[Show abstract][Hide abstract] ABSTRACT: With this issue of the JCI, we celebrate the 80th anniversary of the Journal. While 80 years is not a century, we still feel it is important to honor what the JCI has meant to the biomedical research community for 8 decades. To illustrate why the JCI is the leading general-interest translational research journal edited by and for biomedical researchers, we have asked former JCI editors-in-chief to reflect on some of the major scientific advances reported in the pages of the Journal during their tenures.
Journal of Clinical Investigation 11/2004; 114(8):1017-33. · 12.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The heterotetrameric AP-1 adaptor complex is involved in the assembly of clathrin-coated vesicles originating from the trans-Golgi network (TGN). The beta 1 subunit of AP-1 is known to contain a consensus clathrin binding sequence, LLNLD (the so-called clathrin box motif), in its hinge segment through which the beta chain interacts with the N-terminal domains of clathrin trimers. Here, we report that the hinge region of the gamma subunit of human and mouse AP-1 contains two copies of a new variant, LLDLL, of the clathrin box motif that also bind to the terminal domain of the clathrin heavy chain. High-affinity binding of the gamma hinge to clathrin trimers requires both LLDLL sequences to be present and the spacing between them to be maintained. We also identify an independent clathrin-binding site within the appendage domain of the gamma subunit that interacts with a region of clathrin other than the N-terminal domain. Clathrin polymerization is promoted by glutathione S-transferase (GST)-gamma hinge, but not by GST-gamma appendage. However, the hinge and appendage domains of gamma function in a cooperative manner to recruit and polymerize clathrin, suggesting that clathrin lattice assembly at the TGN involves multivalent binding of clathrin by the gamma and beta1 subunits of AP-1.
Molecular Biology of the Cell 08/2001; 12(7):1925-35. · 4.60 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The GGAs are a multidomain protein family implicated in protein trafficking between the Golgi and endosomes. Here, the VHS domain of GGA2 was shown to bind to the acidic cluster-dileucine motif in the cytoplasmic tail of the cation-independent mannose 6-phosphate receptor (CI-MPR). Receptors with mutations in this motif were defective in lysosomal enzyme sorting. The hinge domain of GGA2 bound clathrin, suggesting that GGA2 could be a link between cargo molecules and clathrin-coated vesicle assembly. Thus, GGA2 binding to the CI-MPR is important for lysosomal enzyme targeting.
[Show abstract][Hide abstract] ABSTRACT: The heterotetrameric adaptor protein complex AP-3 has been shown to function in the sorting of proteins to the endosomal/lysosomal system. However, the mechanism of AP-3 recruitment onto membranes is poorly understood, and it is still uncertain whether AP-3 nucleates clathrin-coated vesicles. Using purified components, we show that AP-3 and clathrin are recruited onto protein-free liposomes and Golgi-enriched membranes by a process that requires ADP-ribosylation factor (ARF) and GTP but no other proteins or nucleotides. The efficiency of recruitment onto the two sources of membranes is comparable and independent of the composition of the liposomes. Clathrin binding occurred in a cooperative manner as a function of the membrane concentration of AP-3. Thin-section electron microscopy of liposomes and Golgi-enriched membranes that had been incubated with AP-3, clathrin, and ARF.GTP showed the presence of clathrin-coated buds and vesicles. These results establish that AP-3-containing clathrin-coated vesicles form in vitro and are consistent with AP-3-dependent protein transport being mediated by clathrin-coated vesicles.
Molecular Biology of the Cell 12/2000; 11(11):3723-36. · 4.60 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Lysosomes contain several integral membrane proteins (termed Lamps and Limps) that are extensively glycosylated with asparagine-linked oligosaccharides. It has been postulated that these glycans protect the underlying polypeptides from the proteolytic environment of the lysosome. Previous attempts to test this hypothesis have been inconclusive because they utilized approaches that prevent initial glycosylation and thereby impair protein folding. We have used endoglycosidase H to remove the Asn-linked glycans from fully folded lysosomal membrane proteins in living cells. Deglycosylation of Lamp-1 and Lamp-2 resulted in their rapid degradation, whereas Limp-2 was relatively stable in the lysosome in the absence of high mannose Asn-linked oligosaccharides. Depletion of Lamp-1 and Lamp-2 had no measurable effect on endosomal/lysosomal pH, osmotic stability, or density, and cell viability was maintained. Transport of endocytosed material to dense lysosomes was delayed in endoglycosidase H treated cells, but the rate of degradation of internalized bovine serum albumin was unchanged. These data provide direct evidence that Asn-linked oligosaccharides protect a subset of lysosomal membrane proteins from proteolytic digestion in intact cells.
Journal of Biological Chemistry 11/1999; 274(43):31039-46. · 4.65 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have reported that bovine DNase I, a secretory glycoprotein, acquires mannose 6-phosphate residues on 12.6% of its Asn-linked oligosaccharides when expressed in COS-1 cells and that the extent of phosphorylation increases to 79.2% when lysines are placed at positions 27 and 74 of the mature protein (Nishikawa, A., Gregory, W. , Frenz, J., Cacia, J., and Kornfeld, S. (1997) J. Biol. Chem. 272, 19408-19412). We now demonstrate that murine DNase I, which contains Lys27 and Lys74, is phosphorylated only 20.9% when expressed in the same COS-1 cell system. This difference is mostly due to the absence of three residues present in bovine DNase I (Tyr54, Lys124, and Ser190) along with the presence of a valine at position 23 that is absent in the bovine species. We show that Val23 inhibits phosphorylation at the Asn18 glycosylation site, whereas Tyr54, Lys124, and Ser190 enhance phosphorylation at the Asn106 glycosylation site. Tyr54 and Ser190 are widely separated from each other and from Asn106 on the surface of DNase I, indicating that residues present over a broad area influence the interaction with UDP-GlcNAc:lysosomal enzyme N-acetylglucosamine-1-phosphotransferase, which is responsible for the formation of mannose 6-phosphate residues on lysosomal enzymes.
Journal of Biological Chemistry 08/1999; 274(27):19309-15. · 4.65 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The assembly of clathrin-coated vesicles on Golgi membranes is initiated by the GTP-binding protein ADP ribosylation factor (ARF), which generates high-affinity membrane-binding sites for the heterotetrameric AP-1 adaptor complex. Once bound, the AP-1 recruits clathrin triskelia, which polymerize to form the coat. We have found that ARF.GTP also recruits AP-1 and clathrin onto protein-free liposomes. The efficiency of this process is modulated by the composition of the liposomes, with phosphatidylserine being the most stimulatory phospholipid. There is also a requirement for cytosolic factor(s) other than ARF. Thin-section electron microscopy shows the presence of clathrin-coated buds and vesicles that resemble those formed in vivo. These results indicate that AP-1-containing clathrin-coated vesicles can form in the absence of integral membrane proteins. Thus, ARF.GTP, appropriate lipids, and cytosolic factor(s) are the minimal components necessary for AP-1 clathrin-coat assembly.
Proceedings of the National Academy of Sciences 05/1999; 96(9):5013-8. · 9.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The GTP-binding protein ADP-ribosylation factor (ARF) initiates clathrin-coat assembly at the trans-Goli network (TGN) by generating high-affinity membrane-binding sites for the AP-1 adaptor complex. Both transmembrane proteins, which are sorted into the assembling coated bud, and novel docking proteins have been suggested to be partners with GTP-bound ARF in generating the AP-1-docking sites. The best characterized, and probably the major transmembrane molecules sorted into the clathrin-coated vesicles that form on the TGN, are the mannose 6-phosphate receptors (MPRs). Here, we have examined the role of the MPRs in the AP-1 recruitment process by comparing fibroblasts derived from embryos of either normal or MPR-negative animals. Despite major alterations to the lysosome compartment in the MPR-deficient cells, the steady-state distribution of AP-1 at the TGN is comparable to that of normal cells. Golgi-enriched membranes prepared from the receptor-negative cells also display an apparently normal capacity to recruit AP-1 in vitro in the presence of ARF and either GTP or GTPgammaS. The AP-1 adaptor is recruited specifically onto the TGN and not onto the numerous abnormal membrane elements that accumulate within the MPR-negative fibroblasts. AP-1 bound to TGN membranes from either normal or MPR-negative fibroblasts is fully resistant to chemical extraction with 1 M Tris-HCl, pH 7, indicating that the adaptor binds to both membrane types with high affinity. The only difference we do note between the Golgi prepared from the MPR-deficient cells and the normal cells is that AP-1 recruited onto the receptor-lacking membranes in the presence of ARF1.GTP is consistently more resistant to extraction with Tris. Because sensitivity to Tris extraction correlates well with nucleotide hydrolysis, this finding might suggest a possible link between MPR sorting and ARF GAP regulation. We conclude that the MPRs are not essential determinants in the initial steps of AP-1 binding to the TGN but, instead, they may play a regulatory role in clathrin-coated vesicle formation by affecting ARF.GTP hydrolysis.
Molecular Biology of the Cell 04/1999; 10(3):537-49. · 4.60 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The cation-independent mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF-II receptor) undergoes constitutive endocytosis, mediating the internalization of two unrelated classes of ligands, mannose 6-phosphate (Man-6-P)-containing acid hydrolases and insulin-like growth factor II (IGF-II). To determine the role of ligand valency in M6P/IGF-II receptor-mediated endocytosis, we measured the internalization rates of two ligands, beta-glucuronidase (a homotetramer bearing multiple Man-6-P moieties) and IGF-II. We found that beta-glucuronidase entered the cell approximately 3-4-fold faster than IGF-II. Unlabeled beta-glucuronidase stimulated the rate of internalization of 125I-IGF-II to equal that of 125I-beta-glucuronidase, but a bivalent synthetic tripeptide capable of occupying both Man-6-P-binding sites on the M6P/IGF-II receptor simultaneously did not. A mutant receptor with one of the two Man-6-P-binding sites inactivated retained the ability to internalize beta-glucuronidase faster than IGF-II. Thus, the increased rate of internalization required a multivalent ligand and a single Man-6-P-binding site on the receptor. M6P/IGF-II receptor solubilized and purified in Triton X-100 was present as a monomer, but association with beta-glucuronidase generated a complex composed of two receptors and one beta-glucuronidase. Neither IGF-II nor the synthetic peptide induced receptor dimerization. These results indicate that intermolecular cross-linking of the M6P/IGF-II receptor occurs upon binding of a multivalent ligand, resulting in an increased rate of internalization.
Journal of Biological Chemistry 02/1999; 274(2):1164-71. · 4.65 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Association of the Golgi-specific adaptor protein complex 1 (AP-1) with the membrane is a prerequisite for clathrin coat assembly on the trans-Golgi network (TGN). The AP-1 adaptor is efficiently recruited from cytosol onto the TGN by myristoylated ADP-ribosylation factor 1 (ARF1) in the presence of the poorly hydrolyzable GTP analog guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS). Substituting GTP for GTPgammaS, however, results in only poor AP-1 binding. Here we show that both AP-1 and clathrin can be recruited efficiently onto the TGN in the presence of GTP when cytosol is supplemented with ARF1. Optimal recruitment occurs at 4 microM ARF1 and with 1 mM GTP. The AP-1 recruited by ARF1.GTP is released from the Golgi membrane by treatment with 1 M Tris-HCl (pH 7) or upon reincubation at 37 degreesC, whereas AP-1 recruited with GTPgammaS or by a constitutively active point mutant, ARF1(Q71L), remains membrane bound after either treatment. An incubation performed with added ARF1, GTP, and AlFn, used to block ARF GTPase-activating protein activity, results in membrane-associated AP-1, which is largely insensitive to Tris extraction. Thus, ARF1. GTP hydrolysis results in lower-affinity binding of AP-1 to the TGN. Using two-stage assays in which ARF1.GTP first primes the Golgi membrane at 37 degreesC, followed by AP-1 binding on ice, we find that the high-affinity nucleating sites generated in the priming stage are rapidly lost. In addition, the AP-1 bound to primed Golgi membranes during a second-stage incubation on ice is fully sensitive to Tris extraction, indicating that the priming stage has passed the ARF1.GTP hydrolysis point. Thus, hydrolysis of ARF1.GTP at the priming sites can occur even before AP-1 binding. Our finding that purified clathrin-coated vesicles contain little ARF1 supports the concept that ARF1 functions in the coat assembly process rather than during the vesicle-uncoating step. We conclude that ARF1 is a limiting factor in the GTP-stimulated recruitment of AP-1 in vitro and that it appears to function in a stoichiometric manner to generate high-affinity AP-1 binding sites that have a relatively short half-life.
Molecular Biology of the Cell 07/1998; 9(6):1323-37. · 4.60 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The urokinase-type plasminogen activator receptor (uPAR) plays an important role on the cell surface in mediating extracellular degradative processes and formation of active TGF-beta, and in nonproteolytic events such as cell adhesion, migration, and transmembrane signaling. We have searched for mechanisms that determine the cellular location of uPAR and may participate in its disposal. When using purified receptor preparations, we find that uPAR binds to the cation-independent, mannose 6-phosphate/insulin-like growth factor-II (IGF-II) receptor (CIMPR) with an affinity in the low micromolar range, but not to the 46-kD, cation-dependent, mannose 6-phosphate receptor (CDMPR). The binding is not perturbed by uPA and appears to involve domains DII + DIII of the uPAR protein moiety, but not the glycosylphosphatidylinositol anchor. The binding occurs at site(s) on the CIMPR different from those engaged in binding of mannose 6-phosphate epitopes or IGF-II. To evaluate the significance of the binding, immunofluorescence and immunoelectron microscopy studies were performed in transfected cells, and the results show that wild-type CIMPR, but not CIMPR lacking an intact sorting signal, modulates the subcellular distribution of uPAR and is capable of directing it to lysosomes. We conclude that a site within CIMPR, distinct from its previously known ligand binding sites, binds uPAR and modulates its subcellular distribution.
The Journal of Cell Biology 06/1998; 141(3):815-28. · 10.82 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The effect of wortmannin on the trafficking of the mannose 6-phosphate/insulin-like growth factor II receptor (Man-6-P/IGF-II receptor) and its ligand beta-glucuronidase has been determined in murine L cells and normal rat kidney cells. The drug induced a 90% decrease in the steady-state level of the Man-6-P/IGF-II receptor at the plasma membrane without affecting the rate of internalization, indicating that the return of receptor from endosomes to the plasma membrane is retarded. Wortmannin also slowed the movement of receptor from endosomes to the trans-Golgi network by about 60%. Such a kinetic block would dramatically reduce the number of Man-6-P/IGF-II receptors in the trans-Golgi network, which could account for the previously described hypersecretion of procathepsin D induced by wortmannin. In addition, the drug slowed delivery of endocytosed beta-glucuronidase from endosomes to dense lysosomes. These data, taken together with the published reports of others, indicate that wortmannin inhibits membrane trafficking out of multiple compartments of the endosomal system and suggest a role for phosphatidylinositol 3-kinase in regulating these processes.
Journal of Biological Chemistry 03/1998; 273(7):3848-53. · 4.65 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Mammalian ADP-ribosylation factor 1 (mARF1) is a small GTP-binding protein that is activated by a Golgi guanine nucleotide exchange factor. Once bound to the Golgi membranes in the GTP form, mARF1 initiates the recruitment of the adaptor protein 1 (AP-1) complex and coatomer (COPI) onto these membranes and activates phospholipase D1 (PLD1). To map the domains of mARF1 that are important for these activities, we constructed chimeras between mARF1 and Saccharomyces cerevisiae ARF2, which functions poorly in all of these processes except COPI recruitment. The carboxyl half of mARF1 (amino acids 95-181) was essential for activation by the Golgi guanine nucleotide exchange factor, whereas a separate domain (residues 35-94) was required to effectively activate PLD1 and to promote efficient AP-1 recruitment. Since residues 35-94 of mARF1 are critical for optimal activity in both PLD1 activation and AP-1 recruitment, we hypothesize that this region of ARF contains residues that interact with effector molecules.
Journal of Biological Chemistry 01/1998; 272(52):33001-8. · 4.65 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The 67-amino acid cytoplasmic tail of the cation-dependent mannose 6-phosphate receptor (CD-MPR) contains a signal(s) that prevents the receptor from entering lysosomes where it would be degraded. To identify the key residues required for proper endosomal sorting, we analyzed the intracellular distribution of mutant forms of the receptor by Percoll density gradients. A receptor with a Trp19 --> Ala substitution in the cytoplasmic tail was highly missorted to lysosomes whereas receptors with either Phe18 --> Ala or Phe13 --> Ala mutations were partially defective in avoiding transport to lysosomes. Analysis of double and triple mutants confirmed the key role of Trp19 for sorting of the CD-MPR in endosomes, with Phe18, Phe13, and several neighboring residues contributing to this function. The addition of the Phe18-Trp19 motif of the CD-MPR to the cytoplasmic tail of the lysosomal membrane protein Lamp1 was sufficient to partially impair its delivery to lysosomes. Replacing Phe18 and Trp19 with other aromatic amino acids did not impair endosomal sorting of the CD-MPR, indicating that two aromatic residues located at these positions are sufficient to prevent the receptor from trafficking to lysosomes. However, alterations in the spacing of the diaromatic amino acid sequence relative to the transmembrane domain resulted in receptor accumulation in lysosomes. These findings indicate that the endosomal sorting of the CD-MPR depends on the correct presentation of a diaromatic amino acid-containing motif in its cytoplasmic tail. Because a diaromatic amino acid sequence is also present in the cytoplasmic tail of other receptors known to be internalized from the plasma membrane, this feature may prove to be a general determinant for endosomal sorting.
Proceedings of the National Academy of Sciences 12/1997; 94(26):14471-6. · 9.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The secretory glycoprotein DNase I acquires mannose 6-phosphate moieties on its Asn-linked oligosaccharides, indicating that it is a substrate for UDP-GlcNAc:lysosomal enzyme N-acetylglucosamine-1-phosphotransferase (phosphotransferase) (Cacia, J., Quan, C., and Frenz, J. (1995) Glycobiology 4, 99). Phosphotransferase recognizes a conformation-dependent protein determinant that is present in lysosomal hydrolases, but absent in most secretory glycoproteins. To identify the amino acid residues of DNase I that are required for interaction with phosphotransferase, wild-type and mutant forms of bovine DNase I were expressed in COS-1 cells and the extent of oligosaccharide phosphorylation determined. Phosphorylation of DNase I oligosaccharides decreased from 12.6% to 2.3% when Lys-50, Lys-124, and Arg-27 were mutated to alanines, indicating that these residues are required for the basal level of phosphorylation. Mutation of lysines at other positions did not impair phosphorylation, demonstrating the selectivity of this process. When Arg-27 was replaced with a lysine, phosphorylation increased to 54%, showing that phosphotransferase prefers lysine residues to arginines. Mutation of Asn-74 to a lysine also increased phosphorylation to 50.3%, and the double mutant (R27K/N74K) was phosphorylated 79%, equivalent to the values obtained with lysosomal hydrolases. Interestingly, Lys-27 and Lys-74 caused selective phosphorylation of the neighboring Asn-linked oligosaccharide. Finally, mutation of Lys-117 to an alanine stimulated phosphorylation, demonstrating that some residues may be negative regulators of this process. We conclude that selected lysine and arginine residues on the surface of DNase I constitute the major elements of the phosphotransferase recognition domain present on this secretory glycoprotein.
Journal of Biological Chemistry 09/1997; 272(31):19408-12. · 4.65 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have compared the abilities of mammalian ADP-ribosylation factors (ARFs) 1, 5, and 6 and Saccharomyces cerevisiae ARF2 to serve as substrates for the rat liver Golgi membrane guanine nucleotide exchange factor and to initiate the formation of clathrin- and coatomer protein (COP) I-coated vesicles on these membranes. While Golgi membranes stimulated the exchange of GTPgammaS for GDP on all of the ARFs tested, mammalian ARF1 was the best substrate, with an apparent Km of 5 microM. In all cases myristoylation of ARF was required for stimulation. Agents that inhibit the Golgi membrane guanine nucleotide exchange factor (the fungal metabolite brefeldin A and trypsin treatment) selectively inhibited the guanine nucleotide exchange on mammalian ARF1. Taken together, these data indicate that of the ARFs tested, only mammalian ARF1 is activated efficiently by the Golgi guanine nucleotide exchange factor. The other ARFs are activated mainly by another mechanism, possibly phospholipid-mediated. Once activated, all of the membrane-associated, myristoylated ARFs promoted the recruitment of coatomer to about the same extent. Mammalian ARFs 1 and 5 were the most effective in promoting the recruitment of the AP-1 adaptor complex, whereas yeast ARF2 was the least active. These data indicate that the specificity for ARF action on the Golgi membranes is primarily determined by the Golgi guanine nucleotide exchange factor, which has a strong preference for myristoylated mammalian ARF1.
Journal of Biological Chemistry 03/1997; 272(7):4141-8. · 4.65 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Coat proteins appear to play a general role in intracellular protein trafficking by coordinating a membrane budding event with cargo selection. Here we show that the AP-2 adaptor, a clathrin-associated coat-protein complex that nucleates clathrin-coated vesicle formation at the cell surface, can also initiate the assembly of normal polyhedral clathrin coats on dense lysosomes under physiological conditions in vitro. Clathrin coat formation on lysosomes is temperature dependent, displays an absolute requirement for ATP, and occurs in both semi-intact cells and on purified lysosomes, suggesting that clathrin-coated vesicles might regulate retrograde membrane traffic out of the lysosomal compartment.
The Journal of Cell Biology 01/1997; 135(6 Pt 2):1801-14. · 10.82 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Binding of peptides to MHC class II Ag generates ligands for TCR of Th lymphocytes. We have identified a novel class of peptides bound to MHC class II: mannose 6- phosphate (Man-6-P) containing glycopeptides from lysosomal enzymes. These species were identified in the process of characterizing mannose 6-phosphate/insulin-like growth factor II (M-6-P/IGF-II) receptor binding to the surface of B lymphoblasts. Surface iodination and Man-6-P/IGF-II receptor affinity chromatography implicated MHC class II as a carrier of Man-6-P-modified oligosaccharides. These oligosaccharides were found to be primarily associated with the bound peptide. Peptides eluted from the Man-6-P/IGF-II receptor-binding fraction of immunoaffinity-purified MHC class II from the Swei cell line contained a sequence derived from the propiece of lysosomal acid lipase. Partial sequences were also obtained for peptides from other HLA-DR alleles but none of these were attributable to known proteins. This study defines a novel approach for isolating rare glycan-modified peptides from MHC class II and demonstrates that very large secondary modifications are tolerated in peptides bound to MHC class II.
The Journal of Immunology 04/1996; 156(5):1841-7. · 5.52 Impact Factor