Mutations in the cytoplasmic domain of the 275 kd mannose 6-phosphate receptor differentially alter lysosomal enzyme sorting and endocytosis

Tyrosine 569 in the c-Fms juxtamembrane domain is essential for kinase activity and macrophage colony-stimulating factor-dependent internalization

The receptor (Fms) for macrophage colony-stimulating factor (M-CSF) is a member of the tyrosine kinase class of growth factor receptors. It maintains survival, stimulates growth, and drives differentiation of the macrophage lineage of hematopoietic cells. Fms accumulates on the cell surface and becomes activated for signal transduction after M-CSF binding and is then internalized via endocytosis for eventual degradation in lysosomes. We have investigated the mechanism of endocytosis as part of the overall signaling process of this receptor and have identified an amino acid segment near the cytoplasmic juxtamembrane region surrounding tyrosine 569 that is important for internalization. Mutation of tyrosine 569 to alanine (Y569A) eliminates ligand-induced rapid endocytosis of receptor molecules. The mutant Fms Y569A also lacks tyrosine kinase activity; however, tyrosine kinase activity is not essential for endocytosis because the kinase inactive receptor Fms K614A does undergo ligand-induced endocytosis, albeit at a reduced rate. Mutation of tyrosine 569 to phenylalanine had no effect on the M-CSF-induced endocytosis of Fms, and a four-amino-acid sequence containing Y-569 could support endocytosis when transferred into the cytoplasmic juxtamembrane region of a glycophorin A construct. These results indicate that tyrosine 569 within the juxtamembrane region of Fms is part of a signal recognition sequence for endocytosis that does not require tyrosine phosphorylation at this site and that this domain also influences the kinase activity of the receptor. These results are consistent with a ligand-dependent step in recognition of the potential cryptic internalization signal.

Increased cytotoxicity of interleukin 2-pseudomonas exotoxin (IL2-PE) chimeric proteins containing a targeting signal for lysosomal membranes

IL2-PE40 is a chimeric protein composed of human interleukin 2 (IL2) genetically fused to the amino terminus of a truncated form of pseudomonas exotoxin lacking its cell recognition domain (PE40). IL2-PE40 is extremely cytotoxic to IL2 receptor positive cells. This chimeric protein was found to be an effective and selective immunosuppressive agent for IL2 receptor targeted therapy in many models of disorders of the immune response where activated T-cells play a crucial role. In an attempt to produce an improved IL2-PE40 chimeric protein, we constructed new IL2-PE derivatives. This was done by inserting defined DNA sequences within the chimeric gene encoding IL2-PE40. Inserted sequences represent motifs of other proteins known to be targeted and/or sorted to specific compartments inside or outside the cell. One of the proteins, IL2-PE40(LAP+DUP), containing a targeted signal for lysosomal membrane, was 2-3-fold more active than IL2-PE40. The insertion of the LAP sequence also increased the cytotoxicity of another IL2-PE derivative, IL2-PE664Glu. Our results suggest that a selective targeting of IL2-PE chimeric proteins to lysosomes may enable the proteins to reach the cytosol more efficiently, thus improving its specific cytotoxicity. The LAP (lysosomal alkaline phosphatase) sequence may be used as a common motif for increasing the cytotoxicity of other chimeric proteins to be used for targeted immunotherapy.

A chimera of the cytoplasmic tail of the mannose 6-phosphate/IGF-II receptor and lysozyme localizes to the TGN rather than prelysosomes where the bulk of the endogenous receptor is found

We fused the cytoplasmic and transmembrane domains of the bovine mannose 6-phosphate/IGF-II receptor (MPR) to lysozyme, a monomeric secretory protein thought to be devoid of sorting information. When the resulting chimera (lys/MPR) was transiently expressed in COS cells or stably expressed in CV1 cells, it had a predominantly intracellular distribution in the trans-Golgi region, with less than 10% present on the surface. In contrast, a similar chimera containing the transmembrane and cytoplasmic domains of the low density lipoprotein receptor (lys/LDLR) was localized to the plasma membrane, even though it endocytoses efficiently. Exchanging domains between the lys/MPR and lys/LDLR chimeras indicated that the MPR cytoplasmic domain contains the information necessary to specify the intracellular localization of the chimeric molecule. This signal must be located in the membrane-proximal third of the tail, as deletion of the last 120 residues of the 163 residue tail has no obvious effect on the distribution of lys/MPR. However, the recycling of the lys/MPR does not completely mimic that of the intact endogenous MPR, as immunofluorescence labelling shows that they are predominantly in different locations, indicating a role for the lumenal domain of the MPR in determining the steady-state distribution of the MPR itself.

The amino terminus of GLUT4 functions as an internalization motif but not an intracellular retention signal when substituted for the transferrin receptor cytoplasmic domain

Previous studies have demonstrated that the amino-terminal cytoplasmic domain of GLUT4 contains a phenylalanine-based targeting motif that determines its steady state distribution between the surface and the interior of cells (Piper, R. C., C. Tai, P. Kuleza, S. Pang, D. Warnock, J. Baenziger, J. W. Slot, H. J. Geuze, C. Puri, and D. E. James. 1993. J. Cell Biol. 121:1221). To directly measure the effect that the GLUT4 amino terminus has on internalization and subsequent recycling back to the cell surface, we constructed chimeras in which this sequence was substituted for the amino-terminal cytoplasmic domain of the human transferrin receptor. The chimeras were stably transfected into Chinese hamster ovary cells and their endocytic behavior characterized. The GLUT4-transferrin receptor chimera was recycled back to the cell surface with a rate similar to the transferrin receptor, indicating that the GLUT4 sequence was not promoting intracellular retention of the chimera. The GLUT4-transferrin receptor chimera was internalized at half the rate of the transferrin receptor. Substitution of an alanine for phenylalanine at position 5 slowed internalization of the chimera by twofold, to a level characteristic of bulk membrane internalization. However, substitution of a tyrosine increased the rate of internalization to the level of the transferrin receptor. Neither of these substitutions significantly altered the rate at which the chimeras were recycled back to the cell surface. These results demonstrate that the major function of the GLUT4 amino-terminal domain is to promote the effective internalization of the protein from the cell surface, via a functional phenylalanine-based internalization motif, rather than retention of the transporter within intracellular structures.

Thrombomodulin lacking the cytoplasmic domain efficiently internalizes thrombin via nonclathrin-coated, pit-mediated endocytosis

Thrombomodulin (TM) is a transmembrane vascular endothelial cell receptor that is a cofactor in a major physiologically relevant natural anticoagulant system. We recently developed a cell model to examine one mechanism of regulation of TM cell surface expression and visually demonstrated that the receptor undergoes internalization predominantly via noncoated pits (Conway et al., 1992, J. Cell. Phys., 151:604-612). We have extended these studies to examine the role of the cytoplasmic domain of TM by deleting this region and expressing the truncated version of the molecule in COS cells (COS.Cyto.Del cells). Electron microscopy demonstrated internalization of gold-labeled anti-TM antibody or thrombin in a time- and temperature-dependent manner, similar to that seen with the wild-type transfected cells (COS.TM-CR). Endocytosis was characterized by initial surface clustering of gold particles, followed by aggregation into noncoated pits, early endosome formation, and, finally, entry into multivesicular bodies and lysosomes. There was a notable absence of gold particles in clathrin-coated pits and vesicles. The kinetics of binding and internalization of 125I-labeled ligand in COS.Cyto.Del cells was compared with that of COS.TM-CR cells and was not significantly different. These studies provide ultrastructural and quantitative data to indicate that TM efficiently undergoes endocytosis via nonclathrin-coated pits when the receptor is lacking the cytoplasmic domain. This finding suggests that there may be alternative regions of the molecule that mediate those signals necessary for internalization.

The MHC class II-associated invariant chain contains two endosomal targeting signals within its cytoplasmic tail

The oligomeric complex formed by major histocompatibility complex (MHC) class II alpha and beta chains and invariant chain (Ii) assembles in the endoplasmic reticulum and is then transported via the Golgi complex to compartments of the endocytic pathway. When Ii alone is expressed in CV1 cells it is sorted to endosomes. The Ii cytoplasmic tail has been found to be essential for targeting to these compartments. In order to characterize further the signals responsible for endosomal targeting, we have deleted various segments of the cytoplasmic tail. The Ii mutants were transiently expressed and the cellular location of the proteins was analyzed biochemically and morphologically. The cytoplasmic tail of Ii was found to contain two endosomal targeting sequences within its cytoplasmic tail; one targeting sequence was present within amino acid residues 12–29 and deletion of this segment revealed the presence of a second endosomal targeting sequence, located within the first 11 amino acid residues. The presence of a leucine-isoleucine pair at positions 7 and 8 within this sequence was found to be essential for endosomal targeting. In addition, the presence of this L-I motif lead to accumulation of Ii molecules in large endosomal vacuoles containing lysosomal marker proteins. Both wild type Ii and Ii mutant molecules containing only one endosomal targeting sequence were rapidly internalized from the plasma membrane. When the Ii cytoplasmic tail was fused to the membrane-spanning region of neuraminidase, a resident plasma membrane protein, the resulting chimera (INA) was found in endocytic compartments containing lysosomal marker proteins. Thus the cytoplasmic tail of Ii is sufficient for targeting to the endocytic/lysosomal pathway.

Assembly and targeting of adaptin chimeras in transfected cells

Adaptors are the components of clathrincoated pits and vesicles that attach the clathrin to the membrane. There are two types of adaptors in the cell: one associated with the plasma membrane and one associated with the TGN. Both adaptors are heterotetramers consisting of two adaptins (alpha and beta for the plasma membrane; gamma and beta' for the TGN), plus two smaller proteins. The COOH-terminal domains of the adaptins form appendages that resemble ears, connected by flexible hinges. Unlike the other adaptor components, the COOH termini of the alpha- and gamma-adaptins show no homology with each other, suggesting that they might provide the signal that directs the adaptors to the appropriate membrane. To test this possibility, the COOH-terminal ears were switched between alpha- and gamma-adaptins and were also deleted. All of the constructs contained the bovine gamma-adaptin hinge, enabling them to be detected with a species-specific antibody against this region when transfected into rat fibroblasts. Immunoprecipitation indicated that the engineered adaptins were still fully capable of assembling into adaptor complexes. Immunofluorescence revealed that in spite of their modified ears, the constructs were still able to be recruited onto the appropriate membrane; however, the ear-minus constructs gave increased cytoplasmic staining, and replacing the gamma-adaptin ear with the alpha-adaptin ear caused a small amount of colocalization with endogenous alpha-adaptin in some cells. Thus, the major targeting determinant appears to reside in the adaptor "head," while the ears may stabilize the association of adaptors with the membrane.

Cell surface HLA-DR-invariant chain complexes are targeted to endosomes by rapid internalization

Class II molecules of the major histocompatibility complex (MHC) bind peptides derived from protein antigens delivered into endocytic compartments and present these peptides to CD4+ T cells. The precursors to functional MHC class II molecules loaded with peptides are complexes of the invariant chain associated with class II alpha beta heterodimers. Targeting of newly synthesized MHC class II molecules to endosomes is mediated by the invariant chain, but the intracellular transport route is not known. This study demonstrates that in a human B-cell line a large population of MHC class II-invariant chain complexes reaches endosomes by rapid internalization from the cell surface. Quantitation of cell surface MHC class II-invariant chain complexes and of their surface half-life revealed that 3000 complexes internalized per minute into endosomes. This highly efficient endocytosis was mediated by the cytoplasmic tail of the invariant chain. After internalization, the invariant chain dissociated from the MHC class II-invariant chain complexes. This pathway may represent an important mechanism for loading class II molecules with immunogenic peptides from several endocytic compartments.

Two steps of insulin receptor internalization depend on different domains of the beta-subunit

The internalization of signaling receptors such as the insulin receptor is a complex, multi-step process. The aim of the present work was to determine the various steps in internalization of the insulin receptor and to establish which receptor domains are implicated in each of these by the use of receptors possessing in vitro mutations. We find that kinase activation and autophosphorylation of all three regulatory tyrosines 1146, 1150, and 1151, but not tyrosines 1316 and 1322 in the COOH-terminal domain, are required for the ligand-specific stage of the internalization process; i.e., the surface redistribution of the receptor from microvilli where initial binding occurs to the nonvillous domain of the cell. Early intracellular steps in insulin signal transduction involving the activation of phosphatidylinositol 3'-kinase are not required for this redistribution. The second step of internalization consists in the anchoring of the receptors in clathrin-coated pits. In contrast to the first ligand specific step, this step is common to many receptors including those for transport proteins and occurs in the absence of kinase activation and receptor autophosphorylation, but requires a juxta-membrane cytoplasmic segment of the beta-subunit of the receptor including a NPXY sequence. Thus, there are two independent mechanisms controlling insulin receptor internalization which depend on different domains of the beta-subunit.

Divergent fates of P- and E-selectins after their expression on the plasma membrane

P-selectin and E-selectin are related adhesion receptors for monocytes and neutrophils that are expressed by stimulated endothelial cells. P-selectin is stored in Weibel-Palade bodies, and it reaches the plasma membrane after exocytosis of these granules. E-selectin is not stored, and its synthesis is induced by cytokines. We studied the fate of the two proteins after their surface expression by following the intracellular routing of internalized antibodies to the selectins. By immunofluorescent staining, P-selectin antibody was first seen in endosomes, then in the Golgi region, and finally in Weibel-Palade bodies. In contrast, the E-selectin antibody was detected only in endosomes and lysosomes. Subcellular fractionation of cells after 4 h chase confirmed the localization of P-selectin antibody in storage granules and of the E-selectin antibody in lysosomes. In AtT-20 cells, a mouse pituitary cell line, transfected with P- or E-selectin, only P-selectin was delivered to the endogenous adrenocorticotrophic hormone storage granules after endocytosis. Deletion of the cytoplasmic domain abolished internalization. In summary, after a brief surface exposure, internalized E-selectin is degraded in the lysosomes, whereas P-selectin returns to the storage granules from where it can be reused.

Mutation of tyrosine-350 impairs the coupling of the beta 2-adrenergic receptor to the stimulatory guanine nucleotide binding protein without interfering with receptor down-regulation

Long-term stimulation of the beta 2-adrenergic receptor (beta 2AR) leads to an internalization and degradation of the receptor. This down-regulation of the beta 2AR number contributes to the desensitization of the adenylyl cyclase activity induced by chronic exposure to agonists. It was proposed that two tyrosine residues (Tyr-350 and Tyr-354) located in the cytoplasmic tail of the beta 2AR play a crucial role in agonist-induced down-regulation. In addition to perturbation of the down-regulation, the substitution of these tyrosines for alanines also led to a functional uncoupling of the receptor from Gs [Valiquette et al. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 5089-5093]. To further characterize the relative contribution of Tyr-350 and Tyr-354 to the receptor interaction with Gs and agonist-promoted down-regulation, both tyrosines were individually replaced by alanines and mutant receptors expressed in CHW cells. We show here that mutation of Tyr-350 but not that of Tyr-354 significantly decreased the ability of the beta 2AR to be functionally coupled to Gs and thereby to stimulate the adenylyl cyclase. Moreover, in contrast to the double tyrosine mutation, neither of the single-point mutations affected the agonist-induced down-regulation pattern. These data suggest that the presence of either Tyr-350 or Tyr-354 is sufficient to maintain normal agonist-induced down-regulation whereas the integrity of Tyr-350 is required for an appropriate coupling to Gs.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorylation of the cation-independent mannose 6-phosphate receptor is closely associated with its exit from the trans-Golgi network

We have previously shown that two serine residues present in two conserved regions of the bovine cation-independent mannose 6-phosphate receptor (CI-MPR) cytoplasmic domain are phosphorylated in vivo (residues 2421 and 2492 of the full length bovine CI-MPR precursor). In this study, we have used CHO cells to investigate the phosphorylation state of these two serines along the different steps of the CI-MPR exocytic and endocytic recycling pathways. Transport and phosphorylation of the CI-MPR in the biosynthetic pathway were examined using deoxymannojirimycin (dMM), a specific inhibitor of the cis-Golgi processing enzyme alpha-mannosidase I which leads to the accumulation of N-linked high mannose oligosaccharides on glycoproteins. Upon removal of dMM, normal processing to complex-type oligosaccharides (galactosylation and then sialylation) occurs on the newly synthesized glycoproteins, including the CI-MPR which could then be purified and analyzed on lectin affinity columns. Phosphorylation of the newly synthesized CI-MPR was concomitant with the sialylation of its oligosaccharides and appeared as a major albeit transient modification. Phosphorylation of the cell surface CI-MPR was examined during its endocytosis as well as its return to the Golgi using antibody tagging and exogalactosylation. The cell surface CI-MPR was not phosphorylated when it entered clathrin-coated pits or when it moved to the early and late endosomes. In contrast, the surface CI-MPR was phosphorylated when it had been resialylated upon its return to the trans-Golgi network. Subcellular fractionation experiments showed that the phosphorylated CI-MPR and the corresponding kinase were found in clathrin-coated vesicles. Collectively, these results indicate that phosphorylation of the two serines in the CI-MPR cytoplasmic domain is associated with a single step of transport of its recycling pathways and occurs when this receptor is in the trans-Golgi network and/or has left this compartment via clathrin-coated vesicles.

Yeast Kex1p is a Golgi-associated membrane protein: deletions in a cytoplasmic targeting domain result in mislocalization to the vacuolar membrane

We have investigated the localization of Kex1p, a type I transmembrane carboxypeptidase involved in precursor processing within the yeast secretory pathway. Indirect immunofluorescence demonstrated the presence of Kex1p in a punctate organelle resembling the yeast Golgi apparatus as identified by Kex2p and Sec7p (Franzusoff, A., K. Redding, J. Crosby, R. S. Fuller, and R. Schekman. 1991. J. Cell Biol. 112:27-37). Glycosylation studies of Kex1p were consistent with a Golgi location, as Kex1p was progressively N-glycosylated in an MNN1-dependent manner. To address the basis of Kex1p targeting to the Golgi apparatus, we examined the cellular location of a series of carboxy-terminal truncations of the protein. The results indicate that a cytoplasmically exposed carboxy-terminal domain is required for retention of this membrane protein within the Golgi apparatus. Deletions of the retention region or overproduction of wild-type Kex1p led to mislocalization of Kex1p to the vacuolar membrane. This unexpected finding is discussed in terms of models involving either the vacuole as a default destination for membrane proteins, or by endocytosis to the vacuole following their default localization to the plasma membrane.

The effect of staurosporine, a protein kinase inhibitor, on asialoglycoprotein receptor endocytosis

Receptor-mediated endocytosis via coated pits is modulated by the activity of protein kinases and protein phosphorylation. We examined the effects of the potent protein kinase inhibitor staurosporine (SSP) on endocytosis of the asialoglycoprotein (ASGP) receptor in HepG2 cells. Staurosporine caused a rapid (< 2 min) inhibition of ligand internalization from the cell surface. In contrast the rate of receptor exocytosis from intracellular compartments to the cell surface was not altered (t1/2 = 8 min). This resulted in increased ASGP receptors at the plasma membrane (140% of control) while the total number of receptors per cell was unchanged. Receptor up-regulation was half-maximal at 30 nM SSP. At this concentration staurosporine also inhibited the internalization of iodinated transferrin by HepG2 cells and SK Hep-1 cells, another human hepatoma-derived cell line. Staurosporine was without effect on the non-receptor-mediated uptake of Lucifer yellow by pinocytosis. We investigated the possible involvement of protein kinase C in the inhibitory effects of staurosporine on receptor endocytosis. The active protein kinase C inhibitor H7 did not inhibit ASGP receptor internalization. Furthermore depletion of cellular protein kinase C by overnight incubation with 1 microM phorbol myristate acetate did not abrogate the SSP effect. Together these data suggest that the mechanism of SSP action is independent of the inhibition of protein kinase C. In conclusion staurosporine is a potent and rapid inhibitor of receptor trafficking which is specific for receptor internalization from the plasma membrane.

Basolateral sorting of LDL receptor in MDCK cells: the cytoplasmic domain contains two tyrosine-dependent targeting determinants

In MDCK cells, transport of membrane proteins to the basolateral plasma membrane has been shown to require a distinct cytoplasmic domain determinant. Although the determinant is often related to signals used for localization in clathrin-coated pits, inactivation of the coated pit domain in the human LDL receptor did not affect basolateral targeting. By expressing mutant and chimeric LDL receptors, we have now identified two independently acting signals that are individually sufficient for basolateral targeting. The two determinants mediate basolateral sorting with different efficiencies, but both contain tyrosine residues critical for activity. The first determinant was colinear with, but distinct from, the coated pit domain of the receptor. The second was found in the C-terminal region of the cytoplasmic domain of the receptor and, although tyrosine-dependent, did not mediate endocytosis. The results suggest that membrane proteins can have functionally redundant signals for basolateral transport and that a tyrosine-containing motif may be a common feature of multiple intracellular sorting events.

The cytoplasmic tail of the mannose 6-phosphate/insulin-like growth factor-II receptor has two signals for lysosomal enzyme sorting in the Golgi

The mannose 6-phosphate/insulin-like growth factor-II (Man-6-P/IGF-II) receptor is known to cycle between the Golgi, endosomes, and the plasma membrane. In the Golgi the receptor binds newly synthesized lysosomal enzymes and transports them directly to an endosomal (prelysosomal) compartment without traversing the plasma membrane. Deletion of the carboxyl-terminal Leu-Leu-His-Val residues of the 163 amino acid cytoplasmic tail of the bovine Man-6-P/IGF-II receptor partially impaired this function, resulting in the diversion of a portion of the receptor-ligand complexes to the cell surface, where they were endocytosed. The same phenotype was observed when 134 residues of the cytoplasmic tail were deleted from the carboxyl terminus. Disruption of the Tyr24-Lys-Tyr-Ser-Lys-Val29 plasma membrane internalization signal alone had little effect on Golgi sorting, but when combined with either deletion resulted in a complete loss of this function. The mutant receptors retained the ability to recycle to the Golgi and bind cathepsin D. These results indicate that the cytoplasmic tail of the Man-6-P/IGF-II receptor contains two signals that contribute to Golgi sorting, presumably by interacting with the Golgi clathrin-coated pit adaptor proteins. The Leu-Leu-containing sequence represents a novel motif for mediating interaction with Golgi adaptor proteins.

Biosynthesis and endocytosis of lysosomal enzymes in human colon carcinoma SW 1116 cells: impaired internalization of plasma membrane-associated cation-independent mannose 6-phosphate receptor

The human colon adenocarcinoma cell lines SW 948, SW 1116, and SW 1222 were tested for their ability to sort and internalize lysosomal enzymes. The biosynthesis of the lysosomal enzymes cathepsin B, arylsulfatase A, and beta-hexosaminidase in these cell lines exhibits no significant differences to that in human fibroblasts. The intracellular targeting of newly synthesized hydrolases to the lysosomes relies in colon carcinoma cells on the mannose 6-phosphate receptor system. Both the cation-independent mannose 6-phosphate receptor (CI-MPR) and the cation-dependent mannose 6-phosphate receptor are expressed in all colon carcinoma cell lines investigated. Endocytosis of lysosomal enzymes via mannose 6-phosphate receptors is reduced in colon carcinoma cells as compared with human fibroblasts. SW 1116 cells were shown to be deficient in receptor-mediated endocytosis of mannose 6-phosphate containing ligands. Ligands of other endocytic receptors as well as the fluid-phase marker horseradish peroxidase were internalized at normal rates. While antibodies against CI-MPR bind to the surface of SW 1116 cells, these antibodies cannot be internalized. These data suggest that the cycling of CI-MPR is specifically impaired in SW 1116 cells.

Membrane protein sorting in the yeast secretory pathway: evidence that the vacuole may be the default compartment

The targeting signals of two yeast integral membrane dipeptidyl aminopeptidases (DPAPs), DPAP B and DPAP A, which reside in the vacuole and the Golgi apparatus, respectively, were analyzed. No single domain of DPAP B is required for delivery to the vacuolar membrane, because removal or replacement of either the cytoplasmic, transmembrane, or lumenal domain did not affect the protein's transport to the vacuole. DPAP A was localized by indirect immunofluorescence to non-vacuolar, punctate structures characteristic of the yeast Golgi apparatus. The 118-amino acid cytoplasmic domain of DPAP A is sufficient for retention of the protein in these structures, since replacement of the cytoplasmic domain of DPAP B with that of DPAP A resulted in an immunolocalization pattern indistinguishable from that of wild type DPAP A. Overproduction of DPAP A resulted in its mislocalization to the vacuole, because cells expressing high levels of DPAP A exhibited vacuolar as well as Golgi staining. Deletion of 22 residues of the DPAP A cytoplasmic domain resulted in mislocalization of the mutant protein to the vacuole. Thus, the cytoplasmic domain of DPAP A is both necessary and sufficient for Golgi retention, and removal of the retention signal, or saturation of the retention apparatus by overproducing DPAP A, resulted in transport to the vacuole. Like wild type DPAP B, the delivery of mutant membrane proteins to the vacuole was unaffected in the secretory vesicle-blocked sec1 mutant; thus, transport to the vacuole was not via the plasma membrane followed by endocytosis. These data are consistent with a model in which membrane proteins are delivered to the vacuole along a default pathway.

The pathway and targeting signal for delivery of the integral membrane glycoprotein LEP100 to lysosomes

A complete set of chimeras was made between the lysosomal membrane glycoprotein LEP100 and the plasma membrane-directed vesicular stomatitis virus G protein, combining a glycosylated lumenal or ectodomain, a single transmembrane domain, and a cytosolic carboxyl-terminal domain. These chimeras, the parent molecules, and a truncated form of LEP100 lacking the transmembrane and cytosolic domains were expressed in mouse L cells. Only LEP100 and chimeras that included the cytosolic 11 amino acid carboxyl terminus of LEP100 were targeted to lysosomes. The other chimeras accumulated in the plasma membrane, and truncated LEP100 was secreted. Chimeras that included the extracellular domain of vesicular stomatitis G protein and the carboxyl terminus of LEP100 were targeted to lysosomes and very rapidly degraded. Therefore, in chimera-expressing cells, virtually all the chimeric molecules were newly synthesized and still in the biosynthesis and lysosomal targeting pathways. The behavior of one of these chimeras was studied in detail. After its processing in the Golgi apparatus, the chimera entered the plasma membrane/endosome compartment and rapidly cycled between the plasma membrane and endosomes before going to lysosomes. In pulse-expression experiments, a large population of chimeric molecules was observed to appear transiently in the plasma membrane by immunofluorescence microscopy. Soon after protein synthesis was inhibited, this surface population disappeared. When lysosomal proteolysis was inhibited, chimeric molecules accumulated in lysosomes. These data suggest that the plasma membrane/early endosome compartment is on the pathway to the lysosomal membrane. This explains why mutations that block endocytosis result in the accumulation of lysosomal membrane proteins in the plasma membrane.

The insulin receptor juxtamembrane region contains two independent tyrosine/beta-turn internalization signals

We have investigated the role of tyrosine residues in the insulin receptor cytoplasmic juxtamembrane region (Tyr953 and Tyr960) during endocytosis. Analysis of the secondary structure of the juxtamembrane region by the Chou-Fasman algorithms predicts that both the sequences GPLY953 and NPEY960 form tyrosine-containing beta-turns. Similarly, analysis of model peptides by 1-D and 2-D NMR show that these sequences form beta-turns in solution, whereas replacement of the tyrosine residues with alanine destabilizes the beta-turn. CHO cell lines were prepared expressing mutant receptors in which each tyrosine was mutated to phenylalanine or alanine, and an additional mutant contained alanine at both positions. These mutations had no effect on insulin binding or receptor autophosphorylation. Replacements with phenylalanine had no effect on the rate of [125I]insulin endocytosis, whereas single substitutions with alanine reduced [125I]insulin endocytosis by 40-50%. Replacement of both tyrosines with alanine reduced internalization by 70%. These data suggest that the insulin receptor contains two tyrosine/beta-turns which contribute independently and additively to insulin-stimulated endocytosis.

Sorting of protein A to the staphylococcal cell wall

The cell wall of gram-positive bacteria can be thought of as representing a unique cell compartment, which contains anchored surface proteins that require specific sorting signals. Some biologically important products are anchored in this way, including protein A and fibronectin binding protein of Staphylococcus aureus and streptococcal M protein. Studies of staphylococcal protein A and Escherichia coli alkaline phosphatase show that the signal both necessary and sufficient for cell wall anchoring consists of an LPXTGX motif, a C-terminal hydrophobic domain, and a charged tail. These sequence elements are conserved in many surface proteins from different gram-positive bacteria. We propose the existence of a hitherto undescribed sorting mechanism that positions proteins on the surface of gram-positive bacteria.

A novel di-leucine motif and a tyrosine-based motif independently mediate lysosomal targeting and endocytosis of CD3 chains

Partial complexes of the T cell antigen receptor lacking zeta chains are delivered to lysosomes. Chimeric proteins composed of the Tac antigen fused to the cytoplasmic domains of each CD3 chain has allowed the identification of lysosomal targeting sequences. Tac-gamma and Tac-delta chimeras are retained in the endoplasmic reticulum because of the presence of basic residues reminiscent of sequences responsible for the localization of endoplasmic reticulum resident proteins. Truncation of these retention motifs revealed lysosomal targeting of both Tac-gamma and delta chimeras. A di-leucine- and a tyrosine-based motif are individually sufficient to induce both endocytosis and delivery to lysosomes of Tac. In contrast with chimeras containing only one of these motifs, the chimera containing both was predominantly delivered directly to lysosomes without going through the cell surface. These two sequences may represent two families of targeting motifs that determine the fate of proteins within the peripheral membrane system.

Expression cloning of a diphtheria toxin receptor: identity with a heparin-binding EGF-like growth factor precursor

A monkey cDNA (pDTS) encoding a diphtheria toxin (DT) sensitivity determinant was isolated by expression cloning in mouse L-M cells. Mouse cells are naturally resistant to DT, because they lack functional cell surface receptors for the toxin. Unlike wild-type L-M cells, pDTS-transfected mouse cells are extremely toxin sensitive and specifically bind radioiodinated DT. Intoxication of the transfected cells requires receptor-mediated endocytosis of the bound toxin. The cDNA is predicted to encode an integral membrane protein that is identical to the precursor of a heparin-binding EGF-like growth factor. The DT sensitivity protein is thus a growth factor precursor that DT exploits as a receptor.

Two forms of the low-affinity Fc receptor for IgE differentially mediate endocytosis and phagocytosis: identification of the critical cytoplasmic domains

We have previously identified two species of the low-affinity human Fc receptor for IgE, Fc epsilon RIIa and Fc epsilon RIIb, which differ only in a short stretch of amino acids at the N-terminal cytoplasmic end. Their differential expressions on B cells and monocytes suggest that Fc epsilon RIIa and Fc epsilon RIIb are involved in B-cell function and IgE-mediated immunity, respectively. Here we show that Fc epsilon RII-mediated endocytosis is observed only in Fc epsilon RIIa-expressing cells, whereas IgE-dependent phagocytosis is observed only in Fc epsilon RIIb-expressing cells, demonstrating the functional difference between Fc epsilon RIIa and Fc epsilon RIIb. Furthermore, site-directed mutagenesis revealed that the tyrosine residue in the Fc epsilon RIIa-specific region is important for endocytosis, and the Asn-Pro residues in the Fc epsilon RIIb-specific region are required for phagocytosis. These findings suggest that endocytosis and phagocytosis are functionally separable phenomena involving distinct amino acid residues.

Solid phase synthesis of peptides containing the non-hydrolysable analog of (O)phosphotyrosine, p(CH2PO3H2)Phe. Application to the synthesis of 344-357 sequences of the beta 2 adrenergic receptor

Studies about phosphorylation-dephosphorylation mechanisms require the development of probes capable of being used in in vitro and in vivo conditions. We show in this work that the chemically and enzymatically stable p(CH2PO3H2)Phe analog of (O)phosphotyrosine can be easily introduced in peptides by the solid-phase method. It has been incorporated in the 344-357 sequence of the beta 2 adrenergic receptor in place of the Tyr residue in position 350 and/or 354 in order to investigate the role of tyrosine phosphorylation in the receptor agonist-induced down-regulation. Since p(CH2PO3H2)Phe is an ionized hydrophilic residue, peptides containing this amino acid do not easily permeate the cellular membranes. Therefore the modified amino acid was introduced in the synthetic pathway in its N-Boc-p(CH2PO3Et2)Phe form, which could be partially or completely deprotected. Coupling steps, including that of the new amino acid, were performed with good yields (approximately 60% total yield) and further deprotections provided both the p(CH2PO3H2)Phe and p(CH2PO3HEt)Phe containing peptides with yields of around 20% each. The structure of the peptides was assessed by NMR, mass spectroscopy and amino acid analysis and the new amino acid was characterized under its phenylthiocarbamyl form (PTC).

In vitro mutagenesis and the search for structure-function relationships among G protein-coupled receptors

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Low density lipoprotein receptor and cation-independent mannose 6-phosphate receptor are transported from the cell surface to the Golgi apparatus at equal rates in PC12 cells

Efficient transport of cell surface glycoproteins to the Golgi apparatus has been previously demonstrated for a limited number of proteins, and has been proposed to require selective sorting in the endocytic pathway after internalization. We have studied the endocytic fate of several glycoproteins that accumulate in different organelles in a variant clone of PC12, a regulated secretory cell line. The cation-independent mannose 6-phosphate receptor and the low density lipoprotein receptor, both rapidly internalized from the cell surface, and the synaptic vesicle membrane protein synaptophysin, were transported to the Golgi apparatus with equivalent, nonlinear kinetics. Transport to the Golgi apparatus (t1/2 = 2.5-3.0 h) was several times faster than turnover of these proteins (t1/2 greater than or equal to 20 h), indicating that transport of these proteins to the Golgi apparatus occurred on average several times for each protein. In contrast, Thy-1, a protein anchored in the membrane by a glycosylphosphoinositide group, was internalized and transported to the Golgi apparatus more slowly than the three transmembrane proteins. Since each of the transmembrane proteins studied showed the same t1/2 for transport to the Golgi apparatus, we conclude that transport of these proteins from the cell surface to the Golgi apparatus does not require sorting information specific to any one of these proteins. These results suggest that one of the functions of late endosomes is constitutive recycling of cell surface receptors through the Golgi apparatus if they fail to recycle to the cell surface directly from early endosomes, and that the late endosome recycling pathway is followed frequently by many rapidly internalized proteins.

Transport of the lysosomal membrane glycoprotein lgp120 (lgp-A) to lysosomes does not require appearance on the plasma membrane

We have used stably transfected CHO cell lines to characterize the pathway of intracellular transport of the lgp120 (lgp-A) to lysosomes. Using several surface labeling and internalization assays, our results suggest that lgp120 can reach its final destination with or without prior appearance on the plasma membrane. The extent to which lgp120 was transported via the cell surface was determined by two factors: expression level and the presence of a conserved glycine-tyrosine motif in the cytoplasmic tail. In cells expressing low levels of wild-type lgp120, the majority of newly synthesized molecules reached lysosomes without becoming accessible to antibody or biotinylation reagents added extracellularly at 4 degrees C. With increased expression levels, however, an increased fraction of transfected lgp120, as well as some endogenous lgp-B, appeared on the plasma membrane. The fraction of newly synthesized lgp120 reaching the cell surface was also increased by mutations affecting the cytoplasmic domain tyrosine or glycine residues. A substantial fraction of both mutants reached the surface even at low expression levels. However, only the lgp120G----A7 mutant was rapidly internalized and delivered from the plasma membrane to lysosomes. Taken together, our results show that the majority of newly synthesized wild-type lgp120 does not appear to pass through the cell surface en route to lysosomes. Instead, it is likely that lysosomal targeting involves a saturable intracellular sorting site whose affinity for lgp's is dependent on a glycine-tyrosine motif in the lgp120 cytoplasmic tail.