The cytoplasmic domain of P-selectin contains a sorting determinant that mediates rapid degradation in lysosomes

The targeting of Lamp1 to lysosomes is dependent on the spacing of its cytoplasmic tail tyrosine sorting motif relative to the membrane

Lamp1 is a type I transmembrane glycoprotein that is localized primarily in lysosomes and late endosomes. Newly synthesized molecules are mostly transported from the trans-Golgi network directly to endosomes and then to lysosomes. A minor pathway involves transport via the plasma membrane. The 11-amino acid cytoplasmic tail of lamp1 contains a tyrosine-based motif that has been previously shown to mediate sorting in the trans-Golgi network and rapid internalization at the plasma membrane. We studied whether this motif also mediates sorting in endosomes. We found that mutant forms of lamp1 in which all the amino acids of the cytoplasmic tail were modified except for the RKR membrane anchor and the YXXI sorting motif still localized to dense lysosomes, indicating that the YXXI motif is sufficient to confer proper intracellular targeting. However, when the spacing of the YXXI motif relative to the membrane was changed by deleting one amino acid or adding five amino acids, lysosomal targeting was almost completely abolished. Kinetic studies showed that these mutants were trapped in a recycling pathway, involving trafficking between the plasma membrane and early endocytic compartments. These findings indicate that the YXXI signal of lamp1 is recognized at several sorting sites, including the trans-Golgi network, the plasma membrane, and the early/sorting endosomes. Small changes in the spacing of this motif relative to the membrane dramatically impair sorting in the early/sorting endosomes but have only a modest effect on internalization at the plasma membrane. The spacing of sorting signals relative to the membrane may prove to be an important determinant in the functioning of these signals.

Degradation of mutant influenza virus hemagglutinins is influenced by cytoplasmic sequences independent of internalization signals

A mutant influenza virus hemagglutinin, HA+8, having a carboxyl-terminal extension of 8 amino acids that included 4 aromatic residues, was internalized within 2 min of arriving at the cell surface and was degraded quickly by a process that was inhibited by ammonium chloride. Through second-site mutagenesis, the internalization sequence of HA+8 was found to closely resemble the internalization signals of the transferrin receptor or large mannose 6-phosphate receptor. Comparison of the intracellular traffic of HA+8 and a series of other HA mutants that differed in their rates of internalization revealed a relation between the amount of the protein on the plasma membrane at steady state and the internalization rate that would be predicted if most of each protein recycled to the cell surface. However, there was no simple correlation between the internalization rate and the rate of degradation, indicating that transport to the compartment where degradation occurred was not simply a function of the concentration of the proteins in early endosomes. The internal populations of both HA+8, which was degraded with a t1/2 of 1.9 h, and HA-Y543, which was degraded with a t1/2 of 2.9 h, were found by cell fractionation and density-shift experiments to reside in early endosomes with little accumulation in lysosomes. A fluid-phase marker reached lysosomes 3-4-fold faster than these proteins were degraded. Degradation of these mutant HAs involved a rate-determining step in early endosomes that was sensitive to some feature of the protein that depended upon sequence differences in the cytoplasmic domain unrelated to the internalization signal.

Residues throughout the cytoplasmic domain affect the internalization efficiency of P-selectin

The cytoplasmic domains of many membrane proteins have short sequences, usually including a tyrosine or a di-leucine, that function as sorting signals. P-selectin is an adhesion receptor for leukocytes that is expressed on activated platelets and endothelial cells. Its 35-residue cytoplasmic domain contains signals for sorting into regulated secretory granules, for endocytosis, and for movement from endosomes to lysosomes. The domain has a membrane-distal sequence, YGVFTNAAF, that resembles some tyrosine-based signals. We studied the effects of deletions and mutations in the cytoplasmic tail of human P-selectin on its internalization in clathrin-coated pits of transfected Chinese hamster ovary cells. Mutations and deletions in the putative tyrosine-based motif did not clearly implicate these residues as critical components of a short internalization signal. Indeed, a construct containing a truncated 18-residue cytoplasmic domain with a single substitution (K761A/H773Stop) was internalized nearly three times as fast as wild-type P-selectin; this construct contained no di-leucine, tyrosine, or other known sorting motif. Substitution of residues throughout the cytoplasmic domain affected the internalization rate of P-selectin. Furthermore, the cytoplasmic domain of P-selectin mediated faster internalization when attached to the extracellular and transmembrane domains of the low density lipoprotein receptor than when attached to the corresponding domains of P-selectin. Thus, we were unable to identify a short internalization signal in the cytoplasmic tail of P-selectin. Residues throughout the cytoplasmic domain, and perhaps the transmembrane sequence to which the domain is attached, affect the efficiency of internalization.

The gap junction protein connexin43 is degraded via the ubiquitin proteasome pathway

We investigated the degradation of the gap junction protein connexin43 in E36 Chinese hamster ovary cells and rat cardiomyocyte-derived BWEM cells. Treatment of E36 cells with the lysosomotropic amine, primaquine, for 16 h doubled the amount of connexin43 detected by immunoblotting and modestly increased the half-life of connexin43 in pulse-chase studies, suggesting that the lysosome played a minor role in connexin43 proteolysis. In contrast, treatment with the proteasomal inhibitor N-acetyl-L-leucyl-L-leucinyl-norleucinal led to a 6-fold accumulation of connexin43 and increased the half-life of connexin43 to approximately 9 h. The role of ubiquitin in connexin43 degradation was examined in an E36-derived mutant, ts20, which contains a thermolabile ubiquitin-activating enzyme, E1. E36 and ts20 cells grown at the permissive temperature contained similar amounts of connexin43 detectable by immunoblotting. Heat treatment dramatically reduced the amount of connexin43 detected in E36 cells, while connexin43 levels in heat-treated ts20 cells did not change. E36 cells that were heat-treated in the presence of N-acetyl-L-leucyl-L-leucinyl-norleucinal did not lose their connexin43. Pulse-chase experiments showed the reversibility of the block to connexin43 degradation in ts20 cells that were returned to the permissive temperature. Finally, sequential immunoprecipitation using anti-connexin43 and anti-ubiquitin antibodies demonstrated polyubiquitination of connexin43. These results indicate that ubiquitin-mediated proteasomal proteolysis may be the major mechanism of degradation of connexin43.

P-selectin expression in myocardium of children undergoing cardiopulmonary bypass

Cardiopulmonary bypass is a planned support technique that results in a period of myocardial ischemia and reperfusion. In addition, it is associated with an inflammatory response likely involving endothelial cell activation. In previous studies, we showed that E-selectin and intercellular adhesion molecule-1 (ICAM-1) messenger ribonucleic acid (mRNA) are increased in human myocardium after cardiopulmonary bypass. We have now examined the expression of P-selectin mRNA by ribonuclease protection in paired atrial biopsy specimens from 12 patients before and after cardiopulmonary bypass. By means of immunocytochemistry, we have also examined the endothelial cell surface expression of P-selectin protein, as well as that of E-selectin and ICAM-1 in three additional patients. Patient ages ranged from 1 day to 8.5 years (median 12 months), and cardiopulmonary bypass times ranged from 46 to 196 minutes (median 144 minutes). By ribonuclease protection, there was marked variability in the expression of P-selectin in biopsy specimens before bypass. However, when compared with prebypass levels, P-selectin mRNA decreased modestly in 10 of 12 patients after bypass (median decrease 1.5-fold, p = 0.016). As seen with immunocytochemistry, P-selectin protein was distributed diffusely through the vascular bed on large vessels and small vessels before bypass but was virtually absent on capillaries in specimens taken after bypass. E-selectin, which was absent in prebypass biopsy specimens, was induced in one of the three specimens after bypass, but no change in ICAM-1 protein expression above baseline was noted. We also find that cultured human endothelial cells treated with tumor necrosis factor-alpha in doses which induce ICAM-1 mRNA simultaneously decrease their expression of P-selectin mRNA as compared with untreated cells. These observations suggest that endothelial P-selectin is transcriptionally downregulated after cardiopulmonary bypass at times when E-selectin and ICAM-1 are induced. Furthermore, we find that E-selectin and ICAM-1 are expressed at times and at sites where P-selectin is absent. Although it is possible that P-selectin may have been induced and lost at early times before reperfusion, these data suggest that endothelial P-selectin plays a limited role in the inflammatory response that ensues after cardiopulmonary bypass.

A determinant in the cytoplasmic tail of the cation-dependent mannose 6-phosphate receptor prevents trafficking to lysosomes

The bovine cation-dependent mannose 6-phosphate receptor (CD-MPR) is a type 1 transmembrane protein that cycles between the trans-Golgi network, endosomes, and the plasma membrane. When the terminal 40 residues were deleted from the 67-amino acid cytoplasmic tail of the CD-MPR, the half-life of the receptor was drastically decreased and the mutant receptor was recovered in lysosomes. Analysis of additional cytoplasmic tail truncation mutants and alanine-scanning mutants implicated amino acids 34-39 as being critical for avoidance of lysosomal degradation. The cytoplasmic tail of the CD-MPR was partially effective in preventing the lysosomal membrane protein Lamp1 from entering lysosomes. Complete exclusion required both the CD-MPR cytoplasmic tail and transmembrane domain. The transmembrane domain alone had just a minor effect on the distribution of Lamp1. These findings indicate that the cytoplasmic tail of the CD-MPR contains a signal that prevents the receptor from trafficking to lysosomes. The transmembrane domain of the CD-MPR also contributes to this function.

Membrane transport in the endocytic pathway

Despite controversies and debates, some fundamental properties of endosomes become apparent when comparing results from in vivo and in vitro strategies used to study endosomal membrane traffic. In addition, recent studies are starting to unravel the complex organization of early endosomes, in particular along the route followed by recycling receptors.

Mutational analysis of the membrane-proximal cleavage site of L-selectin: relaxed sequence specificity surrounding the cleavage site

L-selectin expression is regulated in part by membrane-proximal cleavage from the cell surface of leukocytes and L-selectin-transfected cells. The downregulation of L-selectin from the surface of neutrophils is speculated to be a process involved in the adhesion cascade leading to neutrophil recruitment to sites of inflammation. We previously reported that L-selectin is cleaved between Lys321 and Ser322 in a region that links the second short consensus repeat (SCR) and the transmembrane domain. We demonstrate that replacing this cleavage domain of L-selectin with the corresponding region of E-selectin prevents L-selectin shedding, as judged by inhibiting the generation of the 68-kD soluble and 6-kD transmembrane cleavage products of L-selectin. Unexpectedly, we found that point mutations of the cleavage site, as well as mutations of multiple conserved amino acids within the cleavage domain, do not significantly affect L-selectin shedding. However, short deletions of four or five amino acids in the L-selectin cleavage domain inhibit L-selectin downregulation. Mutations that appeared to inhibit L-selectin shedding resulted in higher levels of cell surface expression, consistent with a lack of apparent proteolysis from the cell membrane. One deletion mutant, I327 delta N332, retains the native cleavage site yet inhibits L-selectin proteolysis as well. Restoring the amino acids deleted between I327 and N332 with five alanine residues restores L-selectin proteolysis. Thus, the proteolytic processing of L-selectin appears to have a relaxed sequence specificity at the cleavage site, and it may depend on the physical length or other secondary structural characteristics of the cleavage domain.

The P-selectin glycoprotein ligand functions as a common human leukocyte ligand for P- and E-selectins

P- and E-selectins belong to a family of Ca(2+)-dependent lectins and function as receptors for myeloid leukocytes. We have described a panel of monoclonal antibodies which recognize a sialoglycoprotein from human neutrophils and HL-60 promyelocytic cells and inhibit adhesion of these cells to P-selectin. In this study, we show that the E-selectin receptor-globulin (E-selectin Rg) affinity chromatography can isolate specifically only one glycoprotein from [3H]glucosamine-labeled HL-60 cells in a Ca(2+)-dependent manner. This protein has a molecular mass of approximately 120 kDa under reducing conditions, which appears to be identical with the previously characterized glycoprotein ligand for P-selectin. The molecule can be cross-depleted by and cross-bound to the E- and P-selectin columns. The chromatographic profile of desialylated O-linked carbohydrates from molecules purified by P- and E-selectin affinity chromatography are identical. Both have five structures at 12.8, 9.8, 6.3, 3.5, and 2.5 glucose units. PL5 monoclonal antibody to the P-selectin sialoglycoprotein ligand, E-selectin Rg, and antiserum to P-selectin glycoprotein ligand-1 (PSGL-1) all recognize the purified P-selectin ligand on ligand blots and immunoblots. Furthermore, PL5 monoclonal antibody blocks adhesion of HL-60 cells and human neutrophils to E-selectin Rg. Taken together, our results demonstrate that the P- and E-selectin ligand defined in this study is PSGL-1 and suggest that this molecule is an important leukocyte ligand for both P- and E-selectins.

Leukocyte trafficking mediated by selectin-carbohydrate interactions

The selectins have attracted intense interest because of their carbohydrate-recognition properties and their pivotal roles in leukocyte trafficking. Future studies will center on the mechanisms for regulating the expression of the selectins and their ligands, the molecular details of selectin binding to glycoprotein ligands and small carbohydrates, and the biophysical principles that selectins employ to mediate attachment and rolling of leukocytes under flow.

Endocytosis of interleukin 2 receptors in human T lymphocytes: distinct intracellular localization and fate of the receptor alpha, beta, and gamma chains

Members of the cytokine receptor family are composed of several noncovalently linked chains with sequence and structure homologies in their extracellular domain. Receptor subfamily members share at least one component: thus the receptors for interleukin (IL) 2 and IL15 have common beta and gamma chains, while those for IL2, 4, 7, and 9 have a common gamma chain. The intracellular pathway followed by IL2 receptors after ligand binding and endocytosis was analyzed by immunofluorescence and confocal microscopy in a human T lymphocytic cell line. Surprisingly, the alpha, beta, and gamma chains had different intracellular localizations after being endocytosed together. The alpha chain was always in transferrin-positive compartments (early/recycling endosomes), both at early and late internalization times, but was never detected in rab7-positive compartments (late endosomes). On the other hand, at late internalization times, the beta and gamma chains were excluded from transferrin-positive organelles and did not colocalize with alpha. Furthermore, beta could be found in rab7-positive vesicles. These differences suggest that the alpha chain recycles to the plasma membrane, while the beta and gamma chains are sorted towards the degradation pathway. The half-lives of these three chains on the cell surface also reflect their different intracellular fates after endocytosis. The beta and gamma chains are very short-lived polypeptides since their half-life on the surface is only approximately 1 h, whereas alpha is a much more stable surface protein. This shows for the first time that components of a multimeric receptor can be sorted separately along the endocytic pathway.

Endocytosis and lysosomal targeting of epidermal growth factor receptors are mediated by distinct sequences independent of the tyrosine kinase domain

Ligand-induced internalization of the epidermal growth factor receptor (EGFR) leads to accelerated receptor degradation. Two models have been proposed to explain this. In the first model, induced internalization expands the intracellular pool of receptors, leading to enhanced lysosomal targeting. The second model proposes that activation of intrinsic receptor kinase activity induces inward vesiculation of endosomes, thus interrupting receptor recycling. To test these models, we created EGFR mutants that lack the conserved tyrosine kinase domain, but retain different parts of the distal carboxyl terminus regulatory region. Mutants lacking all distal regulatory sequences underwent slow internalization (0.02 min-1) and turnover (t1/2 approximately 24 h), similar to unoccupied, holo-EGFR. Mutant receptors that lacked the kinase domain, but retained the entire distal regulatory domain, were constitutively internalized and targeted to lysosomes, even in the absence of EGF. The turnover of these receptors (t1/2 approximately 11 h) was similar to that of occupied, kinase-active holo-EGFR (t1/2 approximately 9.5 h). These results show that receptor tyrosine kinase activity is not required for the targeting of EGFR to lysosomes. Receptor mutants which expressed previously identified endocytic sequences underwent rapid internalization. Unexpectedly, enhanced turnover of EGFR mutants required additional sequences located between residues 945 and 991 in the holo-EGFR. Thus, internalization and lysosomal targeting of EGFR are separate processes mediated by distinct sequences. Our results indicate that induced internalization is necessary, but not sufficient, for enhanced EGFR degradation. Instead, down-regulation requires exposure of previously cryptic internalization and lysosomal targeting sequences. Occupied EGFR thus appear to be handled by the endocytic machinery in the same fashion as other constitutively internalized or lysosomally targeted receptors.

A distinct class of intracellular storage vesicles, identified by expression of the glucose transporter GLUT4

Some cell types have cytoplasmic storage vesicles whose fusion with the cell surface is triggered by an extracellular signal. To explore the relationship between different classes of storage vesicles, we expressed, in the neuro-endocrine cell line PC12, the facilitative glucose transporter GLUT4, which is stored in small cytoplasmic vesicles in fat and muscle cells and mobilized to the cell surface when insulin is present. PC12 cells have two known types of storage vesicles, secretory granules and synaptic vesicles, but GLUT4 is targeted to neither. It is recovered, however, in a class of small vesicles that sediment approximately twice as fast as synaptic vesicles. Immunoelectron microscopy confirmed the presence of such small vesicles in transfected PC12 cells. By velocity sedimentation analysis, GLUT4 vesicles efficiently exclude the synaptic vesicle markers synaptophysin, SV2, and synaptobrevin; the transferrin receptor, a marker of conventional endocytosis; and the polymeric immunoglobulin receptor, a marker of transcytosis. The exclusion of synaptophysin and the transferrin receptor from most of the GLUT4-containing structures was confirmed by confocal immunofluorescence microscopy. Like synaptic vesicles, therefore, GLUT4 vesicles of PC12 cells appear to be a unique type of organelle. A GLUT4-containing organelle of identical sedimentation properties was found in transfected fibroblast cell lines and in rat adipocytes. On stimulation of the adipocytes with insulin, GLUT4 was translocated from the peak of small vesicles to faster sedimenting membranes. We propose that the class of vesicles described here is present in a wide range of cell types and is involved in transient modification of the cell surface.

The polymeric immunoglobulin receptor accumulates in specialized endosomes but not synaptic vesicles within the neurites of transfected neuroendocrine PC12 cells

We have expressed in neuroendocrine PC12 cells the polymeric immunoglobulin receptor (pIgR), which is normally targeted from the basolateral to the apical surface of epithelial cells. In the presence of nerve growth factor, PC12 cells extend neurites which contain synaptic vesicle-like structures and regulated secretory granules. By immunofluorescence microscopy, pIgR, like the synaptic vesicle protein synaptophysin, accumulates in both the cell body and the neurites. On the other hand, the transferrin receptor, which normally recycles at the basolateral surface in epithelial cells, and the cation-independent mannose 6-phosphate receptor, a marker of late endosomes, are largely restricted to the cell body. pIgR internalizes ligand into endosomes within the cell body and the neurites, while uptake of ligand by the low density lipoprotein receptor occurs primarily into endosomes within the cell body. We conclude that transport of membrane proteins to PC12 neurites as well as to specialized endosomes within these processes is selective and appears to be governed by similar mechanisms that dictate sorting in epithelial cells. Additionally, two types of endosomes can be identified in polarized PC12 cells by the differential uptake of ligand, a housekeeping type in the cell bodies and a specialized endosome in the neurites. Recent findings suggest that specialized axonal endosomes in neurons are likely to give rise to synaptic vesicles (Mundigl, O., M. Matteoli, L. Daniell, A. Thomas-Reetz, A. Metcalf, R. Jahn, and P. De Camilli. 1993. J. Cell Biol. 122:1207-1221). Although pIgR reaches the specialized endosomes in the neurites of PC12 cells, we find by subcellular fractionation that under a variety of conditions it is efficiently excluded from synaptic vesicle-like structures as well as from secretory granules.

Preferential localization of a vesicular monoamine transporter to dense core vesicles in PC12 cells

Neurons and endocrine cells have two types of secretory vesicle that undergo regulated exocytosis. Large dense core vesicles (LDCVs) store neural peptides whereas small clear synaptic vesicles store classical neurotransmitters such as acetylcholine, gamma-aminobutyric acid (GABA), glycine, and glutamate. However, monoamines differ from other classical transmitters and have been reported to appear in both LDCVs and smaller vesicles. To localize the transporter that packages monoamines into secretory vesicles, we have raised antibodies to a COOH-terminal sequence from the vesicular amine transporter expressed in the adrenal gland (VMAT1). Like synaptic vesicle proteins, the transporter occurs in endosomes of transfected CHO cells, accounting for the observed vesicular transport activity. In rat pheochromocytoma PC12 cells, the transporter occurs principally in LDCVs by both immunofluorescence and density gradient centrifugation. Synaptic-like microvesicles in PC12 cells contain relatively little VMAT1. The results appear to account for the storage of monoamines by LDCVs in the adrenal medulla and indicate that VMAT1 provides a novel membrane protein marker unique to LDCVs.