Phosphorylation-dependent down-modulation of CD4 requires a specific structure within the cytoplasmic domain of CD4

A Novel Biallelic LCK Variant Resulting in Profound T-Cell Immune Deficiency and Review of the Literature

Lymphocyte-specific protein tyrosine kinase (LCK) is an SRC-family kinase critical for initiation and propagation of T-cell antigen receptor (TCR) signaling through phosphorylation of TCR-associated CD3 chains and recruited downstream molecules. Until now, only one case of profound T-cell immune deficiency with complete LCK deficiency [1] caused by a biallelic missense mutation (c.1022T>C, p.L341P) and three cases of incomplete LCK deficiency [2] caused by a biallelic splice site mutation (c.188-2A>G) have been described. Additionally, deregulated LCK expression has been associated with genetically undefined immune deficiencies and hematological malignancies. Here, we describe the second case of complete LCK deficiency in a 6-month-old girl born to consanguineous parents presenting with profound T-cell immune deficiency. Whole exome sequencing (WES) revealed a novel pathogenic biallelic missense mutation in LCK (c.1393T>C, p.C465R), which led to the absence of LCK protein expression and phosphorylation, and a consecutive decrease in proximal TCR signaling. Loss of conventional CD4+ and CD8+ αβT-cells and homeostatic T-cell expansion was accompanied by increased γδT-cell and Treg percentages. Surface CD4 and CD8 co-receptor expression was reduced in the patient T-cells, while the heterozygous mother had impaired CD4 and CD8 surface expression to a lesser extent. We conclude that complete LCK deficiency is characterized by profound T-cell immune deficiency, reduced CD4 and CD8 surface expression, and a characteristic TCR signaling disorder. CD4 and CD8 surface expression may be of value for early detection of mono- and/or biallelic LCK deficiency.

Interactions between the Hepatitis C Virus Nonstructural 2 Protein and Host Adaptor Proteins 1 and 4 Orchestrate Virus Release

Hepatitis C virus (HCV) spreads via secreted cell-free particles or direct cell-to-cell transmission. Yet, virus-host determinants governing differential intracellular trafficking of cell-free- and cell-to-cell-transmitted virus remain unknown. The host adaptor proteins (APs) AP-1A, AP-1B, and AP-4 traffic in post-Golgi compartments, and the latter two are implicated in basolateral sorting. We reported that AP-1A mediates HCV trafficking during release, whereas the endocytic adaptor AP-2 mediates entry and assembly. We demonstrated that the host kinases AAK1 and GAK regulate HCV infection by controlling these clathrin-associated APs. Here, we sought to define the roles of AP-4, a clathrin-independent adaptor; AP-1A; and AP-1B in HCV infection. We screened for interactions between HCV proteins and the μ subunits of AP-1A, AP-1B, and AP-4 by mammalian cell-based protein fragment complementation assays. The nonstructural 2 (NS2) protein emerged as an interactor of these adaptors in this screening and by coimmunoprecipitations in HCV-infected cells. Two previously unrecognized dileucine-based motifs in the NS2 C terminus mediated AP binding and HCV release. Infectivity and coculture assays demonstrated that while all three adaptors mediate HCV release and cell-free spread, AP-1B and AP-4, but not AP-1A, mediate cell-to-cell spread. Live-cell imaging revealed HCV cotrafficking with AP-1A, AP-1B, and AP-4 and that AP-4 mediates HCV trafficking in a post-Golgi compartment. Lastly, HCV cell-to-cell spread was regulated by AAK1 and GAK and thus susceptible to treatment with AAK1 and GAK inhibitors. These data provide a mechanistic understanding of HCV trafficking in distinct release pathways and reveal a requirement for APs in cell-to-cell viral spread.

HCV spreads via cell-free infection or cell-to-cell contact that shields it from antibody neutralization, thereby facilitating viral persistence. Yet, factors governing this differential sorting remain unknown. By integrating proteomic, RNA interference, genetic, live-cell imaging, and pharmacological approaches, we uncover differential coopting of host adaptor proteins (APs) to mediate HCV traffic at distinct late steps of the viral life cycle. We reported that AP-1A and AP-2 mediate HCV trafficking during release and assembly, respectively. Here, we demonstrate that dileucine motifs in the NS2 protein mediate AP-1A, AP-1B, and AP-4 binding and cell-free virus release. Moreover, we reveal that AP-4, an adaptor not previously implicated in viral infections, mediates cell-to-cell spread and HCV trafficking. Lastly, we demonstrate cell-to-cell spread regulation by AAK1 and GAK, host kinases controlling APs, and susceptibility to their inhibitors. This study provides mechanistic insights into virus-host determinants that facilitate HCV trafficking, with potential implications for pathogenesis and antiviral agent design.

Viral channel proteins in intracellular protein-protein communication: Vpu of HIV-1, E5 of HPV16 and p7 of HCV

Viral channel forming proteins are known for their capability to make the lipid membrane of the host cell and its subcellular compartments permeable to ions and small compounds. There is increasing evidence that some of the representatives of this class of proteins are also strongly interacting with host proteins and the effectiveness of this interaction seems to be high. Interaction of viral channel proteins with host factors has been proposed by bioinformatics approaches and has also been identified experimentally. An overview of the interactions with host proteins is given for Vpu from HIV-1, E5 from HPV-16 and p7 from HCV. This article is part of a Special Issue entitled: Viral Membrane Proteins - Channels for Cellular Networking.

Identification of a novel binding site between HIV type 1 Nef C-terminal flexible loop and AP2 required for Nef-mediated CD4 downregulation

HIV-1 Nef is an accessory protein necessary for HIV-1 virulence and rapid AIDS development. Nef promotes viral replication and infection by connecting CD4 and several other cell surface receptors to the clathrin adaptor protein AP2, resulting in the internalization and degradation of the receptors interacting with Nef. We investigated how Nef can mediate constitutive receptor endocytosis through the interaction of the dileucine motif in its C-terminal flexible loop (C-loop) with AP2, whereas AP2 binding of the transmembrane receptors usually results in an equilibrated (recycled) endocytosis. Our results indicated that in addition to the dileucine motif, there is a second motif in the Nef C-loop involved in the Nef-AP2 interaction. Nef-mediated CD4 downregulation was impaired when the residue in the hydrophobic region in the Nef C-loop (LL165HPMSLHGM173) was mutated to a basic residue K/R or an acidic residue E/D or to the rigid residue P, or when M168L170, L170H171, or G172M173 was mutated to AA. A pull-down assay indicated that AP2 was not coprecipitated with Nef mutants that did not downregulate CD4. Molecular modeling of the Nef C-terminal flexible loop in complex with AP2 suggests that M168L170 occupies a pocket in the AP2 σ2 subunit. Our data suggest a new model in the Nef-AP2 interaction in which the hydrophobic region in the Nef C-loop with the dileucine (L164L165) motif and M168L170 motif binds to AP2(σ2), while the acidic motif E174 and D175 binds to AP2(α), which explains how Nef through the flexible loop connects CD4 to AP2 for constitutive CD4 downregulation.

Protein kinase C and NF-κB-dependent CD4 downregulation in macrophages induced by T cell-derived soluble factors: consequences for HIV-1 infection

Upon activation, CD4(+) T cells release cytokines, chemokines, and other soluble factors that influence the kinetics of HIV-1 replication in macrophages (M). In this article, we show that activation of human primary T cells suppresses the early stages of HIV-1 replication in human primary Mφ by downregulating the main cellular receptor for the virus CD4. The secreted factors responsible for this effect have a molecular mass greater than conventional cytokines, are independent of Th1 or Th2 polarization, and are not IFN-γ, IL-16, RANTES, or macrophage inhibitory factor, as revealed by cytokine array analysis and neutralization assays. CD4 downregulation is entirely posttranslational and involves serine phosphorylation of CD4 and its targeting to an intracellular compartment destined for acidification and degradation. CD4 downregulation is dependent on the activities of both protein kinase C and NF-κB as well as the proteasomes. Using high-resolution liquid chromatography-tandem mass spectrometry analysis in conjugation with label-free protein quantitation software, we found that proteins that promote Mφ adherence and spreading, such as attractin, fibronectin, and galectin-3-binding protein, were significantly overrepresented in the activated T cell supernatant fractions. These results reveal the existence of previously unreported anti-HIV-1 proteins, released by activated T cells that downregulate CD4 expression, and are of fundamental importance to understand the kinetics of HIV infection in vivo.

Human immunodeficiency virus (HIV) type-1, HIV-2 and simian immunodeficiency virus Nef proteins

The genomes of all retroviruses encode the Gag Pol and Env structural proteins. Human and simian lentiviruses have acquired non-structural proteins among which Nef plays a major role in the evolution of viral infection towards an immunodeficiency syndrome. Indeed, in the absence of a functional nef gene, primate lentiviruses are far less pathogenic than their wild type counterparts. The multiple protein-protein interactions in which Nef is involved all contribute to explain the role played by Nef in HIV- and SIV-associated disease progression. This review summarizes common and distinct features among Nef proteins and how they contribute to increasing HIV and SIV fitness towards their respective hosts.

Nef-induced CD4 endocytosis in human immunodeficiency virus type 1 host cells: role of p56lck kinase

Human immunodeficiency virus type 1 (HIV-1) Nef interferes with the endocytic machinery to modulate the cell surface expression of CD4. However, the basal trafficking of CD4 is governed by different rules in the target cells of HIV-1: whereas CD4 is rapidly internalized from the cell surface in myeloid cells, CD4 is stabilized at the plasma membrane through its interaction with the p56(lck) kinase in lymphoid cells. In this study, we showed that Nef was able to downregulate CD4 in both lymphoid and myeloid cell lines but that an increase in the internalization rate of CD4 could be observed only in lymphoid cells. Expression of p56(lck) in nonlymphoid CD4-expressing cells restores the ability of Nef in order to increase the internalization rate of CD4. Concurrent with this observation, the expression of a p56(lck)-binding-deficient mutant of CD4 in lymphoid cells abrogates the Nef-induced acceleration of CD4 internalization. We also show that the expression of Nef causes a decrease in the association of p56(lck) with cell surface-expressed CD4. Regardless of the presence of p56(lck), the downregulation of CD4 by Nef was followed by CD4 degradation. Our results imply that Nef uses distinct mechanisms to downregulate the cell surface expression levels of CD4 in either lymphoid or myeloid target cells of HIV-1.

Molecular characterisation and expression of CD4 in two distantly related marsupials: the gray short-tailed opossum (Monodelphis domestica) and tammar wallaby (Macropus eugenii)

The gene and corresponding cDNA for CD4 in the gray short-tailed opossum, Monodelphis domestica, and the cDNA sequence for CD4 in the tammar wallaby, Macropus eugenii, have been characterised. The opossum CD4 homolog reveals conserved synteny, preserved genomic organisation and analogous structural arrangement to human and mouse CD4. Opossum and tammar CD4 exhibit typical eutherian CD4 features including the highly conserved p56(lck) binding motif in the cytoplasmic region and the invariant cysteine residues in extracellular domains 1 and 4. Interestingly, the marsupial CD4 sequences substitute a tryptophan for the first cysteine in domain 2 negating the formation of a disulphide bond as seen in other eutherian CD4 sequences except human and mouse. Overall the marsupial CD4 sequences share amino acid identity of 59% to each other and 37-41% with eutherian mammals. However, in contrast to eutherian homologs, the marsupial CD4 sequences were found to be truncated at the terminal end of the cytoplasmic tail. This is the first report confirming the presence of CD4 in a marsupial and describing its key features.

CD4 Phosphorylation Partially Reverses Nef Down-Regulation of CD4

HIV Nef down-regulates CD4 from the cell surface in the absence of CD4 phosphorylation, whereas PMA down-regulates CD4 through a phosphorylation-dependent pathway. In this study we show that the down-regulation of CD4 in human Jurkat T cells expressing Nef was nearly complete (approximately 95%), whereas that induced by PMA was partial (approximately 40%). Unexpectedly, treating T cells expressing Nef with PMA restored the surface CD4 up to 35% of the steady state level. Both mutating the phosphorylation sites in the CD4 cytoplasmic tail (Ser408 and Ser415) and the use of a protein kinase C inhibitor, bisindolylmaleimide1, abolished the restoration of surface CD4, suggesting that the restoration required CD4 phosphorylation. CD4 and Nef could be cross-linked by a chemical cross-linker, 3,3-dithiobis[sulfosuccinimidyl-propionate], in control T cell membranes, but not in PMA-treated T cell membrane, suggesting that CD4 and Nef interacted with each other in T cells, and the phosphorylation disrupted the CD4-Nef interaction. We propose that this dissociation switches CD4 internalization from the Nef-mediated, nearly complete down-regulation to a phosphorylation-dependent, partial down-regulation, resulting in a net gain of CD4 on the T cell surface.

Structure and regulation of Src family kinases

Src family kinases are prototypical modular signaling proteins. Their conserved domain organization includes a myristoylated N-terminal segment followed by SH3, SH2, and tyrosine kinase domains, and a short C-terminal tail. Structural dissection of Src kinases has elucidated the canonical mechanisms of phosphotyrosine recognition by the SH2 domain and proline-motif recognition by the SH3 domain. Crystallographic analysis of nearly intact Src kinases in the autoinhibited state has shown that these protein interaction motifs turn inward and lock the kinase in an inactive conformation via intramolecular interactions. The autoinhibited Src kinase structures reveal a mode of domain assembly used by other tyrosine kinases outside the Src family, including Abl and likely Tec family kinases. Furthermore, they illustrate the underlying regulatory principles that have proven to be general among diverse modular signaling proteins. Although there is considerable structural information available for the autoinhibited conformation of Src kinases, how they may assemble into active signaling complexes with substrates and regulators remains largely unexplored.

Targeting of membrane proteins to endosomes and lysosomes

The pathways involved in targeting membrane proteins to lysosomes are extraordinarily complex. Newly synthesized proteins in the ER are transported to the Golgi complex, and upon arrival at the trans Golgi network (TGN) are targeted either directly to endosomes, or first to the cell surface from where they can be rapidly internalized into the endocytic pathway for delivery to lysosomes. The routes to endosomes are specified by sorting motifs in the cytoplasmic tails of the proteins that are recognized at the TGN or plasma membrane. The molecular details of these processes are just emerging.

Degradation of origin recognition complex large subunit by the anaphase-promoting complex in Drosophila

The initiation of DNA synthesis is thought to occur at sites bound by a heteromeric origin recognition complex (ORC). Previously, we have shown that in Drosophila, the level of the large subunit, ORC1, is modulated during cell cycle progression and that changes in ORC1 concentration alter origin utilization during development. Here, we investigate the mechanisms underlying cell cycle-dependent degradation of ORC1. We show that signals in the non-conserved N-terminal domain of ORC1 mediate its degradation upon exit from mitosis and in G1 phase by the anaphase-promoting complex (APC) in vivo. Degradation appears to be the result of direct action of the APC, as the N-terminal domain is ubiquitylated by purified APC in vitro. This regulated proteolysis is potent, sufficient to generate a normal temporal distribution of protein even when transcription of ORC1 is driven by strong constitutive promoters. These observations suggest that in Drosophila, ORC1 regulates origin utilization much as does Cdc6 in budding yeast.

Ubiquitin system-dependent regulation of growth hormone receptor signal transduction

The growth hormone (GH) receptor is a key regulator of cellular metabolism. Unlike most growth factor receptors, its downregulation is not initiated by its ligand. Like many growth factor receptors, specific molecular mechanisms guarantee that a receptor can signal only once in its lifetime. Three features render the GH receptor unique: (a) an active ubiquitination system is required for both uptake (endocytosis) and degradation in the lysosomes; (b) uptake of the receptor is a continuous process, independent of both GH binding and Jak2 signal transduction; (c) only the cell surface expression of dimerised GH receptors is controlled by the ubiquitin system. This system enables two independent regulatory mechanisms for the endocrinology of the GH/GHR axis: the pulsatile secretion of GH by the pituitary and the GH sensitivity of individual cells of the body by the effects of the ubiquitin system on GH receptor availability.

Receptor activation and 2 distinct COOH-terminal motifs control G-CSF receptor distribution and internalization kinetics

We have studied the intracellular distribution and internalization kinetics of the granulocyte colony-stimulating factor receptor (G-CSF-R) in living cells using fusion constructs of wild-type or mutant G-CSF-R and enhanced green fluorescent protein (EGFP). Under steady-state conditions the G-CSF-R localized predominantly to the Golgi apparatus, late endosomes, and lysosomes, with only low expression on the plasma membrane, resulting from spontaneous internalization. Internalization of the G-CSF-R was significantly accelerated by addition of G-CSF. This ligand-induced switch from slow to rapid internalization required the presence of G-CSF-R residue Trp650, previously shown to be essential for its signaling ability. Both spontaneous and ligand-induced internalization depended on 2 distinct amino acid stretches in the G-CSF-R COOH-terminus: 749-755, containing a dileucine internalization motif, and 756-769. Mutation of Ser749 at position -4 of the dileucine motif to Ala significantly reduced the rate of ligand-induced internalization. In contrast, mutation of Ser749 did not affect spontaneous G-CSF-R internalization, suggesting the involvement of a serine-threonine kinase specifically in ligand-accelerated internalization of the G-CSF-R. COOH-terminal truncation mutants of G-CSF-R, found in severe congenital neutropenia, lack the internalization motifs and were completely defective in both spontaneous and ligand-induced internalization. As a result, these mutants showed constitutively high cell-surface expression.

A Zinc Clasp Structure Tethers Lck to T Cell Coreceptors CD4 and CD8

The T cell coreceptors CD4 and CD8 both associate via their cytoplasmic tails with the N-terminus of the Src-family tyrosine kinase Lck. These interactions require zinc and are critical for T cell development and activation. We examined the folding and solution structures of ternary CD4-Lck-Zn2+ and CD8alpha-Lck-Zn2+ complexes. The coreceptor tails and the Lck N-terminus are unstructured in isolation but assemble in the presence of zinc to form compactly folded heterodimeric domains. The cofolded complexes have similar "zinc clasp" cores that are augmented by distinct structural elements. A dileucine motif required for clathrin-mediated endocytosis of CD4 is masked by Lck.

HIV coreceptors: role of structure, posttranslational modifications, and internalization in viral-cell fusion and as targets for entry inhibitors

The human immunodeficiency virus (HIV) envelope glycoprotein forms trimers on the virion surface, with each monomer consisting of two subunits, gp120 and gp41. The gp120 envelope component binds to CD4 on target cells and undergoes conformational changes that allow gp120 to interact with certain G-protein-coupled receptors (GPCRs) on the same target membranes. The GPCRs that function as HIV coreceptors were found to be chemokine receptors. The primary coreceptors are CCR5 and CXCR4, but several other chemokine receptors were identified as "minor coreceptors", indicating their ability support entry of some HIV strains in tissue cultures. Formation of the tri-molecular complexes stabilizes virus binding and triggers a series of conformational changes in gp41 that facilitate membrane fusion and viral cell entry. Concerted efforts are underway to decipher the specific interactions between gp120/CD4, gp120/coreceptors, and their contributions to the subsequent membrane fusion process. It is hoped that some of the transient conformational intermediates in gp120 and gp41 would serve as targets for entry inhibitors. In addition, the CD4 and coreceptors are primary targets for several classes of inhibitors currently under testing. Our review summarizes the current knowledge on the interactions of HIV gp120 with its receptor and coreceptors, and the important properties of the chemokine receptors and their regulation in primary target cells. We also summarize the classes of coreceptor inhibitors under development.

Cooperative interactions of simian immunodeficiency virus Nef, AP-2, and CD3-zeta mediate the selective induction of T-cell receptor-CD3 endocytosis

The Nef proteins of human immunodeficiency virus and simian immunodeficiency virus (SIV) bind the AP-1 and AP-2 clathrin adaptors to downmodulate the expression of CD4 and CD28 by recruiting them to sites of AP-2 clathrin-dependent endocytosis. Additionally, SIV Nef directly binds the CD3-zeta subunit of the CD3 complex and downmodulates the T-cell receptor (TCR)-CD3 complex. We report here that SIV mac239 Nef induces the endocytosis of TCR-CD3 in Jurkat T cells. SIV Nef also induces the endocytosis of a chimeric CD8-CD3-zeta protein containing only the CD3-zeta cytoplasmic domain (8-zeta), in the absence of other CD3 subunits. Thus, the interaction of SIV Nef with CD3-zeta likely mediates the induction of TCR-CD3 endocytosis. In cells expressing SIV Nef and 8-zeta, both proteins colocalize with AP-2, indicating that Nef induces 8-zeta internalization via this pathway. Surprisingly, deletion of constitutively strong AP-2 binding determinants (CAIDs) in SIV Nef had little effect on its ability to induce TCR-CD3, or 8-zeta endocytosis, even though these determinants are required for the induction of CD4 and CD28 endocytosis via this pathway. Fluorescent microscopic analyses revealed that while neither the mutant SIV Nef protein nor 8-zeta colocalized with AP-2 when expressed independently, both proteins colocalized with AP-2 when coexpressed. In vitro binding studies using recombinant SIV Nef proteins lacking CAIDs and recombinant CD3-zeta cytoplasmic domain demonstrated that SIV Nef and CD3-zeta cooperate to bind AP-2 via a novel interaction. The fact that Nef uses distinct AP-2 interaction surfaces to recruit specific membrane receptors demonstrates how Nef independently selects distinct types of target receptors and recruits them to AP-2 for endocytosis.

Constitutive GABAA receptor endocytosis is dynamin-mediated and dependent on a dileucine AP2 adaptin-binding motif within the beta 2 subunit of the receptor

Receptor endocytosis is an important mechanism for regulating the synaptic efficacy of neurotransmitters. There is strong evidence that GABA(A) receptor endocytosis is clathrin-dependent; however, this process is not well understood. Here we demonstrate that in HEK 293 cells, endocytosis of GABA(A) receptors composed of either alpha1beta2gamma2Lor alpha1beta2 subunits is blocked by the dominant negative dynamin construct K44A. Furthermore, we identify a dileucine AP2 adaptin-binding motif within the receptor beta2 subunit that is critical for endocytosis. Internalization of GABAA receptors lacking this motif is dramatically inhibited, and the receptors appear to accumulate on the cell surface. Patch clamp analysis of receptors lacking the dileucine motif show that there is an increase in the peak amplitude of GABA-gated chloride currents compared with wild-type receptors. Additionally, GABA-gated chloride currents in HEK 293 cells expressing wild-type receptors are increased by introduction of a peptide corresponding to the dileucine motif region of the receptor beta2 subunit but not by a control peptide containing alanine substitutions for the dileucine motif. In mouse brain cerebral cortical neurons, the dileucine motif peptide increases GABA-gated chloride currents of native GABA(A) receptors. This is the first report to our knowledge that an AP2 adaptin dileucine recognition motif is critical for the endocytosis of ligand-gated ion channels belonging to this superfamily.

Involvement of CD4 D3-D4 membrane proximal extracellular domain for the inhibitory effect of oxidative stress on activation-induced CD4 down-regulation and its possible role for T cell activation

During antigen presentation, CD4 functions to stabilize T cell receptor (TCR)-class II MHC interactions and coordinate Ag-induced T cell activation signals. These activation signals cause CD4 down-regulation, presumably acting to optimize T cell activation. We previously reported that oxidative stress interferes with activation-induced CD4 down-regulation in T cells. In this study, we have further investigated inhibition of CD4 down-regulation by oxidative stress and its role for T cell activation. A construct comprised of the mouse FcgammaRIIB extracellular domain and the transmembrane/cytoplasmic domains of human CD4 (FcgammaR/CD4) was expressed in a human T cell line. Oxidant actually potentiated down-regulation of the FcgammaR/CD4 chimera and induced Lck dissociation from both CD4 and FcgammaR/CD4, which is a crucial intracellular process for activation-induced CD4 down-regulation, suggesting a critical role of CD4 ectodomain in the inhibition of CD4 down-regulation by oxidative stress. Furthermore, insertion of CD4 D3-D4 membrane proximal extracellular region between FcgammaR extracellular domain and CD4 transmembrane/cytoplasmic domains in FcgammaR/CD4 chimera made this molecule behave like native CD4 molecule under oxidative stress condition. These data imply that the inhibitory effect of oxidative stress on CD4 down-regulation is executed via D3-D4 domain of CD4 ectodomain. As to its role for T cell activation, CD4 coaggregation with CD3 under the oxidative conditions enhanced activation signal induced by CD3 aggregation. Our results demonstrate that Ag-induced T cell activation which is normally concomitant with CD4 down-regulation may be disturbed through the aberrant regulation of CD4 expression by oxidative stress.

Protein kinase C activation decreases cell surface expression of the GLT-1 subtype of glutamate transporter. Requirement of a carboxyl-terminal domain and partial dependence on serine 486

Na(+)-dependent glutamate transporters are required for the clearance of extracellular glutamate and influence both physiological and pathological effects of this excitatory amino acid. In the present study, the effects of a protein kinase C (PKC) activator on the cell surface expression and activity of the GLT-1 subtype of glutamate transporter were examined in two model systems, primary co-cultures of neurons and astrocytes that endogenously express GLT-1 and C6 glioma cells transfected with GLT-1. In both systems, activation of PKC with phorbol ester caused a decrease in GLT-1 cell surface expression. This effect is opposite to the one observed for the EAAC1 subtype of glutamate transporter (Davis, K. E., Straff, D. J., Weinstein, E. A., Bannerman, P. G., Correale, D. M., Rothstein, J. D., and Robinson, M. B. (1998) J. Neurosci. 18, 2475-2485). Several recombinant chimeric proteins between GLT-1 and EAAC1 transporter subtypes were generated to identify domains required for the subtype-specific redistribution of GLT-1. We identified a carboxyl-terminal domain consisting of 43 amino acids (amino acids 475-517) that is required for PKC-induced GLT-1 redistribution. Mutation of a non-conserved serine residue at position 486 partially attenuated but did not completely abolish the PKC-dependent redistribution of GLT-1. Although we observed a phorbol ester-dependent incorporation of (32)P into immunoprecipitable GLT-1, mutation of serine 486 did not reduce this signal. We also found that chimeras containing the first 446 amino acids of GLT-1 were not functional unless amino acids 475-517 of GLT-1 were also present. These non-functional transporters were not as efficiently expressed on the cell surface and migrated to a smaller molecular weight, suggesting that a subtype-specific interaction is required for the formation of functional transporters. These studies demonstrate a novel effect of PKC on GLT-1 activity and define a unique carboxyl-terminal domain as an important determinant in cellular localization and regulation of GLT-1.

Identification of distinct roles for a dileucine and a tyrosine internalization motif in the interleukin (IL)-13 binding component IL-13 receptor alpha 2 chain

Interleukin (IL)-13 receptor alpha2 (IL-13Ralpha2) chain is an essential binding component for IL-13-mediated ligand binding. Recently, we have demonstrated that this receptor chain also plays an important role in the internalization of IL-13. To study the mechanism of IL-13 internalization, we generated mutated IL-13Ralpha2 chains that targeted trileucine residues (Leu(335), Leu(336), and Leu(337)) in the transmembrane domain and a tyrosine motif (Tyr(343)) in the intracellular domain and transfected these cDNAs in COS-7 cells. Cells that expressed a C-terminally truncated IL-13Ralpha2 chain (Delta335) did not bind IL-13, suggesting that the trileucine region modulates IL-13 binding. Truncation of IL-13Ralpha2 chain with a mutation in the trileucine region resulted in significantly decreased internalization compared with wild type IL-13Ralpha2 chain transfected cells. COS-7 cells transfected with tyrosine motif mutants exhibited a similar internalization level compared with wild type IL-13Ralpha2 chain transfected cells; however, dissociation of cell surface IL-13 was faster compared with wild type IL-13Ralpha2 transfectants. These results were further confirmed by determining the cytotoxicity of a chimeric protein composed of IL-13 and a mutated form of Pseudomonas exotoxin (IL13-PE38QQR) to cells that expressed IL-13Ralpha2 chain mutants. We further demonstrate that the IL-13Ralpha2 chain is not ubiquitinated and that internalization of IL-13Ralpha2 did not depend on ubiquitination. Together, our findings suggest that the dileucine motif in the trileucine region and tyrosine motif participate in IL-13Ralpha2 internalization in distinct manners.

Distinct reading of different structural determinants modulates the dileucine-mediated transport steps of the lysosomal membrane protein LIMPII and the insulin-sensitive glucose transporter GLUT4

Leucine-based motifs mediate the sorting of membrane proteins at such cellular sites as the trans-Golgi network, endosomes, and plasma membrane. A Leu paired with a second Leu, Ile, or Met, while itself lacking the ability to mediate transport, is the key structural feature in these motifs. Here we have studied the structural differences between the leucine-based motifs contained in the COOH tails of LIMPII and GLUT4, two membrane proteins that are transported through the secretory pathway and are targeted to lysosomes () and to a perinuclear compartment adjacent to the Golgi complex (), respectively. LIMPII and GLUT4 display negatively (Asp(470)/Glu(471)) and positively (Arg(484)/Arg(485)) charged residues, respectively, at positions -4 and -5 upstream from the critical Leu residue. The change in the charge sign of residues -4 and -5 results in missorting of LIMPII and GLUT4. We note that the acidic Glu residue at position -4 is critical for efficient intracellular sorting of LIMPII to lysosomes, but is dispensable for its surface internalization by endocytosis. Efficient intracellular sorting and endocytosis of GLUT4 require an Arg pair between positions -4 and -7. These results are consistent with the existence of distinct leucine-based motifs and provide evidence of their different readings at different cellular sites.

Phosphorylation of human gp130 at Ser-782 adjacent to the Di-leucine internalization motif. Effects on expression and signaling

The receptor for leukemia inhibitory factor (LIF) consists of two polypeptides, the LIF receptor and gp130. Agonist stimulation has been shown previously to cause phosphorylation of gp130 on serine, threonine, and tyrosine residues. We found that gp130 fusion proteins were phosphorylated exclusively on Ser-782 by LIF- and growth factor-stimulated 3T3-L1 cell extracts. Ser-780 was required for phosphorylation of Ser-782 but was not itself phosphorylated. Ser-782 is located immediately N-terminal to the di-leucine motif of gp130, which regulates internalization of the receptor. Transient expression of chimeric granulocyte colony-stimulating factor receptor (G-CSFR)-gp130(S782A) receptors resulted in increased cell surface expression in COS-7 cells and increased ability to induce vasoactive intestinal peptide gene expression in IMR-32 neuroblastoma cells when compared with expression of chimeric receptors containing wild-type gp130 cytoplasmic domains. These results identify Ser-782 as the major phosphorylated serine residue in human gp130 and indicate that this site regulates cell surface expression of the receptor polypeptide.

Selective targeting of avidin/mannose 6-phosphate receptor chimeras to early or late endosomes

In this study we have used the Semliki forest virus expression system to transiently express chimeric proteins that contain transmembrane and cytoplasmic domains of the cation-independent mannose 6-phosphate receptor (CI-MPR) fused to chicken avidin. Immunofluorescence and electron microscopy studies showed that the chimeric protein with the entire cytoplasmic domain of CI-MPR was transported to late endosomes, where it accumulated. We made use of the biotin-binding capacity of lumenal avidin, and found that, in agreement with this distribution, the chimeric protein could be labelled with biotinylated HRP endocytosed for a long, but not a brief, period of time. However, truncation of the C-terminal tail distal to the rapid endocytosis motif (YKYSKV), caused the truncated chimera to be transported to, and accumulated within, early endosomes. This truncated chimera did not reach recycling early endosomes labelled with internalised transferrin, to any significant extent, but was accessible to biotinylated HRP internalised for 5 min (or for longer periods at 19 degrees C). Coinfection of these chimeras showed that they follow the same route from the TGN to the early endosomes. We conclude that the sequence distal to the endocytosis motif contains the signals which are required for efficient transport to late endosomes. Our results also suggest that the YKYSKV sequence close to the CI-MPR transmembrane segment is sufficient for targeting to sorting early endosomes.

A phosphorylation site regulates sorting of the vesicular acetylcholine transporter to dense core vesicles

Vesicular transport proteins package classical neurotransmitters for regulated exocytotic release, and localize to at least two distinct types of secretory vesicles. In PC12 cells, the vesicular acetylcholine transporter (VAChT) localizes preferentially to synaptic-like microvesicles (SLMVs), whereas the closely related vesicular monoamine transporters (VMATs) localize preferentially to large dense core vesicles (LDCVs). VAChT and the VMATs contain COOH-terminal, cytoplasmic dileucine motifs required for internalization from the plasma membrane. We now show that VAChT undergoes regulated phosphorylation by protein kinase C on a serine (Ser-480) five residues upstream of the dileucine motif. Replacement of Ser-480 by glutamate, to mimic the phosphorylation event, increases the localization of VAChT to LDCVs. Conversely, the VMATs contain two glutamates upstream of their dileucine-like motif, and replacement of these residues by alanine conversely reduces sorting to LDCVs. The results provide some of the first information about sequences involved in sorting to LDCVs. Since the location of the transporters determines which vesicles store classical neurotransmitters, a change in VAChT trafficking due to phosphorylation may also influence the mode of transmitter release.

Shedding of syndecan-1 and -4 ectodomains is regulated by multiple signaling pathways and mediated by a TIMP-3-sensitive metalloproteinase

The syndecan family of four transmembrane heparan sulfate proteoglycans binds a variety of soluble and insoluble extracellular effectors. Syndecan extracellular domains (ectodomains) can be shed intact by proteolytic cleavage of their core proteins, yielding soluble proteoglycans that retain the binding properties of their cell surface precursors. Shedding is accelerated by PMA activation of protein kinase C, and by ligand activation of the thrombin (G-protein-coupled) and EGF (protein tyrosine kinase) receptors (Subramanian, S.V., M.L. Fitzgerald, and M. Bernfield. 1997. J. Biol. Chem. 272:14713-14720). Syndecan-1 and -4 ectodomains are found in acute dermal wound fluids, where they regulate growth factor activity (Kato, M., H. Wang, V. Kainulainen, M.L. Fitzgerald, S. Ledbetter, D.M. Ornitz, and M. Bernfield. 1998. Nat. Med. 4:691-697) and proteolytic balance (Kainulainen, V., H. Wang, C. Schick, and M. Bernfield. 1998. J. Biol. Chem. 273:11563-11569). However, little is known about how syndecan ectodomain shedding is regulated. To elucidate the mechanisms that regulate syndecan shedding, we analyzed several features of the process that sheds the syndecan-1 and -4 ectodomains. We find that shedding accelerated by various physiologic agents involves activation of distinct intracellular signaling pathways; and the proteolytic activity responsible for cleavage of syndecan core proteins, which is associated with the cell surface, can act on unstimulated adjacent cells, and is specifically inhibited by TIMP-3, a matrix-associated metalloproteinase inhibitor. In addition, we find that the syndecan-1 core protein is cleaved on the cell surface at a juxtamembrane site; and the proteolytic activity responsible for accelerated shedding differs from that involved in constitutive shedding of the syndecan ectodomains. These results demonstrate the existence of highly regulated mechanisms that can rapidly convert syndecans from cell surface receptors or coreceptors to soluble heparan sulfate proteoglycan effectors. Because the shed ectodomains are found and function in vivo, regulation of syndecan ectodomain shedding by physiological mediators indicates that shedding is a response to specific developmental and pathophysiological cues.

Accurate intracellular localization of HLA-DM requires correct spacing of a cytoplasmic YTPL targeting motif relative to the transmembrane domain

HLA-DM is an MHC class II-related heterodimer that is targeted to lysosomal compartments by a tyrosine-based signal YTPL, present in the cytoplasmic tail of the beta chain. Similar signals in other proteins control transport to different intracellular locations and can be recognized at several sorting sites within the cell including the trans-Golgi network, the plasma membrane and the early or sorting endosome. Therefore, in addition to recognizing the basic tyrosine motif, the sorting machinery must be sensitive to additional features associated with these elements. Here we show that efficient trafficking of HLA-DM to lysosomal compartments is dependent upon the proximity of its tyrosine motif to the transmembrane domain. Constructs in which the spacing is altered are rapidly internalized but are expressed at the cell surface. We conclude that the spacing of the HLA-DMB-encoded tyrosine motif relative to the transmembrane domain is an important feature controlling DM sorting in endosomes.

Membrane trafficking of neurotransmitter transporters in the regulation of synaptic transmission

Many psychoactive drugs influence the transport of neurotransmitters across biological membranes, suggesting that the physiological regulation of neurotransmitter transport might contribute to normal and perhaps abnormal behaviour. Over the past few years, molecular characterization of the neurotransmitter transporters has enabled investigation of their subcellular location and regulation. The analysis of location suggests that membrane trafficking has an important role in the normal function of these proteins. One of the major regulatory mechanisms also involves changes in localization that might contribute to synaptic plasticity. This article discusses recent work on the membrane trafficking of neurotransmitter transporters and its role in regulating their activity.

The C terminus of SUR1 is required for trafficking of KATP channels

In beta cells from the pancreas, ATP-sensitive potassium channels, or KATP channels, are composed of two subunits, SUR1 and KIR6.2, assembled in a (SUR1/KIR6.2)4 stoichiometry. The correct stoichiometry of channels at the cell surface is tightly regulated by the presence of novel endoplasmic reticulum (ER) retention signals in SUR1 and KIR6.2; incompletely assembled KATP channels fail to exit the ER/cis-Golgi compartments. In addition to these retrograde signals, we show that the C terminus of SUR1 has an anterograde signal, composed in part of a dileucine motif and downstream phenylalanine, which is required for KATP channels to exit the ER/cis-Golgi compartments and transit to the cell surface. Deletion of as few as seven amino acids, including the phenylalanine, from SUR1 markedly reduces surface expression of KATP channels. Mutations leading to truncation of the C terminus of SUR1 are one cause of a severe, recessive form of persistent hyperinsulinemic hypoglycemia of infancy. We propose that the complete loss of beta cell KATP channel activity seen in this form of hyperinsulinism is a failure of KATP channels to traffic to the plasma membrane.

Protein Kinase C Activation Inhibits Tyrosine Phosphorylation of Cbl and Its Recruitment of Src Homology 2 Domain-Containing Proteins

One of the major proteins that is rapidly tyrosine phosphorylated upon stimulation of the TCR/CD3 complex is the 120-kDa product of the c-cbl protooncogene (Cbl). Upon activation, tyrosine-phosphorylated Cbl interacts with the Src homology 2 (SH2) domains of several signaling proteins, e.g., phosphatidylinositol 3-kinase (PI3-K) and CrkL. In the present study, we report that pretreatment of Jurkat T cells with PMA reduced the anti-CD3-induced tyrosine phosphorylation of Cbl and, consequently, its activation-dependent association with PI3-K and CrkL. A specific protein kinase C (PKC) inhibitor (GF-109203X) reversed the effect of PMA on tyrosine phosphorylation of Cbl and restored the activation-dependent association of Cbl with PI3-K and CrkL. We also provide evidence that PKCα and PKCθ can physically associate with Cbl and are able to phosphorylate it in vitro and in vivo. Furthermore, a serine-rich motif at the C terminus of Cbl, which is critical for PMA-induced 14-3-3 binding, is also phosphorylated by PKCα and PKCθ in vitro. These results suggest that, by regulating tyrosine and serine phosphorylation of Cbl, PKC is able to control the association of Cbl with signaling intermediates, such as SH2 domain-containing proteins and 14-3-3 proteins, which may consequently result in the modulation of its function.

The ubiquitin-proteasome system and endocytosis

Internalization of membrane proteins has been studied for more than three decades without solving all the underlying mechanisms. Our knowledge of clathrin-mediated endocytosis is certainly sufficient to understand the basic principles. However, more detailed insight is required to recognize why different proteins enter clathrin-coated pits with different rates and affinities. In addition to clathrin coat components, at least two adaptor systems and even more accessory proteins have been described to preselect membrane proteins before they can enter cells. Recent experimental data have identified the ubiquitin-proteasome system as a regulatory system for endocytosis. This system is well-known for its basic regulatory function in protein degradation, and controls a magnitude of key events. The ubiquitin-proteasome system is now identified as a regulator of the endocytosis of selected membrane proteins. In this review, we will discuss the complexity and implications of this mechanism for receptor-mediated endocytosis.

Phorbol ester-induced disruption of the CD4-Lck complex occurs within a detergent-resistant microdomain of the plasma membrane. Involvement of the translocation of activated protein kinase C isoforms

Recent studies have highlighted the existence of discrete microdomains at the cell surface that are distinct from caveolae. The function of these microdomains remains unknown. However, recent evidence suggests that they may participate in a subset of transmembrane signaling events. In hematopoietic cells, these low density Triton-insoluble (LDTI) microdomains (also called caveolae-related domains) are dramatically enriched in signaling molecules, such as cell surface receptors (CD4 and CD55), Src family tyrosine kinases (Lyn, Lck, Hck, and Fyn), heterotrimeric G proteins, and gangliosides (GM1 and GM3). Human T lymphocytes have become a well established model system for studying the process of phorbol ester-induced down-regulation of CD4. Here, we present evidence that phorbol 12-myristate 13-acetate (PMA)-induced down-regulation of the cell surface pool of CD4 occurs within the LDTI microdomains of T cells. Localization of CD4 in LDTI microdomains was confirmed by immunoelectron microscopy. PMA-induced disruption of the CD4-Lck complex was rapid (within 5 min), and this disruption occurred within LDTI microdomains. Because PMA is an activator of protein kinase C (PKC), we next evaluated the possible roles of different PKC isoforms in this process. Our results indicate that PMA induced the rapid translocation of cytosolic PKCs to LDTI microdomains. We identified PKCalpha as the major isoform involved in this translocation event. Taken together, our results support the hypothesis that LDTI microdomains represent a functionally important plasma membrane compartment in T cells.

Phosphorylation of the cytoplasmic domain of Alzheimer's beta-amyloid precursor protein at Ser655 by a novel protein kinase

The cytoplasmic domain of Alzheimer's beta-amyloid precursor protein (APP) can be phosphorylated at Thr654, Ser655, and Thr668 (APP695 isoform numbering). Previous studies demonstrated that Ser655 of APP was phosphorylated by protein kinase C (PKC) and calmodulin-dependent protein kinase II (CaMKII) in vitro and by unidentified protein kinase(s) in vivo. We report here the characterization of a novel protein kinase (designated APP kinase I) which phosphorylates Ser655 of APP. APP kinase I was partially purified over 7,000-fold from rat brain and identified as a approximately 43 kDa protein that is distinct from a number of known protein kinases, including PKC and extracellular signal-regulated kinases (ERKs). The identification of a novel protein kinase that phosphorylates Ser655 will hopefully contribute to our understanding of the metabolism and/or function of APP in the pathogenesis of Alzheimer's disease (AD).

Molecular cloning of a lipolysis-stimulated remnant receptor expressed in the liver

The lipolysis-stimulated receptor (LSR) is a lipoprotein receptor primarily expressed in the liver and activated by free fatty acids. Antibodies inhibiting LSR functions showed that the receptor is a heterotrimer or tetramer consisting of 68-kDa (alpha) and 56-kDa (beta) subunits associated through disulfide bridges. Screening of expression libraries with these antibodies led to identification of mRNAs derived by alternate splicing from a single gene and coding for proteins with molecular masses matching that of LSR alpha and beta. Antibodies directed against a synthetic peptide of LSR alpha and beta putative ligand binding domains inhibited LSR activity. Western blotting identified two liver proteins with the same apparent molecular mass as that of LSR alpha and beta. Transient transfections of LSR alpha alone in Chinese hamster ovary cells increased oleate-induced binding and uptake of lipoproteins, while cotransfection of both LSR alpha and beta increased oleate-induced proteolytic degradation of the particles. The ligand specificity of LSR expressed in cotransfected Chinese hamster ovary cells closely matched that previously described using fibroblasts from subjects lacking the low density lipoprotein receptor. LSR affinity is highest for the triglyceride-rich lipoproteins, chylomicrons, and very low density lipoprotein. We speculate that LSR is a rate-limiting step for the clearance of dietary triglycerides and plays a role in determining their partitioning between the liver and peripheral tissues.

The downregulation of CD4 and MHC-I by primate lentiviruses: a paradigm for the modulation of cell surface receptors

The human and simian immunodeficiency viruses (HIV and SIV) downregulate the cell surface expression of CD4, their primary receptor, and of class I histocompatibility complex (MHC-I), a critical mediator of immune recognition. While the first of these effects seems important to preserve viral infectivity, the second likely promotes immune evasion. Three HIV-1 proteins, Nef, Env and Vpu, contribute to downregulate CD4, Env forms a complex with CD4 in the endoplasmic reticulum, thereby retaining the receptor in this compartment. Nef and Vpu, on the other hand, act as connectors between CD4 and specific intracellular trafficking pathways, targeting the receptor for degradation in the lysosome and the proteasome, respectively. Some of the downstream partners of the viral proteins in these events have been identified, and include the adaptor complex of clathrin-coated pits, the beta subunit of COP-I coatomer, and the ubiquitin pathway-related h-beta TrCP protein. HIV-induced MHC-I downregulation, mostly the effect of Nef, also reflects a redistribution of this receptor, with its accumulation in the Golgi. The modalities of this process, however, are as yet imperfectly understood. New evidence indicates that the mechanisms employed by primate lentiviruses to downmodulate CD4 and MHC-I are also exploited by a number of cellular regulatory processes.

Cloning and Modeling of the First Nonmammalian CD4

We have cloned and sequenced the first nonmammalian CD4 cDNA from the chicken using the COS cell expression method. Chicken CD4 contains four extracellular Ig domains that, in analogy to mammalian CD4, are in the order V, C2, V, and C2. The molecule is 24% identical with both human and mouse sequences. The extracellular domains were modeled using human and rat CD4 crystal structures as templates. In the first domain there are two extra Cys residues that are at suitable distance to form an intra-β-sheet disulfide bridge in addition to the canonical one in the V domain. The region responsible for the interaction with MHC class II is relatively nonconserved in chicken. However, there are positively charged amino acids in the C″ region of the N-terminal domain that may mediate the association to the negatively charged residues of the MHC class II β-chain. Molecular modeling also implies that the membrane-proximal domain mediates dimerization of chicken CD4 in a similar way as it does for human CD4. Furthermore, the cytoplasmic tail is highly conserved, containing the protein tyrosine kinase p56lck recognition site that is preceded by an adjacent di-leucine motif for the internalization of the molecule. Interestingly, there are no Ser residues in the cytoplasmic part, which may explain the slow down-regulation of chicken CD4 after phorbol ester stimulation.

Cluster of differentiation antigen 4 (CD4) endocytosis and adaptor complex binding require activation of the CD4 endocytosis signal by serine phosphorylation

Cluster of differentiation antigen 4 (CD4), the T lymphocyte antigen receptor component and human immunodeficiency virus coreceptor, is down-modulated when cells are activated by antigen or phorbol esters. During down-modulation CD4 dissociates from p56(lck), undergoes endocytosis through clathrin-coated pits, and is then sorted in early endosomes to late endocytic organelles where it is degraded. Previous studies have suggested that phosphorylation and a dileucine sequence are required for down-modulation. Using transfected HeLa cells, in which CD4 endocytosis can be studied in the absence of p56(lck), we show that the dileucine sequence in the cytoplasmic domain is essential for clathrin-mediated CD4 endocytosis. However, this sequence is only functional as an endocytosis signal when neighboring serine residues are phosphorylated. Phosphoserine is required for rapid endocytosis because CD4 molecules in which the cytoplasmic domain serine residues are substituted with glutamic acid residues are not internalized efficiently. Using surface plasmon resonance, we show that CD4 peptides containing the dileucine sequence bind weakly to clathrin adaptor protein complexes 2 and 1. The affinity of this interaction is increased 350- to 700-fold when the peptides also contain phosphoserine residues.

A plasma membrane localization signal in the HIV-1 envelope cytoplasmic domain prevents localization at sites of vesicular stomatitis virus budding and incorporation into VSV virions

Previous studies showed that the HIV-1 envelope (Env) protein was not incorporated into vesicular stomatitis virus (VSV) virions unless its cytoplasmic tail was replaced with that of the VSV glycoprotein (G). To determine whether the G tail provided a positive incorporation signal for Env, or if sequences in the Env tail prevented incorporation, we generated mutants of Env with its 150-amino-acid tail shortened to 29, 10, or 3 amino acids (Envtr mutants). Cells infected with VSV recombinants expressing these proteins or an Env-G tail hybrid showed similar amounts of Env protein at the surface. The Env-G tail hybrid or the Envtr3 mutant were incorporated at the highest levels into budding VSV virions. In contrast, the Envtr29 or Envtr10 mutants were incorporated poorly. These results defined a signal preventing incorporation within the 10 membrane-proximal amino acids of the Env tail. Confocal microscopy revealed that this signal functioned by causing localization of human immunodeficiency virus type 1 Env to plasma membrane domains distinct from the VSV budding sites, where VSV proteins were concentrated.

Molecular characterization of the signal responsible for the targeting of the interleukin 2 receptor beta chain toward intracellular degradation

During receptor-mediated endocytosis, most growth factor receptors are transported to late endocytic compartments and degraded. This process is important to control their expression on the cell surface and requires sorting in early endocytic compartments. Little is known about the mechanisms and the signals involved. We have studied the signal involved in targeting the interleukin 2 receptor beta chain (IL2Rbeta), a member of the cytokine receptor superfamily, toward degradation after internalization. We show that a motif of 8 amino acids in the cytosolic tail of IL2Rbeta is sufficient to target a normally recycling receptor toward degradation. Deletion of this signal strongly impairs IL2Rbeta degradation. Further molecular characterization of the motif shows that it does not resemble the well documented tyrosine and dileucine families of trafficking signals.

Differential regulation of CXCR4 and CCR5 endocytosis

The chemokine receptors CCR5 and CXCR4 are major co-receptors/receptors for the CD4-dependent and CD4-independent entry of human and simian immunodeficiency viruses. The chemokines that bind and activate these receptors can inhibit the entry of viruses that use the respective co-receptor molecules. Chemokine-induced co-receptor internalisation is a significant component of the mechanism through which chemokines inhibit virus entry. CXCR4 internalisation is induced by the CXCR4 ligand stromal cell derived factor-1 (SDF-1), phorbol esters and, in T cells, cellular activation. Here we show that CXCR4 endocytosis can be mediated through either one of two distinct internalisation signals. A COOH-terminal serine rich domain is required for ligand- but not phorbol ester- induced CXCR4 internalisation. However, a Ser/IleLeu motif, similar to that required for the endocytosis of CD4 and the T cell receptor/CD3 complex, is required for phorbol ester-induced, but not ligand-induced, CXCR4 endocytosis. By contrast, CCR5 internalisation is induced by the beta-chemokine RANTES but not by phorbol esters. CCR5 lacks the Ser/IleLeu sequence required for phorbol ester-induced uptake of CXCR4. Together these results indicate that distinct mechanisms can regulate CXCR4 and CCR5 endocytosis and trafficking.

Mutational analysis of the carboxy-terminal phosphorylation site of GLUT-4 in 3T3-L1 adipocytes

The carboxy terminus of GLUT-4 contains a functional internalization motif (Leu-489Leu-490) that helps maintain its intracellular distribution in basal adipocytes. This motif is flanked by the major phosphorylation site in this protein (Ser-488), which may play a role in regulating GLUT-4 trafficking in adipocytes. In the present study, the targeting of GLUT-4 in which Ser-488 has been mutated to alanine (SAG) has been examined in stably transfected 3T3-L1 adipocytes. The trafficking of SAG was not significantly different from that of GLUT-4 in several respects. First, in the absence of insulin, the distribution of SAG was similar to GLUT-4 in that it was largely excluded from the cell surface and was enriched in small intracellular vesicles. Second, SAG exhibited insulin-dependent movement to the plasma membrane (4- to 5-fold) comparable to GLUT-4 (4- to 5-fold). Finally, okadaic acid, which has previously been shown to stimulate both GLUT-4 translocation and its phosphorylation at Ser-488, also stimulated the movement of SAG to the cell surface similarly to GLUT-4. Using immunoelectron microscopy, we have shown that GLUT-4 is localized to intracellular vesicles containing the Golgi-derived gamma-adaptin subunit of AP-1 and that this localization is enhanced when Ser-488 is mutated to alanine. We conclude that the carboxy-terminal phosphorylation site in GLUT-4 (Ser-488) may play a role in intracellular sorting at the trans-Golgi network but does not play a major role in the regulated movement of GLUT-4 to the plasma membrane in 3T3-L1 adipocytes.

Leucine-based receptor sorting motifs are dependent on the spacing relative to the plasma membrane

Many integral membrane proteins contain leucine-based motifs within their cytoplasmic domains that mediate internalization and intracellular sorting. Two types of leucine-based motifs have been identified. One type is dependent on phosphorylation, whereas the other type, which includes an acidic amino acid, is constitutively active. In this study, we have investigated how the spacing relative to the plasma membrane affects the function of both types of leucine-based motifs. For phosphorylation-dependent leucine-based motifs, a minimal spacing of 7 residues between the plasma membrane and the phospho-acceptor was required for phosphorylation and thereby activation of the motifs. For constitutively active leucine-based motifs, a minimal spacing of 6 residues between the plasma membrane and the acidic residue was required for optimal activity of the motifs. In addition, we found that the acidic residue of leucine-based motifs must be located amino-terminal to the dileucine sequence for proper function of the motifs and that residues surrounding the motifs affect the activity of the motifs. Thus, our observations suggest that the position, the exact sequence, and surrounding residues are major determinants of the function of leucine-based receptor sorting motifs.

A leucine-based motif mediates the endocytosis of vesicular monoamine and acetylcholine transporters

Specific transport proteins mediate the packaging of neurotransmitters into secretory vesicles and consequently require targeting to the appropriate intracellular compartment. To identify residues in the neuron-specific vesicular monoamine transporter (VMAT2) responsible for endocytosis, we examined the effect of amino (NH2-) and carboxyl (COOH-)-terminal mutations on steady state distribution and internalization. Deletion of a critical COOH-terminal domain sequence (AKEEKMAIL) results in accumulation of VMAT2 at the plasma membrane and a 50% reduction in endocytosis. Site-directed mutagenesis shows that replacement of the isoleucine-leucine pair within this sequence by alanine-alanine alone reduces endocytosis by 50% relative to wild type VMAT2. Furthermore, the KEEKMAIL sequence functions as an internalization signal when transferred to the plasma membrane protein Tac, and the mutation of the isoleucine-leucine pair also abolishes internalization of this protein. The closely related vesicular acetylcholine transporter (VAChT) contains a similar di-leucine sequence within the cytoplasmic COOH-terminal domain that when mutated results in accumulation of VAChT at the plasma membrane. The VAChT di-leucine sequence also confers internalization when appended to two other proteins and in one of these chimeras, conversion of the di-leucine sequence to di-alanine reduces the internalization rate by 50%. Both VMAT2 and VAChT thus use leucine-based signals for efficient endocytosis and as such are the first synaptic vesicle proteins known to use this motif for trafficking.

Di-leucine-mediated internalization of ligand by a truncated growth hormone receptor is independent of the ubiquitin conjugation system

The growth hormone receptor (GHR) is a member of the cytokine receptor family. Its function is to mediate cellular responses upon binding of growth hormone. Ligand binding induces dimerization and activation of the GHR. One mechanism by which the GHR is rapidly inactivated involves the ubiquitin conjugation system, a system implicated in the degradation of cytosolic and nuclear proteins. We have shown previously that the ubiquitin-conjugating system mediates internalization of the GHR. Here, we present evidence that in addition to the ubiquitin-dependent endocytosis signal, the cytosolic tail of the GHR contains a di-leucine motif. Upon truncation of the GHR at amino acid residue 349, this di-leucine motif is activated and mediates ubiquitin-independent internalization of the receptor. Di-leucine-mediated GHR internalization requires functional clathrin-coated pits and results in GHR transport to the lysosome. Although the full-length GHR internalizes independent of the di-leucine motif, this motif may function in internalization of GHR isoforms.

Nitrogen-regulated ubiquitination of the Gap1 permease of Saccharomyces cerevisiae

Addition of ammonium ions to yeast cells growing on proline as the sole nitrogen source induces rapid inactivation and degradation of the general amino acid permease Gap1 through a process requiring the Npi1/Rsp5 ubiquitin (Ub) ligase. In this study, we show that NH4+ induces endocytosis of Gap1, which is then delivered into the vacuole where it is degraded. This down-regulation is accompanied by increased conversion of Gap1 to ubiquitinated forms. Ubiquitination and subsequent degradation of Gap1 are impaired in the npi1 strain. In this mutant, the amount of Npi1/Rsp5 Ub ligase is reduced >10-fold compared with wild-type cells. The C-terminal tail of Gap1 contains sequences, including a di-leucine motif, which are required for NH4+-induced internalization and degradation of the permease. We show here that mutant Gap1 permeases affected in these sequences still bind Ub. Furthermore, we provide evidence that only a small fraction of Gap1 is modified by Ub after addition of NH4+ to mutants defective in endocytosis.

Phosphorylation Regulates the Delivery of MHC Class II Invariant Chain Complexes to Antigen Processing Compartments

Transport of newly synthesized MHC class II glycoproteins to endosomal Ag processing compartments is mediated by their association with the invariant chain (Ii). Targeting to these compartments is dependent upon recognition of leucine-based endosomal/lysosomal targeting motifs in the Ii cytosolic domain. Ii, like many molecules that contain leucine-based endosomal targeting motifs, is phosphorylated in vivo. In this report we demonstrate that the cytosolic domain of the p35 Ii isoform is phosphorylated in class II Ii complexes isolated from human B lymphoblastoid cell lines or freshly obtained PBMC. Mutation of serine residue 6 or 8 prevents phosphorylation of Ii-p35 expressed in HeLa cells. Treatment of B lymphoblastoid cell lines with the serine/threonine kinase inhibitor staurosporine prevented Ii phosphorylation and significantly delayed trafficking of newly synthesized class II Ii complexes to endosomal Ag processing compartments. By contrast, staurosporine had no effect on the rate of transport of class I or class II glycoproteins through the Golgi apparatus and did not inhibit the delivery of the chimeric molecule Tac-DMβ to endocytic compartments, suggesting that staurosporine does not nonspecifically inhibit protein transport to the endocytic pathway. These results demonstrate that phosphorylation regulates the efficient targeting of MHC class II Ii complexes to Ag processing compartments and strongly suggest that this effect is mediated by phosphorylation of the MHC class II-associated Ii chain.