cargo into transport vesicles and serve as nexuses for the formation of large multiprotein complexes on the nascent vesicles. The gamma-adaptin ear (GAE) domains of the AP-1 adaptor protein complex and the GGA adaptor proteins recruit accessory proteins to these multiprotein complexes by binding to a hydrophobic motif. We determined the structure of the GAE domain of human GGA3 in complex with a peptide based on the DFGPLV sequence of the accessory protein Rabaptin-5 and refined it at a resolution of 2.2 A. The leucine and valine residues of the peptide are partly buried in two contiguous shallow, hydrophobic depressions. The anchoring phenylalanine is buried in a deep pocket formed by the aliphatic portions of two conserved arginine residues, along with an alanine and a proline, illustrating the unusual function of a cluster of basic residues in binding a hydrophobic motif.

Divalent interaction of the GGAs with the Rabaptin-5-Rabex-5 complex

Cargo transfer from trans-Golgi network (TGN)-derived transport carriers to endosomes involves a still undefined set of tethering/fusion events. Here we analyze a molecular interaction that may play a role in this process. We demonstrate that the GGAs, a family of Arf-dependent clathrin adaptors involved in selection of TGN cargo, interact with the Rabaptin-5-Rabex-5 complex, a Rab4/Rab5 effector regulating endosome fusion. These interactions are bipartite: GGA-GAE domains recognize an FGPLV sequence (residues 439-443) in a predicted random coil of Rabaptin-5 (a sequence also recognized by the gamma1- and gamma2-adaptin ears), while GGA-GAT domains bind to the C-terminal coiled-coils of Rabaptin-5. The GGA-Rabaptin-5 interaction decreases binding of clathrin to the GGA-hinge domain, and expression of green fluorescent protein (GFP)-Rabaptin-5 shifts the localization of endogenous GGA1 and associated cargo to enlarged early endosomes. These observations thus identify a binding sequence for GAE/gamma-adaptin ear domains and reveal a functional link between proteins regulating TGN cargo export and endosomal tethering/fusion events.

Increased Release of Arachidonic Acid and Eicosanoids in Iron-Overloaded Cardiomyocytes

Background —Patients with transfusional iron overload may develop a life-limiting cardiomyopathy. The sensitivity of lipid-metabolizing enzymes to peroxidative injury, as well as the reported effects of arachidonic acid (AA) and metabolites on cardiac rhythm, led us to hypothesize that iron-overloaded cardiomyocytes display alterations in the release of AA and prostaglandins.

Methods and Results —Neonatal rat ventricular myocytes (NRVMs) cultured for 72 hours in the presence of 80 μg/mL ferric ammonium citrate displayed an increased rate of AA release, both under resting conditions and after stimulation with agonists such as [Sar 1 ]Ang II. Although iron treatment did not affect overall incorporation of [ 3 H]AA into NRVM phospholipids, it caused a 2-fold increase in the distribution of precursor in phosphatidylcholine species, with a proportional decrease in phosphatidylinositol, phosphatidylserine, and phosphatidylethanolamine. Increased release of AA in iron-overloaded NRVMs was reduced by the diacylglycerol lipase inhibitor RHC80267 but was largely insensitive to inhibitors of phospholipases A 2 and C. Iron-overloaded cardiomyocytes also displayed increased production of eicosanoids and induction of cyclooxygenase-2 after stimulation with interleukin-1α.

Conclusions —Iron overload enhances AA release and incorporation of AA into phosphatidylcholine, as well as cyclooxygenase-2 induction and eicosanoid production, in NRVMs. The effects of AA and metabolites on cardiomyocyte rhythmicity suggest a causal connection between these signals and electromechanical alterations in iron-overload–induced cardiomyopathy.

Epidermal growth factor regulates astrocyte expression of the interleukin-4 receptor via a MAPK-independent pathway

Human astrocytes express the interleukin (IL)-4 receptor alpha chain (IL-4R alpha) in vitro and in vivo but mechanisms governing astrocyte IL-4R alpha expression have not been established. We hypothesized that epidermal growth factor (EGF) and IL-4, agents that profoundly affect astrocyte proliferation, might also alter IL-4R alpha expression. Exposure to EGF for 24 h enhanced IL-4R alpha mRNA levels; in contrast, IL-4 yielded no increase. Immunoblotting demonstrated that EGF but not IL-4 increased astrocyte IL-4R alpha protein after 2--4 days of exposure. Similarly, EGF but not IL-4 strongly activated phosphorylation of p42/p44 extracellular regulated kinase isoforms, a reaction blocked by the mitogen-activated protein kinase (MAPK) inhibitor, PD98059. PD98059 also blocked EGF-stimulated DNA synthesis but not IL-4R alpha mRNA levels, while antibody to the EGF receptor (erbB1) blocked both EGF effects. Data suggest that astrocyte IL-4R alpha expression is upregulated by EGF but not by IL-4 in an EGF-receptor-dependent manner and that mechanisms are independent of MAPK activation.

Endothelins regulate arachidonic acid release and mitogen-activated protein kinase activity in Schwann cells

Immortalized rat Schwann cells (iSC) express endothelin (ET) receptors coupled to inhibition of adenylyl cyclase and stimulation of phospholipase C (PLC). These effects precede phenotypic changes and increased DNA synthesis. We have investigated the role of ETs in the regulation of arachidonic acid (AA) release and mitogen-activated protein kinases (MAPKs). Both ET-1 and ET-3 increased AA release in iSC. This effect was sensitive to the phospholipase A(2) (PLA(2)) inhibitors E:-6-(bromomethylene)tetrahydro-3-(1-naphthalenyl)-2H:-pyran-2-one and arachidonyl-trifluoromethyl ketone but was insensitive to inhibitors of PLC or phospholipase D-dependent diacylglycerol generation. ET-1-dependent AA release was also unaffected by removal of extracellular Ca(2+) and blocking the concomitant elevation in [Ca(2+)](i), consistent with participation of a Ca(2+)-independent PLA(2). Treatment of iSC with ETs also resulted in activation of extracellular signal-regulated kinase, c-Jun-NH(2)-terminal kinase (JNK), and p38 MAPK. A cause-effect relationship between agonist-dependent AA release and stimulation of MAPKs, but not the opposite, was suggested by activation of JNK by exogenous AA and by the observation that inhibition of MAPK kinase or p38 MAPK was inconsequential to ET-1-induced AA release. Similar effects of ETs on AA release and MAPK activity were observed in cultures expanded from primary SC and in iSC. Regulation of these effectors may mediate the control of proliferation and differentiation of SC by ETs during peripheral nerve development and regeneration.

PLA(2) dependence of diaphragm mitochondrial formation of reactive oxygen species

Contraction-induced respiratory muscle fatigue and sepsis-related reductions in respiratory muscle force-generating capacity are mediated, at least in part, by reactive oxygen species (ROS). The subcellular sources and mechanisms of generation of ROS in these conditions are incompletely understood. We postulated that the physiological changes associated with muscle contraction (i.e., increases in calcium and ADP concentration) stimulate mitochondrial generation of ROS by a phospholipase A(2) (PLA(2))-modulated process and that sepsis enhances muscle generation of ROS by upregulating PLA(2) activity. To test these hypotheses, we examined H(2)O(2) generation by diaphragm mitochondria isolated from saline-treated control and endotoxin-treated septic animals in the presence and absence of calcium and ADP; we also assessed the effect of PLA(2) inhibitors on H(2)O(2) formation. We found that 1) calcium and ADP stimulated H(2)O(2) formation by diaphragm mitochondria from both control and septic animals; 2) mitochondria from septic animals demonstrated substantially higher H(2)O(2) formation than mitochondria from control animals under basal, calcium-stimulated, and ADP-stimulated conditions; and 3) inhibitors of 14-kDa PLA(2) blocked the enhanced H(2)O(2) generation in all conditions. We also found that administration of arachidonic acid (the principal metabolic product of PLA(2) activation) increased mitochondrial H(2)O(2) formation by interacting with complex I of the electron transport chain. These data suggest that diaphragm mitochondrial ROS formation during contraction and sepsis may be critically dependent on PLA(2) activation.

Signal transduction pathways of the human V1-vascular, V2-renal, V3-pituitary vasopressin and oxytocin receptors

Vasopressin (VP) and oxytocin (OT) are cyclic nonapeptides whose actions are mediated by stimulation of specific G protein-coupled receptors (GPCRs) currently classified into V1-vascular (V1R), V2-renal (V2R) and V3-pituitary (V3R) VP receptors and OT receptors (OTR). The recent cloning of the different members of the VP/OT family of receptors now allows the extensive characterization of the molecular determinants involved in ligand binding and signal transduction pathways coupled to a given VP/OT receptor subtype in stably transfected mammalian cell lines. In this article, we review the present knowledge of the signal transduction pathways coupled to the different VP/OT receptor subtypes and we present new observations derived from the study of each human VP or OT receptor subtype stably expressed in CHO cells.

Molecular pharmacology of human vasopressin receptors

Vasopressin (AVP) and oxytocin (OT) are cyclic nonapeptides whose actions are mediated by activation of specific G protein-coupled receptors (GPCRs) currently classified into V1-vascular (V1R), V2-renal (V2R) and V3-pituitary (V3R) AVP receptors and OT receptors (OTR). The cloning of the different members of the AVP/OT family of receptors now allows the extensive molecular pharmacological characterization of a single AVP/OT receptor subtype in stably transfected mammalian cell lines. The human V1-vascular (CHO-V1), V2-renal (CHO-V2), V3-pituitary (CHO-V3) and oxytocin (CHO-OT) receptors stably expressed in CHO cells display distinct binding profiles for 18 peptide and 5 nonpeptide AVP/OT analogs. Several peptide and nonpeptide compounds have a greater affinity for the V1R than AVP itself. V2R peptide agonists and antagonists tend to be non-selective ligands whereas nonpeptide V2R antagonists are potent and subtype-selective. None of the 22 AVP/OT analogs tested has a better affinity for the human V3R than AVP itself. Several peptide antagonists do not select well between V1R and OTR. These results underscore the need for developing specific and potent analogs interacting specifically with a given human AVP/OT receptor subtype.

Agonist-specific alterations in receptor-phospholipase coupling following inactivation of Gi2alpha gene

Different forms of phospholipase A2, together with pertussis toxin-sensitive G-proteins, [Ca2+]i (intracellular Ca2+ concentration), protein kinase C, calmodulin, protein tyrosine kinases, mitogen-activated protein kinases and Ca2+/calmodulin-dependent protein kinase appear to play a role in agonist-mediated release of arachidonic acid. Here we report that fibroblasts from 14-day-old mouse embryos with inactivated Gi2alpha (alpha-subunit of the heterotrimeric G-protein Gi2) gene display a marked decrease in the ability of lysophosphatidic acid, thrombin and Ca2+ ionophores to release arachidonic acid compared with their normal counterparts. The requirement for Gi2alpha in the release of arachidonic acid following increased [Ca2+]i may be explained by the incomplete translocation of cytosolic phospholipase A2 observed in Gi2alpha-deficient cells. Paradoxically, inactivation of the Gi2alpha gene resulted in up-regulation of bradykinin receptors and their coupling to increased arachidonic acid release, phospholipase C activity and [Ca2+]i. A concomitant increase in basal phospholipase C activity was also observed in the Gi2alpha-deficient cells. These observations establish a pleiotropic and essential role for Gi2alpha in receptor-phospholipase coupling that contrasts with its less obligatory participation in agonist-mediated inhibition of adenylate cyclase.

The human V3 pituitary vasopressin receptor: ligand binding profile and density-dependent signaling pathways

The vasopressin (AVP) V3 pituitary receptor (V3R) is a G protein-coupled corticotropic phenotypic marker that is overexpressed in ACTH-hypersecreting tumors. Studies of the agonist/antagonist binding profile and signal transduction pathways linked to the human V3R have been limited because of the scarcity of this protein. To define the signals activated by V3Rs and the eventual changes triggered by developmental or pathological receptor regulation, we developed Chinese hamster ovary (CHO)-V3 cells stably expressing low, medium, or high levels of human V3Rs (binding capacity, <10, 10-25, and 25-100 pmol/mg, respectively). The affinity of the V3R for 21 peptide and nonpeptide AVP analogs was clearly distinct from that exhibited by the human V1R and V2R. AVP triggered stimulation of phospholipase C in CHO-V3 cells (partially sensitive to treatment with pertussis toxin) with a potency directly proportional to receptor density. V3R-mediated arachidonic acid release also was also sensitive to pertussis toxin and more efficacious in cells exhibiting medium than in those with high receptor density. AVP also stimulated the pertussis toxin-insensitive uptake of [3H]thymidine in CHO-V3 cells. The concentration-response curves for this effect were monophasic in cells expressing low and medium levels of V3Rs; on the contrary, a biphasic curve was observed in cells with high V3R density. Coupling of V3R to increased production of cAMP was only observed in CHOV3 high cells, suggesting a negative relationship between increased cAMP production and DNA synthesis. Activation of mitogen-activated protein kinases by V3R was pertussis toxin insensitive, but was dependent on activation of phospholipase C and protein kinase C; both the level and duration of activation were a function of the receptor density. Thus, the human V3R has a pharmacological profile clearly distinct from that of the human V1R and V2R and activates several signaling pathways via different G proteins, depending on the level of receptor expression. The increased synthesis of DNA and cAMP levels observed in cells expressing medium and high levels of V3Rs, respectively, may represent important events in the tumorigenesis of corticotroph cells.

Ca2+ signalling in K562 human erythroleukaemia cells: effect of dimethyl sulphoxide and role of G-proteins in thrombin- and thromboxane A2-activated pathways

The human leukaemic cell line K562 is a pluripotent stem cell with the potential to mature along a megakaryocytic or erythroid line. In these cells, thrombin and U46619 (9,11-dideoxy-9 alpha, 11 alpha-methanoepoxy prostaglandin F2 alpha), a thromboxane A2 analogue, increased intracellular Ca2+ in a rapid and concentration-dependent manner. The peak transient observed with both thrombin and U46619 was preserved upon stimulation in the absence of extracellular calcium and blunted with phorbol myristate acetate, suggestive of activation of phospholipase C. Short-term treatment with leupeptin abolished the calcium response to thrombin, but did not alter that to U46619. Both pertussis toxin (PT) and DMSO pretreatment inhibited thrombin- but not U46619-stimulated intracellular calcium elevation, indicating that these agonists signal through different G-proteins. Western blot analysis of crude membranes from K562 cells revealed the presence of G12 alpha and G13 alpha; the other known PT-substrates, Gi1 alpha and G0 alpha, were not detected. Consistent with this observation, ADP-ribosylation experiments revealed the presence of two PT substrates which co-migrated with human erythrocyte G12 alpha and G13 alpha. An antibody raised against Gq/11 alpha, a subfamily of G-protein alpha subunits unmodified by PT, specifically recognized 42 kDa protein(s) in K562 cells. PCR amplification of reverse-transcribed K562 RNA followed by DNA sequencing showed that these cells express messages for both Gq alpha and G11 alpha. Treatment of K562 cells with DMSO reduced the levels of thrombin receptor mRNA, without simultaneous changes in the expression of G12 alpha and G13 alpha. We have thus identified Ca(2+)-mobilizing agonists and related G-proteins in K562 cells, together with changes induced by DMSO in this signalling pathway.

Regulation of phospholipase A2 activity in undifferentiated and neutrophil-like HL60 cells. Linkage between impaired responses to agonists and absence of protein kinase C-dependent phosphorylation of cytosolic phospholipase A2

We compared the regulation of cytosolic phospholipase A2 (cPLA2) activity in undifferentiated and neutrophil-like HL60 cells. Although Ca(2+)-mobilizing P2-purinergic receptors are expressed in both cell types, arachidonic acid (AA) release stimulated by P2-purinergic agonists was 5-7-fold higher in the differentiated cells. Similarly, the stimulation of AA release by AlF4- in intact cells or by ATP and guanosine 5'-3-O-(thio)triphosphate (GTP gamma S) in electropermeabilized cells was significantly higher in the differentiated cells. Treatment with phorbol 12-myristate 13-acetate (PMA) enhanced A23187-stimulated AA release in intact HL60 granulocytes with minimal effects in the undifferentiated cells. Immunoblotting experiments showed similar levels of cPLA2 and of agonist-mediated activation of mitogen-activated protein kinase in both cell types. Experiments measuring stimulation of AA release by either melittin, using endogenously labeled intact cells, or Ca2+, using homogenates and exogenous substrate, indicated that undifferentiated cells do not lack an activatable PLA2. The stimulatory effects of GTP gamma S and Ca2+ on AA release in homogenates from endogenously labeled cells suggested that undifferentiated cells display G protein-cPLA2 coupling. Basal and PMA-stimulated phosphorylation of cPLA2 was detected in differentiated, but not in undifferentiated cells. However, the two cell types displayed only subtle differences in the time courses of phosphorylation of mitogen-activated protein kinase triggered by agonists and PMA. The observed defect in cPLA2 phosphorylation may represent the alteration preventing agonist-mediated stimulation of AA release in undifferentiated HL60 cells.

Time-resolved resonance Raman study on the binding of carbon monoxide to recombinant human myoglobin and its distal histidine mutants

Time-resolved resonance Raman (RR) spectra of the recombined species of photodissociated CO with recombinant human myoglobin (Mb) and several E7 mutants, in which distal His was replaced by Gly (H64G), Gln (H64Q), Ala (H64A), Ile (H64I), Val (H64V), and Leu (H64L) through site-directed mutagenesis, were observed in the time range -20 ns to 1 ms following photolysis. The Fe-CO stretching (VFe-CO) RR band was observed successfully with pulse excitation when the laser power was greatly reduced. H64H, H64G, and H64Q gave the VFe-CO band at 505-510 cm-1 in their stationary states. In their recovery processes 1-100 microseconds after photodissociation, a broad transient band was observed at slightly lower frequencies than those of their equilibrium structures for H64G and H64Q, but a transient VFe-CO band corresponding to the so-called "open" form was not identified around 490 cm-1 for any of the three species. A second group, H64A, H64I, H64V, and H64L, gave the main VFe-CO band at 490-495 cm-1 with a shoulder around 510 cm-1 (except for H64L) in the stationary state and exhibited a much faster recovery than the first group. These latter four species gave a broad transient band around 492-500 cm-1 in the time range of 100-1000 ns, while the approximately 510 cm-1 shoulder appeared much later. The equilibrium relative intensity of the two bands was attained at 500 microseconds, suggesting that the interconversion between the two forms is slower than 100 microseconds.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorylation-dependent regulation of phospholipase A2 by G-proteins and Ca2+ in HL60 granulocytes

We studied the regulation of arachidonic acid (AA) release by guanosine 5'-O-(3-thiotriphosphate (GTP gamma S) and Ca2+ in electropermeabilized HL60 granulocytes. Stimulation of AA release by GTP gamma S and Ca2+ was mediated by phospholipase A2 (PLA2) and required the presence of MgATP (EC50: 100-250 microM). The nucleotide effects were Ca(2+)-dependent (maximal effects detected at 1 microM free cation). UTP and ATP gamma S, which stimulate AA release in intact HL60 granulocytes with potencies and efficacies similar to those of ATP, were ineffective in supporting the effects of GTP gamma S in electropermeabilized cells. Pretreatment with pertussis toxin affected stimulation of AA release by ATP in intact cell, without altering the nucleotide effects in permeabilized cells. We observed the protein kinase C-dependent phosphorylation of PLA2 in permeabilized HL60 granulocytes, together with a correlation between the effects of phorbol esters and staurosporine on this reaction and on AA release. ATP-independent activation of PLA2 by GTP gamma S and/or Ca2+ was measured in subcellular fractions prepared from HL60 granulocytes. These data appear consistent with a model in which PLA2 activity in resting HL60 granulocytes is subjected to an inhibitory constraint that prevents its activation by Ca2+ and G-proteins. Removal of this constraint, either by the protein kinase C-dependent phosphorylation of the enzyme in vivo or physical disruption of the regulatory assembly (e.g. by N2 cavitation), allows its activation by Ca2+ and G-proteins.

A truncated recombinant alpha subunit of Gi3 with a reduced affinity for beta gamma dimers and altered guanosine 5'-3-O-(thio)triphosphate binding

The baculovirus-based expression system was adapted to express alpha subunits of the complete (alpha i3) and an amino-terminally truncated (alpha i3') form of Gi3 and of two complete forms of Gs (alpha s-L and alpha s-S). Subunits encoded in full length cDNAs were obtained with yields of 40-60 mg of recombinant protein/liter of cells, of which alpha i3 was between 30 and 50% soluble, but alpha s subunits were only 5-10% soluble. Only the complete alpha i3 was myristoylated. alpha i3 was purified in four steps. The purified protein bound 0.8-0.9 mol of guanosine 5'-3-O-(thio)triphosphate (GTP gamma S) per mol of protein and had one predominant contaminant which was identified as a truncated form that begins with methionine 18 instead of methionine 1. Both the full length alpha i3 and the truncated alpha i3' formed trimers with human erythrocyte beta gamma as seen by their migration in sucrose density gradients and by an increased rate of ADP ribosylation by pertussis toxin, but compared to alpha i3, alpha i3' interacted with beta gamma with a reduced affinity and dissociated upon warming. At 32 degrees C, only full length alpha i3 was ADP-ribosylated; at 4 degrees C, alpha i3 and alpha i3' were both ADP-ribosylated, with the truncated form requiring approximately 200-fold higher concentrations of beta gamma. A genetically engineered alpha i3' (alpha i3[18-354]) was also expressed in Sf9 cells. Yields, assessed as saturable GTP gamma S binding sites, were 3-5 mg per liter. Scatchard analysis showed that truncation of the amino terminus interferes with the ability of Mg2+ to promote high affinity binding of GTP gamma S. We conclude that the G protein alpha subunit amino terminus is not essential for interaction with beta gamma dimers, but rather is important in determining the affinity of the alpha subunit for both the beta gamma dimers and guanine nucleotide.

Studies on the interaction of alpha subunits of GTP-binding proteins with beta gamma dimers

The interaction of several preparations of purified beta gamma dimers with two types of guanosine-nucleotide-binding-regulatory-(G)-protein alpha subunits, a recombinant bv alpha i3, made in Sf9 Spodoptera frugiperda cells by the baculovirus (bv) expression system, and alpha s, either purified from human erythrocyte Gs-type GTP-binding protein, and activated by NaF/AlCl3, or unpurified as found in a natural membrane, were studied. The beta gamma dimers used were from bovine rod outer segments (ROS), bovine brain, human erythrocytes (hRBC) and human placenta and contained distinct ratios of beta subunits that, upon electrophoresis, migrated as two bands with approximate M(r) of 35,000 and 36,000, as well as distinct complements of at least two gamma subunits each. When tested for their ability to recombine at submaximal concentrations with bv alpha i3, ROS, brain, hRBC and placental beta gamma dimers exhibited apparent affinities that were the same within a factor of two. When bovine brain, placental and ROS beta gamma dimers were tested for their ability to promote deactivation of Gs, brain and placental beta gamma dimers were equipotent and at least 10-fold more potent than that of ROS beta gamma dimers; likewise, brain beta gamma and placental dimers were equipotent in inhibiting GTP-activated and GTP-plus-isoproterenol-activated adenylyl cyclase, while ROS beta gamma dimers were less potent when assayed at the same concentration. The possibility that different alpha subunits may distinguish subsets of beta gamma dimers from a single cell was investigated by analyzing the beta gamma composition of three G proteins, Gs, Gi2 and Gi3, purified to near homogeneity from a single cell type, the human erythrocyte. No evidence for an alpha-subunit-specific difference in beta gamma composition was found. These findings suggests that, in most cells, alpha subunits interact indistinctly with a common pool of beta gamma dimers. However, since at least one beta gamma preparation (ROS) showed unique behavior, it is clear that there may be mechanisms by which some combinations of beta gamma dimers may exhibit selectivity for the alpha subunits they interact with.

Coordination structure of the ferric heme iron in engineered distal histidine myoglobin mutants

Recombinant human myoglobin mutants with the distal His residue (E7, His64) replaced by Leu, Val, or Gln residues were prepared by site-directed mutagenesis and expression in Escherichia coli. Electronic and coordination structures of the ferric heme iron in the recombinant myoglobin proteins were examined by optical absorption, EPR, 1H NMR, magnetic circular dichroism, and x-ray spectroscopy. Mutations, His-->Val and His-->Leu, remove the heme-bound water molecule resulting in a five-coordinate heme iron at neutral pH, while the heme-bound water molecule appears to be retained in the engineered myoglobin with His-->Gln substitution as in the wild-type protein. The distal Val and distal Leu ferric myoglobin mutants at neutral pH exhibited EPR spectra with g perpendicular values smaller than 6, which could be interpreted as an admixture of intermediate (S = 3/2) and high (S = 5/2) spin states. At alkaline pH, the distal Gln mutant is in the same so-called "hydroxy low spin" form as the wild-type protein, while the distal Leu and distal Val mutants are in high spin states. The ligand binding properties of these recombinant myoglobin proteins were studied by measurements of azide equilibrium and cyanide binding. The distal Leu and distal Val mutants exhibited diminished azide affinity and extremely slow cyanide binding, while the distal Gln mutant showed azide affinity and cyanide association rate constants similar to those of the wild-type protein.

Identification of G protein subtypes in peripheral nerve and cultured Schwann cells

In this study, we investigated the expression of various G proteins in whole sciatic nerves, in myelin and nonmyelin fractions from these nerves, and in membranes of immortalized Schwann cells. In myelin, nonmyelin, and Schwann cell membranes we detected two 39-40-kDa pertussis toxin substrates that were resolved on separation on urea-gradient gels. Two cholera toxin substrates with apparent molecular masses of 42 and 47 kDa were present in nerve and brain myelin and in Schwann cell membranes. In these membranes, a third 45-kDa cholera toxin substrate, which displayed the highest labeling, was also present. Immunoblotting with specific antisera allowed the identification of G(o) alpha, Gi1 alpha, Gi2 alpha, Gi3 alpha, Gq/G11 alpha, and the two isoforms of Gs alpha in nerve homogenates, nerve, and brain myelin fractions. In Schwann cell membranes we identified G(o) alpha, Gi2 alpha, Gi3 alpha, and proteins from the Gq family, but no immunoreactivity toward anti-Gi1 alpha antiserum was detected. In these membranes, anti-Gs alpha antibody recognized the three cholera toxin substrates mentioned above, with the 45-kDa band displaying the highest immunoreactivity. Relative to sciatic nerve myelin, the Schwann cell membranes revealed a significantly higher expression of Gi3 alpha and the absence of Gi1 alpha. The different distribution of G proteins among the different nerve compartments might reflect the very specialized function of Schwann cells and myelin within the nerve.

Dual regulation of arachidonic acid release by P2U purinergic receptors in dibutyryl cyclic AMP-differentiated HL60 cells

ATP promoted biphasic effects on both basal and fMLP-stimulated arachidonic acid (AA) release in neutrophil-like HL60 cells: stimulation in the micromolar range (EC50 = 3.2 +/- 0.9 microM) and inhibition at higher concentrations (EC50 = 90 +/- 11 microM). ATP also inhibited UTP- and platelet activating factor-stimulated AA release. Only stimulatory effects of ATP on basal or fMLP-stimulated phospholipase C were observed. The inhibitory effect of ATP on AA release was not due to reacylation of released AA, chelation of extracellular Ca2+, cell permeabilization, or changes in the rise of [Ca2+]i induced by agonist. The inhibition was rapid, being detected within 5-15 s. The inhibitory effect of ATP on fMLP-stimulated AA release could be desensitized by pretreatment of the cells with 2 mM ATP, but not 20 microM ATP, the concentration that resulted in maximal release of AA and inositol phosphates. The inhibition by ATP was neither dependent on generation of adenosine by ATP hydrolysis nor the result of direct interaction of ATP with P1 purinergic receptors. Among other nucleotides tested (CTP, GTP, ITP, TTP, XTP, adenosine 5'-(beta,gamma-methylene)triphosphate (AMP-PCP), adenyl-5'-yl imidodiphosphate (AMP-P(NH)P), ADP, adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S), and UTP), only UTP and ATP gamma S displayed biphasic effects with potencies and efficacies almost identical to those of ATP. The other nucleotides only exhibited stimulatory effects (EC50 = 60-300 microM). The results are consistent with a model of dual regulation of AA release by two distinct subtypes of P2U receptors in HL60 cells.

EPR characterization of the stereochemistry of the distal heme pocket of the engineered human myoglobin mutants

Recombinant human myoglobin mutants with the distal histidine residue replaced by Leu, Val, or Gln residues have been prepared by site-directed mutagenesis and expression in Escherichia coli. The recombinant apomyoglobin proteins have been successfully reconstituted with cobaltous protoporphyrin IX to obtain cobalt myoglobin mutant proteins, and the role of the distal histidine residue on the interaction between the bound ligand and the myoglobin molecule has been studied by EPR spectroscopy. We found that the distal histidine residue is significant in the orientation of the bound oxygen molecule. Low temperature photolysis experiments on both oxy cobalt proteins and ferric nitric oxide complexes indicated that the nature of the photolyzed form depends on the steric crowding of the distal heme pocket. To our surprise, the distal Leu mutant has a less restricted, less sterically crowded distal heme pocket than that of the distal Val mutant myoglobin, despite the fact that Leu has a larger side chain volume than Val. Our results demonstrate that the distal heme pocket steric crowding is not necessarily related to the side chain volume of the E7 residue.

Roles of G proteins in coupling of receptors to ionic channels and other effector systems

Guanine nucleotide binding (G) proteins are heterotrimers that couple a wide range of receptors to ionic channels. The coupling may be indirect, via cytoplasmic agents, or direct, as has been shown for two K+ channels and two Ca2+ channels. One example of direct G protein gating is the atrial muscarinic K+ channel K+[ACh], an inwardly rectifying K+ channel with a slope conductance of 40 pS in symmetrical isotonic K+ solutions and a mean open lifetime of 1.4 ms at potentials between -40 and -100 mV. Another is the clonal GH3 muscarinic or somatostatin K+ channel, also inwardly rectifying but with a slope conductance of 55 pS. A G protein, Gk, purified from human red blood cells (hRBC) activates K+ [ACh] channels at subpicomolar concentrations; its alpha subunit is equipotent. Except for being irreversible, their effects on gating precisely mimic physiological gating produced by muscarinic agonists. The alpha k effects are general and are similar in atria from adult guinea pig, neonatal rat, and chick embryo. The hydrophilic beta gamma from transducin has no effect while hydrophobic beta gamma from brain, hRBCs, or retina has effects at nanomolar concentrations which in our hands cannot be dissociated from detergent effects. An anti-alpha k monoclonal antibody blocks muscarinic activation, supporting the concept that the physiological mediator is the alpha subunit not the beta gamma dimer. The techniques of molecular biology are now being used to specify G protein gating. A "bacterial" alpha i-3 expressed in Escherichia coli using a pT7 expression system mimics the gating produced by hRBC alpha k.

G protein coupling of receptors to ionic channels and other effector systems

1. Four questions raised by previous studies that had shown activation of K+ channels by alpha subunits of the type 3 Gi protein are addressed in the present communication: a) are K+ channels specific for one Gi? b) are there more ionic channels under direct G protein control? c) can we confirm using recombinant G alpha s the results obtained with biochemically resolved G alpha s and continue ascribing the regulatory effector to this part of the alpha beta gamma holo-G protein? and d) can we confirm that a single G alpha, Gs alpha in this case, is able to affect more than one type of effector function? 2. We found Gi alpha s are isoforms, that there exist also Gi-insensitive, Go-responsive K+ channels and that G alpha s can be multifunctional. Thus, a single receptor will elicit cellular responses that will depend on the endogenous G protein as well as the type of effector function expressed in it. 3. In another set of experiments we found that G beta gamma s, be they derived from human erythrocytes, human placenta, bovine brain or bovine retina, all inhibit Gk-gated K+ channel activity as seen in inside out membrane patches with GTP as the driving nucleotide. In addition we noted that inhibition was much more effective under basal (no agonist in the pipette) than agonist stimulated conditions, as reported in earlier experiments in which beta-adrenoceptors, Gs and catalytic unit of adenylyl cyclase had been incorporated into phospholipid vesicles. 4. We propose that one of the roles of G beta gamma s in membranes is to quench ligand independent G protein activation by unoccupied receptors. Other roles of G beta gamma s are: a) by re-associating with GDP-G alpha s, to promote interaction with receptors, and b) by dissociating from activated R.G alpha *GTP.beta gamma, to allow for receptor dissociation from GTP-activated G alpha s, which is required to satisfy the catalytic mode of receptor action.