Polarized trafficking of plasma membrane proteins: emerging roles for coats, SNAREs, GTPases and their link to the cytoskeleton

Basolateral delivery of the type I transforming growth factor beta receptor is mediated by a dominant-acting cytoplasmic motif

A novel motif within the cytoplasmic tail of the type I TGF-β receptor (TβRI) controls basolateral delivery. While this element functions independent of TβRI recycling and heteromeric TGF-β receptor trafficking, it can dominantly direct an apically expressed receptor to the basolateral membrane in polarized epithelial cells.

Delivery of biomolecules to the correct subcellular locales is critical for proper physiological function. To that end, we have previously determined that type I and II transforming growth factor beta (TGF-β) receptors (TβRI and TβRII, respectively) localize to the basolateral domain in polarized epithelia. While TβRII targeting was shown to be regulated by sequences between amino acids 529 and 538, the analogous region(s) within TβRI is unknown. To address that question, sequential cytoplasmic TβRI truncations and point mutations identified a targeting motif between residues 158 and 163 (VxxEED) required for basolateral TβRI expression. Further studies documented that receptor internalization, down-regulation, direct recycling, or Smad signaling were unaffected by motif mutations that caused TβRI mislocalization. However, inclusion of amino acids 148–217 containing the targeting motif was able to direct basolateral expression of the apically sorted nerve growth factor receptor (NGFR, p75; extracellular and transmembrane regions) in a dominant manner. Finally, coexpression of apically targeted type I and type II TGF-β receptors mediated Smad3 signaling from the apical membrane of polarized epithelial cells. These findings demonstrate that the absence of apical TGF-β signaling in normal epithelia is primarily a reflection of domain-specific receptor expression and not an inability to couple with the signaling machinery.

Role of TGF-β receptor III localization in polarity and breast cancer progression

The majority of breast cancers originate from the highly polarized luminal epithelial cells lining the breast ducts. However, cell polarity is often lost during breast cancer progression. The type III transforming growth factor-β cell surface receptor (TβRIII) functions as a suppressor of breast cancer progression and also regulates the process of epithelial-to-mesenchymal transition (EMT), a consequence of which is the loss of cell polarity. Many cell surface proteins exhibit polarized expression, being targeted specifically to the apical or basolateral domains. Here we demonstrate that TβRIII is basolaterally localized in polarized breast epithelial cells and that disruption of the basolateral targeting of TβRIII through a single amino acid mutation of proline 826 in the cytosolic domain results in global loss of cell polarity through enhanced EMT. In addition, the mistargeting of TβRIII results in enhanced proliferation, migration, and invasion in vitro and enhanced tumor formation and invasion in an in vivo mouse model of breast carcinoma. These results suggest that proper localization of TβRIII is critical for maintenance of epithelial cell polarity and phenotype and expand the mechanisms by which TβRIII prevents breast cancer initiation and progression.

Hepatocyte polarity

Hepatocytes, like other epithelia, are situated at the interface between the organism's exterior and the underlying internal milieu and organize the vectorial exchange of macromolecules between these two spaces. To mediate this function, epithelial cells, including hepatocytes, are polarized with distinct luminal domains that are separated by tight junctions from lateral domains engaged in cell-cell adhesion and from basal domains that interact with the underlying extracellular matrix. Despite these universal principles, hepatocytes distinguish themselves from other nonstriated epithelia by their multipolar organization. Each hepatocyte participates in multiple, narrow lumina, the bile canaliculi, and has multiple basal surfaces that face the endothelial lining. Hepatocytes also differ in the mechanism of luminal protein trafficking from other epithelia studied. They lack polarized protein secretion to the luminal domain and target single-spanning and glycosylphosphatidylinositol-anchored bile canalicular membrane proteins via transcytosis from the basolateral domain. We compare this unique hepatic polarity phenotype with that of the more common columnar epithelial organization and review our current knowledge of the signaling mechanisms and the organization of polarized protein trafficking that govern the establishment and maintenance of hepatic polarity. The serine/threonine kinase LKB1, which is activated by the bile acid taurocholate and, in turn, activates adenosine monophosphate kinase-related kinases including AMPK1/2 and Par1 paralogues has emerged as a key determinant of hepatic polarity. We propose that the absence of a hepatocyte basal lamina and differences in cell-cell adhesion signaling that determine the positioning of tight junctions are two crucial determinants for the distinct hepatic and columnar polarity phenotypes.

Directional protein secretion by the retinal pigment epithelium: roles in retinal health and the development of age-related macular degeneration

The structural and functional integrity of the retinal pigment epithelium (RPE) is fundamental for maintaining the function of the neuroretina. These specialized cells form a polarized monolayer that acts as the retinal–blood barrier, separating two distinct environments with highly specialized functions: photoreceptors of the neuroretina at the apical side and Bruch's membrane/highly vascularized choriocapillaris at the basal side. The polarized nature of the RPE is essential for the health of these two regions, not only in nutrient and waste transport but also in the synthesis and directional secretion of proteins required in maintaining retinal homoeostasis and function. Although multiple malfunctions within the RPE cells have been associated with development of age-related macular degeneration (AMD), the leading cause of legal blindness, clear causative processes have not yet been conclusively characterized at the molecular and cellular level. This article focuses on the involvement of directionally secreted RPE proteins in normal functioning of the retina and on the potential association of incorrect RPE protein secretion with development of AMD. Understanding the importance of RPE polarity and the correct secretion of essential structural and regulatory components emerge as critical factors for the development of novel therapeutic strategies targeting AMD.

Immunocytochemical evidence of Tulp1-dependent outer segment protein transport pathways in photoreceptor cells

Tulp1 is a protein of unknown function exclusive to rod and cone photoreceptor cells. Mutations in the gene cause autosomal recessive retinitis pigmentosa in humans and photoreceptor degeneration in mice. In tulp1-/- mice, rod and cone opsins are mislocalized, and rhodopsin-bearing extracellular vesicles accumulate around the inner segment, indicating that Tulp1 is involved in protein transport from the inner segment to the outer segment. To investigate this further, we sought to define which outer segment transport pathways are Tulp1-dependent. We used immunohistochemistry to examine the localization of outer segment proteins in tulp1-/- photoreceptors, prior to retinal degeneration. We also surveyed the condition of inner segment organelles and rhodopsin transport machinery proteins. Herein, we show that guanylate cyclase 1 and guanylate cyclase activating proteins 1 and 2 are mislocalized in the absence of Tulp1. Furthermore, arrestin does not translocate to the outer segment in response to light stimulation. Additionally, data from the tulp1-/- retina adds to the understanding of peripheral membrane protein transport, indicating that rhodopsin kinase and transducin do not co-transport in rhodopsin carrier vesicles and phosphodiesterase does not co-transport in guanylate cyclase carrier vesicles. These data implicate Tulp1 in the transport of selective integral membrane outer segment proteins and their associated proteins, specifically, the opsin and guanylate cyclase carrier pathways. The exact role of Tulp1 in outer segment protein transport remains elusive. However, without Tulp1, two rhodopsin transport machinery proteins exhibit abnormal distribution, Rab8 and Rab11, suggesting a role for Tulp1 in vesicular docking and fusion at the plasma membrane near the connecting cilium.

Regulation of the epithelial sodium channel by membrane trafficking

The epithelial Na(+) channel (ENaC) is a major regulator of salt and water reabsorption in a number of epithelial tissues. Abnormalities in ENaC function have been directly linked to several human disease states including Liddle's syndrome, psuedohypoaldosteronism, and cystic fibrosis and may be implicated in states as diverse as salt-sensitive hypertension, nephrosis, and pulmonary edema. ENaC activity in epithelial cells is highly regulated both by open probability and number of channels. Open probability is regulated by a number of factors, including proteolytic processing, while ENaC number is regulated by cellular trafficking. This review discusses current understanding of apical membrane delivery, cell surface stability, endocytosis, retrieval, and recycling of ENaC and the molecular partners that have so far been shown to participate in these processes. We review known sites and mechanisms of hormonal regulation of trafficking by aldosterone, vasopressin, and insulin. While many details of the regulation of ENaC trafficking remain to be elucidated, knowledge of these mechanisms may provide further insights into ENaC activity in normal and disease states.

A majority of cotton genes are expressed in single-celled fiber

Multicellular eukaryotes contain a diversity of cell types, presumably differing from one another in the suite of genes expressed during development. At present, little is known about the proportion of the genome transcribed in most cell types, nor the degree to which global patterns of expression change during cellular differentiation. To address these questions in a model plant system, we studied the unique and highly exaggerated single-celled, epidermal seed trichomes ("cotton") of cultivated cotton (Gossypium hirsutum). By taking advantage of advances in expression profiling and microarray technology, we evaluated the transcriptome of cotton fibers across a developmental time-course, from a few days post-anthesis through primary and secondary wall synthesis stages. Comparisons of gene expression in populations of developing cotton fiber cells to genetically complex reference samples derived from 6 different cotton organs demonstrated that a remarkably high proportion of the cotton genome is transcribed, with 75-94% of the total genome transcribed at each stage. Compared to the reference samples, more than half of all genes were up-regulated during at least one stage of fiber development. These genes were clustered into seven groups of expression profiles that provided new insight into biological processes governing fiber development. Genes implicated in vesicle coating and trafficking were found to be overexpressed throughout all stages of fiber development studied, indicating their important role in maintaining rapid growth of this unique plant cell.

A unique element in the cytoplasmic tail of the type II transforming growth factor-beta receptor controls basolateral delivery

Transforming growth factor (TGF)-beta receptors stimulate diverse signaling processes that control a wide range of biological responses. In polarized epithelia, the TGFbeta type II receptor (T2R) is localized at the basolateral membranes. Sequential cytoplasmic truncations resulted in receptor missorting to apical surfaces, and they indicated an essential targeting element(s) near the receptor's C terminus. Point mutations in the full-length receptor confirmed this prediction, and a unique basolateral-targeting region was elucidated between residues 529 and 538 (LTAxxVAxxR) that was distinct, but colocalized within a clinically significant signaling domain essential for TGFbeta-dependent activation of the Smad2/3 cascade. Transfer of a terminal 84 amino-acid fragment, containing the LTAxxVAxxR element, to the apically sorted influenza hemagglutinin (HA) protein was dominant and directed basolateral HA expression. Although delivery to the basolateral surfaces was direct and independent of any detectable transient apical localization, fluorescence recovery after photobleaching demonstrated similar mobility for the wild-type receptor and a missorted mutant lacking the targeting motif. This latter finding excludes the possibility that the domain acts as a cell membrane retention signal, and it supports the hypothesis that T2R sorting occurs from an intracellular compartment.

Identification of a novel apical sorting motif and mechanism of targeting of the M2 muscarinic acetylcholine receptor

Previous studies have shown that the M2 receptor is localized at steady state to the apical domain in Madin-Darby canine kidney (MDCK) epithelial cells. In this study, we identify the molecular determinants governing the localization and the route of apical delivery of the M2 receptor. First, by confocal analysis of a transiently transfected glycosylation mutant in which the three putative glycosylation sites were mutated, we determined that N-glycans are not necessary for the apical targeting of the M2 receptor. Next, using a chimeric receptor strategy, we found that two independent sequences within the M2 third intracellular loop can confer apical targeting to the basolaterally targeted M4 receptor, Val270-Lys280 and Lys280-Ser350. Experiments using Triton X-100 extraction followed by OptiPrep density gradient centrifugation and cholera toxin beta-subunit-induced patching demonstrate that apical targeting is not because of association with lipid rafts. 35S-Metabolic labeling experiments with domain-specific surface biotinylation as well as immunocytochemical analysis of the time course of surface appearance of newly transfected confluent MDCK cells expressing FLAG-M2-GFP demonstrate that the M2 receptor achieves its apical localization after first appearing on the basolateral domain. Domain-specific application of tannic acid of newly transfected cells indicates that initial basolateral plasma membrane expression is required for subsequent apical localization. This is the first demonstration that a G-protein-coupled receptor achieves its apical localization in MDCK cells via transcytosis.

Renal expression and localization of the facilitative glucose transporters GLUT1 and GLUT12 in animal models of hypertension and diabetic nephropathy

Renal tubular glucose reabsorption is mediated by facilitative glucose transporter (GLUT) proteins and energy-dependent sodium glucose luminal transporters. Glucose transport in the diabetic kidney is upregulated and has been implicated in the pathogenesis of progressive diabetic nephropathy. Hyperglycemia, hypertension, and activation of the renin-angiotensin system are believed important in the development of the disease. The present study examines the renal expression of the facilitative glucose transporters GLUT1 and GLUT12 in rat models of diabetic nephropathy. Sprague-Dawley and transgenic (mRen-2)27 rats received either streptozotocin-induced diabetes or vehicle. GLUT12 expression and localization were determined by immunohistochemistry, immunoblotting, in situ hybridization, and confocal immunofluorescence. GLUT1 immunolabeling was detected on the basolateral membrane throughout the nephron. GLUT12 was localized to the distal tubules and collecting ducts. A significant increase in GLUT12 immunolabeling was measured in Ren-2 controls and Ren-2 diabetic animals compared with Sprague-Dawley controls. GLUT12 expression was higher in Ren-2 diabetic compared with Sprague-Dawley diabetic rats. Long-term diabetes resulted in significant increases in GLUT1 levels in the renal proximal tubules and expression was higher in Ren-2 diabetic than Sprague-Dawley diabetic rats. GLUT12 protein was localized to the cytoplasm and to the apical membrane of human and rat distal tubules and collecting ducts. The apical localization of GLUT12 in the distal tubules and collecting ducts suggests that it could contribute to additional glucose reabsorption in the late nephron. Levels of both GLUT1 and GLUT12 are elevated in animal models of hypertension and diabetic nephropathy.

Signals regulating trafficking of Menkes (MNK; ATP7A) copper-translocating P-type ATPase in polarized MDCK cells

The Menkes protein (MNK; ATP7A) functions as a transmembrane copper-translocating P-type ATPase and plays a vital role in systemic copper absorption in the gut and copper reabsorption in the kidney. Polarized epithelial cells such as Madin-Darby canine kidney (MDCK) cells are a physiologically relevant model for systemic copper absorption and reabsorption in vivo. In this study, cultured MDCK cells were used to characterize MNK trafficking and enabled the identification of signaling motifs required to target the protein to specific membranes. Using confocal laser scanning microscopy and surface biotinylation we demonstrate that MNK relocalizes from the Golgi to the basolateral (BL) membrane under elevated copper conditions. As previously shown in nonpolarized cells, the metal binding sites in the NH2-terminal domain of MNK were found to be required for copper-regulated trafficking from the Golgi to the plasma membrane. These data provide molecular evidence that is consistent with the presumed role of this protein in systemic copper absorption in the gut and reabsorption in the kidney. Using site-directed mutagenesis, we identified a dileucine motif proximal to the COOH terminus of MNK that was critical for correctly targeting the protein to the BL membrane and a putative PDZ target motif that was required for localization at the BL membrane in elevated copper.

Sorting Competition with Membrane-permeable Peptides in Intact Epithelial Cells Revealed Discrimination of Transmembrane Proteins Not Only at the trans-Golgi Network but Also at Pre-Golgi Stages

Transmembrane proteins destined to the basolateral cell surface of epithelial cells contain in their cytosolic domain at least two classes of sorting signals: one class promotes exit from the endoplasmic reticulum (ER) and transport to the Golgi complex, and the other class operates at the trans-Golgi network (TGN) specifying segregation into basolateral exocytic pathways. Both kinds of addressing motifs are quite diverse among different proteins. It is unclear to what extent this feature reflects alternative decoding mechanisms or variations in motifs recognized by the same sorting factor. Here we applied a novel strategy based on permeable peptide technology and temperature-sensitive model proteins to study competition between cytosolic sorting motifs in the context of mammalian living cells. We used the transduction domain of HIV-1 Tat protein to make a membrane-permeable peptide of the cytosolic tail of GtsO45, which contains a well characterized ER exit di-acidic (DIE) motif and a tyrosine-based basolateral sorting signal (YTDI). This peptide added to the media inhibited transport of GtsO45 from both ER-to-Golgi and TGN-to-basolateral cell surface in transfected Madin-Darby canine kidney cells. Instead, it did not affect the exocytic trafficking of a GtsO45-derived chimeric protein bearing 30 juxtamembrane residues from the cytosolic domain of the epidermal growth factor receptor that contains a variant ER exit motif (ERE) and an unconventional proline-based basolateral sorting signal. These results not only proved the feasibility of competing for sorting events in intact cells but also showed that distinct plasma membrane proteins can be discriminated at pre-TGN stages, and that basolateral sorting involves different recognition elements for tyrosine-based motifs and an unconventional basolateral motif.

Acidification and protein traffic

Acidification of some organelles, including the Golgi complex, lysosomes, secretory granules, and synaptic vesicles, is important for many of their biochemical functions. In addition, acidic pH in some compartments is also required for the efficient sorting and trafficking of proteins and lipids along the biosynthetic and endocytic pathways. Despite considerable study, however, our understanding of how pH modulates membrane traffic remains limited. In large part, this is due to the diversity of methods to perturb and monitor pH, as well as to the difficulties in isolating individual transport steps within the complex pathways of membrane traffic. This review summarizes old and recent evidence for the role of acidification at various steps of biosynthetic and endocytic transport in mammalian cells. We describe the mechanisms by which organelle pH is regulated and maintained, as well as how organelle pH is monitored and quantitated. General principles that emerge from these studies as well as future directions of interest are discussed.

Cholesterol depletion induces PKA-mediated basolateral-to-apical transcytosis of the scavenger receptor class B type I in MDCK cells

Cholesterol-based membrane microdomains, or lipid rafts, are believed to play important, yet poorly defined, roles in protein trafficking and signal transduction. In polarized epithelial cells, the current view is that rafts are involved in apical but not in basolateral protein transport from the trans-Golgi network (TGN). We report here that cholesterol is required in a post-TGN mechanism of basolateral regionalization. Permanently transfected Madin-Darby canine kidney cells segregated the caveolae/raft-associated high-density lipoprotein scavenger receptor class B type I (SR-BI) predominantly to the basolateral domain where it was constitutively internalized and recycled basolaterally. Acute cholesterol depletion did not significantly alter SR-BI internalization, implying a cholesterol depletion-insensitive endocytic process but instead induced its transcytosis through a protein kinase A (PKA)- and microtubule-dependent mechanism. Forskolin also elicited SR-BI transcytosis. The basolateral distribution of endogenous epidermal growth factor receptor remained unaffected. Strikingly, cholesterol depletion induced PKA activity without increasing the cAMP levels. Thus, our results are consistent with a scenario in which cholesterol-based rafts promote internalization and basolateral recycling of internalized SR-BI whereas a PKA pool sensitive to cholesterol depletion mediates SR-BI transcytosis. Regulated transcytosis of SR-BI may provide an additional mechanism to control cholesterol homeostasis. These results disclose relationships between cholesterol-based rafts and PKA activity operating in a post-TGN mechanism of regulated apical-to-basolateral cell surface protein distribution.

Transcytosis: crossing cellular barriers

Transcytosis, the vesicular transport of macromolecules from one side of a cell to the other, is a strategy used by multicellular organisms to selectively move material between two environments without altering the unique compositions of those environments. In this review, we summarize our knowledge of the different cell types using transcytosis in vivo, the variety of cargo moved, and the diverse pathways for delivering that cargo. We evaluate in vitro models that are currently being used to study transcytosis. Caveolae-mediated transcytosis by endothelial cells that line the microvasculature and carry circulating plasma proteins to the interstitium is explained in more detail, as is clathrin-mediated transcytosis of IgA by epithelial cells of the digestive tract. The molecular basis of vesicle traffic is discussed, with emphasis on the gaps and uncertainties in our understanding of the molecules and mechanisms that regulate transcytosis. In our view there is still much to be learned about this fundamental process.

Basolateral sorting of transforming growth factor-alpha precursor in polarized epithelial cells: characterization of cytoplasmic domain determinants

In polarized Madin-Darby canine kidney (MDCK) cells, newly synthesized transforming growth factor-alpha precursor (proTGFalpha) is directly sorted to the basolateral cell surface where it is sequentially cleaved and released into the basolateral conditioned medium (Dempsey, P.J., Coffey, R.J., J. Biol. Chem. 269 (1994) 16878-16889). In the present study, the role of the proTGFalpha cytoplasmic domain in basolateral sorting has been investigated using deletional and site-directed mutagenesis, as well as chimeric analyses of different TGFalpha constructs stably expressed in MDCK cells. The loss of polarized secretion of a proTGFalpha secretory mutant (TGFsec88) indicated that the proTGFalpha transmembrane and/or cytoplasmic domains contain essential basolateral sorting information. Using reporter chimeras with two apically sorted membrane proteins, p75 neurotrophin growth factor receptor and placental alkaline phosphatase, we show that the proTGFalpha cytoplasmic domain contains dominant basolateral sorting information. Analysis of proTGFalpha cytoplasmic domain truncation and internal deletion mutants, together with site-directed mutagenesis studies within the full-length proTGFalpha cytoplasmic domain, revealed redundant basolateral sorting motifs. Importantly, the C-terminal type I PDZ-binding motif was not required for basolateral sorting as determined by the integrity of basolateral sorting in deletion mutants lacking this motif. ProTGFalpha basolateral sorting may have important consequences for ligand presentation and spatial compartmentalization of epidermal growth factor receptor signaling networks in polarized epithelial cells.

Basolateral cycling mediated by a lumenal domain targeting determinant

All identified basolateral sorting signals of integral membrane proteins are cytoplasmically disposed, suggesting that basolateral targeting is mediated exclusively by direct interaction with vesicle coat components. Here, we report that GPP130, a cis-Golgi protein that undergoes endosome-to-Golgi retrieval using the late endosome-bypass pathway in nonpolarized cells, cycles via the basolateral membrane in polarized MDCK cells. Significantly, the membrane-proximal lumenal domain of GPP130, which mediates GPP130 localization and trafficking in nonpolarized cells, was both necessary and sufficient for basolateral cycling in MDCK cells. The use of lumenal determinants for both basolateral cycling and endosome-to-Golgi retrieval suggests that a novel receptor-mediated mechanism operates at both the trans-Golgi network and distal sites to sort GPP130 along the late-endosome-bypass retrieval pathway in polarized cells.

Localization of human equilibrative nucleoside transporters, hENT1 and hENT2, in renal epithelial cells

Nucleoside transporters are important in the disposition of nucleosides and nucleoside analogs in the kidney. Two human equilibrative nucleoside transporters have been cloned and characterized, hENT1 and hENT2. The primary goal of this study was to localize these transporters in polarized renal epithelia. hENT1 and hENT2 were tagged with green fluorescence protein, stably expressed in renal epithelial cells, and localized by immunofluorescence and functional analysis. Our data demonstrated that both transporters are expressed on the basolateral membrane. hENT1 is also present on the apical membrane. Additionally, we examined the importance to basolateral targeting of two COOH-terminal targeting motifs: a RXXV motif for hENT1 and a dileucine repeat for hENT2. Neither motif appeared to affect targeting, but the dileucine repeat was implicated in surface expression of hENT2. In addition, a splice variant of hENT2 was identified that is predicted to result in a 156-residue COOH-terminal truncation. This variant had a tissue distribution similar to wild-type hENT2 but was retained intracellularly. These data suggest that hENT1 and hENT2 on the basolateral membrane function with concentrative nucleoside transporters on the apical membrane to mediate active reabsorption of nucleosides within the kidney.

Characterization of the human sulfate anion transporter (hsat-1) protein and gene (SAT1; SLC26A1)

Sulfate plays an essential role during growth, development, bone/cartilage formation, and cellular metabolism. In this study, we have isolated the human sulfate anion transporter cDNA (hsat-1; SCL26A1) and gene (SAT1), determined its protein function in Xenopus oocytes and characterized SAT1 promoter activity in mammalian renal cell lines. hsat-1 encodes a protein of 75 kDa, with 12 putative transmembrane domains, that induces sulfate, chloride, and oxalate transport in Xenopus oocytes. hsat-1 mRNA is expressed most abundantly in the kidney and liver, with lower levels in the pancreas, testis, brain, small intestine, colon, and lung. The SAT1 gene is comprised of four exons stretching 6 kb in length, with an alternative splice site formed from an optional exon. SAT1 5' flanking region led to promoter activity in renal OK and LLC-PK1 cells. Using SAT1 5' flanking region truncations, the first 135 bp was shown to be sufficient for basal promoter activity. Mutation of the activator protein-1 (AP-1) site at position -52 in the SAT1 promoter led to loss of transcriptional activity, suggesting its requirement for SAT1 basal expression. This study represents the first functional characterization of the human SAT1 gene and protein encoded by the anion transporter hsat-1.

The mouse sulfate anion transporter gene Sat1 (Slc26a1): cloning, tissue distribution, gene structure, functional characterization, and transcriptional regulation thyroid hormone

Sulfate (SO(4)(2-)) is required for bone/cartilage formation and cellular metabolism. sat-1 is a SO(4)(2-) anion transporter expressed on basolateral membranes of renal proximal tubules, and is suggested to play an important role in maintaining SO(4)(2-) homeostasis. As a first step towards studying its tissue-specific expression, hormonal regulation, and in preparation for the generation of knockout mice, we have cloned and characterized the mouse sat-1 cDNA (msat-1), gene (sat1; Slc26a1) and promoter region. msat-1 encodes a 704 amino acid protein (75.4 kDa) with 12 putative transmembrane domains that induce SO(4)(2-) (also oxalate and chloride) transport in Xenopus oocytes. msat-1 mRNA was expressed in kidney, liver, cecum, calvaria, brain, heart, and skeletal muscle. Two distinct transcripts were expressed in kidney and liver due to alternative utilization of the first intron, corresponding to an internal portion of the 5'-untranslated region. The Sat1 gene (~6 kb) consists of 4 exons. Its promoter is ~52% G + C rich and contains a number of well-characterized cis-acting elements, including sequences resembling hormone responsive elements T(3)REs and VDREs. We demonstrate that Sat1 promoter driven basal transcription in OK cells was stimulated by tri-iodothyronine. Site-directed mutagenesis identified an imperfect T(3)RE at -454-bp in the Sat1 promoter to be responsible for this activity. This study represents the first characterization of the structure and regulation of the Sat1 gene encoding a SO(4)(2-)/chloride/oxalate anion transporter.

Functional involvement of VAMP/synaptobrevin-2 in cAMP-stimulated aquaporin 2 translocation in renal collecting duct cells

The involvement of soluble N-ethylmaleimide sensitive factor-attachment protein receptor (SNARE) proteins in the cAMP-induced exocytosis of aquaporin 2 (AQP2)-containing vesicles was investigated in AQP2-transfected renal CD8 cells. RT-PCR and western blot analysis confirmed the presence of the SNARE homologs VAMP/synaptobrevin-2, syntaxin-1, syntaxin-4 and SNAP-23 in CD8 cells. Tetanus neurotoxin (TeNT) was efficient in cleaving synaptobrevin-like protein both in vitro and in intact CD8 cells incubated with the toxin. TeNT treatment in intact CD8 cells completely abolished cAMP-stimulated AQP2 targeting to the plasma membrane, as assessed by quantification of cell-surface immunoreactivity to an anti-AQP2 antibody raised against a peptide reproducing the extracellular AQP2 C-loop. These results represent the first evidence for the functional involvement of VAMP-2 in cAMP-induced AQP2 exocytosis in renal cells.

Differential recognition of tyrosine-based basolateral signals by AP-1B subunit mu1B in polarized epithelial cells

To investigate the importance of tyrosine recognition by the AP-1B clathrin adaptor subunit mu1B for basolateral sorting of integral membrane proteins in polarized epithelial cells, we have produced and characterized a mutant form of mu1B. The mutant (M-mu1B) contains alanine substitutions of each of the four conserved residues, which in the AP-2 adaptor subunit micro2 are critical for interacting with tyrosine-based endocytosis signals. We show M-mu1B is defective for tyrosine binding in vitro, but is nevertheless incorporated into AP-1 complexes in transfected cells. Using LLC-PK1 cells expressing either wild type or M-mu1B, we find that there is inefficient basolateral expression of membrane proteins whose basolateral targeting signals share critical tyrosines with signals for endocytosis. In contrast, membrane proteins whose basolateral targeting signals are distinct from their endocytosis signals (transferrin and low-density lipoprotein receptors) accumulate at the basolateral domain normally, although in a manner that is strictly dependent on mu1B or M-mu1B expression. Our results suggest that mu1B interacts with different classes of basolateral targeting signals in distinct ways.

Functional characterization of three novel tissue-specific anion exchangers SLC26A7, -A8, and -A9

A second distinct family of anion exchangers, SLC26, in addition to the classical SLC4 (or anion exchanger) family, has recently been delineated. Particular interest in this gene family is stimulated by the fact that the SLC26A2, SLC26A3, and SLC26A4 genes have been recognized as the disease genes mutated in diastrophic dysplasia, congenital chloride diarrhea, and Pendred syndrome, respectively. We report the expansion of the SLC26 gene family by characterizing three novel tissue-specific members, named SLC26A7, SLC26A8, and SLC26A9, on chromosomes 8, 6, and 1, respectively. The SLC26A7-A9 proteins are structurally very similar at the amino acid level to the previous family members and show tissue-specific expression in kidney, testis, and lung, respectively. More detailed characterization by immunohistochemistry and/or in situ hybridization localized SLC26A7 to distal segments of nephrons, SLC26A8 to developing spermatocytes, and SLC26A9 to the lumenal side of the bronchiolar and alveolar epithelium of lung. Expression of SLC26A7-A9 proteins in Xenopus oocytes demonstrated chloride, sulfate, and oxalate transport activity, suggesting that they encode functional anion exchangers. The functional characterization of the novel tissue-specific members may provide new insights to anion transport physiology in different parts of body.

Absence of direct delivery for single transmembrane apical proteins or their "Secretory" forms in polarized hepatic cells

The absence of a direct route to the apical plasma membrane (PM) for single transmembrane domain (TMD) proteins in polarized hepatic cells has been inferred but never directly demonstrated. The genes encoding three pairs of apical PM proteins, whose extracellular domains are targeted exclusively to the apical milieu in Madin-Darby canine kidney cells, were packaged into recombinant adenovirus and delivered to WIF-B cells in vitro and liver hepatocytes in vivo. By immunofluorescence and pulse-chase metabolic labeling, we found that the soluble constructs were overwhelmingly secreted into the basolateral milieu, which in vivo is the blood and in vitro is the culture medium. The full-length proteins were first delivered to the basolateral surface but then concentrated in the apical PM. Our results imply that hepatic cells lack trans-Golgi network (TGN)-based machinery for directly sorting single transmembrane domain apical proteins and raise interesting questions about current models of PM protein sorting in polarized and nonpolarized cells.

Characterization of a di-leucine-based signal in the cytoplasmic tail of the nucleotide-pyrophosphatase NPP1 that mediates basolateral targeting but not endocytosis

Enzymes of the nucleotide pyrophosphatase/phosphodiesterase (NPPase) family are expressed at opposite surfaces in polarized epithelial cells. We investigated the targeting signal of NPP1, which is exclusively expressed at the basolateral surface. Full-length NPP1 and different constructs and mutants were transfected into the polarized MDCK cell line. Expression of the proteins was analyzed by confocal microscopy and surface biotinylation. The basolateral signal of NPP1 was identified as a di-leucine motif located in the cytoplasmic tail. Mutation of either or both leucines largely redirected NPP1 to the apical surface. Furthermore, addition of the conserved sequence AAASLLAP redirected the apical nucleotide pyrophosphatase/phosphodiesterase NPP3 to the basolateral surface. Full-length NPP1 was not significantly internalized. However, when the cytoplasmic tail was deleted upstream the di-leucine motif or when the six upstream flanking amino acids were deleted, the protein was mainly found intracellularly. Endocytosis experiments indicated that these mutants were endocytosed from the basolateral surface. These results identify the basolateral signal of NPP1 as a short sequence including a di-leucine motif that is dominant over apical determinants and point to the importance of surrounding amino acids in determining whether the signal will function as a basolateral signal only or as an endocytotic signal as well.

The proamphiregulin cytoplasmic domain is required for basolateral sorting, but is not essential for constitutive or stimulus-induced processing in polarized Madin-Darby canine kidney cells

In this study, the role of the amphiregulin precursor (pro-AR) cytoplasmic domain in the basolateral sorting and cell-surface processing of pro-AR in polarized epithelial cells has been investigated using Madin-Darby canine kidney cells stably expressing various human pro-AR forms. Our results demonstrate that newly synthesized wild-type pro-AR (50 kDa) is delivered directly to the basolateral membrane domain with >95% efficiency, where it is sequentially cleaved within the ectodomain to release several soluble amphiregulin (AR) forms. Analyses of a pro-AR cytoplasmic domain truncation mutant (ARTL27) and two pro-AR secretory mutants (ARsec184 and ARsec190) indicated that the pro-AR cytoplasmic domain is not required for efficient delivery to the plasma membrane, but does contain essential basolateral sorting information. We show that the pro-AR cytoplasmic domain truncation mutant (ARTL27) is not sorted in polarized Madin-Darby canine kidney cells, with approximately 65% of the newly synthesized protein delivered to the apical cell surface. Under base-line conditions, ARTL27 was preferentially cleaved from the basolateral surface with 4-fold greater efficiency compared with cleavage from the apical membrane domain. However, ARTL27 ectodomain cleavage could be stimulated equivalently from either membrane domain by a variety of different stimuli. The metalloprotease inhibitor BB-94 could inhibit both base-line and stimulus-induced ectodomain cleavage of wild-type pro-AR and ARTL27. These results indicate that the pro-AR cytoplasmic domain is required for basolateral sorting, but is not essential for ectodomain processing. Preferential constitutive cleavage of ARTL27 from the basolateral cell surface also suggests that the metalloprotease activity involved in base-line and stimulus-induced ARTL27 ectodomain cleavage may be regulated differently in the apical and basolateral membrane domains of polarized epithelial cells.

Function and spatial distribution of ion channels and transporters in cell migration

Cell migration plays a central role in many physiological and pathophysiological processes, such as embryogenesis, immune defense, wound healing, or the formation of tumor metastases. Detailed models have been developed that describe cytoskeletal mechanisms of cell migration. However, evidence is emerging that ion channels and transporters also play an important role in cell migration. The purpose of this review is to examine the function and subcellular distribution of ion channels and transporters in cell migration. Topics covered will be a brief overview of cytoskeletal mechanisms of migration, the role of ion channels and transporters involved in cell migration, and ways by which a polarized distribution of ion channels and transporters can be achieved in migrating cells. Moreover, a model is proposed that combines ion transport with cytoskeletal mechanisms of migration.

The rat liver Na(+)/bile acid cotransporter. Importance of the cytoplasmic tail to function and plasma membrane targeting

To understand the potential functions of the cytoplasmic tail of Na(+)/taurocholate cotransporter (Ntcp) and to determine the basolateral sorting mechanisms for this transporter, green fluorescent protein-fused wild type and mutant rat Ntcps were constructed and the transport properties and cellular localization were assessed in transfected COS 7 and Madin-Darby canine kidney (MDCK) cells. Truncation of the 56-amino acid cytoplasmic tail demonstrates that the cytoplasmic tail of rat Ntcp is involved membrane delivery of this protein in nonpolarized and polarized cells and removal of the tail does not affect the bile acid transport function of Ntcp. Using site-directed mutagenesis, two tyrosine residues, Tyr-321 and Tyr-307, in the cytoplasmic tail of Ntcp have been identified as important for the basolateral sorting of rat Ntcp in transfected MDCK cells. Tyr-321 appears to be the major basolateral-sorting determinant, and Tyr-307 acts as a supporting determinant to ensure delivery of the transporter to the basolateral surface, especially at high levels of protein expression. When the two Tyr-based basolateral sorting motifs have been removed, the N-linked carbohydrate groups direct the tyrosine to alanine mutants to the apical surface of transfected MDCK cells. The major basolateral sorting determinant Tyr-321 is within a novel beta-turn unfavorable tetrapeptide Y(321)KAA, which has not been found in any naturally occurring basolateral sorting motifs. Two-dimensional NMR spectroscopy of a 24-mer peptide corresponding to the sequence from Tyr-307 to Thr-330 on the cytoplasmic tail of Ntcp confirms that both the Tyr-321 and Tyr-307 regions do not adopt any turn structure. Since the major motif YKAA contains a beta-turn unfavorable structure, the Ntcp basolateral sorting may not be related to the clathrin-adaptor complex pathway, as is the case for many basolateral proteins.

Two conserved amino acid motifs mediate protein targeting to the micronemes of the apicomplexan parasite Toxoplasma gondii

The micronemal protein 2 (MIC2) of Toxoplasma gondii shares sequence and structural similarities with a series of adhesive molecules of different apicomplexan parasites. These molecules accumulate, through a yet unknown mechanism, in secretory vesicles (micronemes), which together with tubular and membrane structures form the locomotion and invasion machinery of apicomplexan parasites. Our findings indicated that two conserved motifs placed within the cytoplasmic domain of MIC2 are both necessary and sufficient for targeting proteins to T. gondii micronemes. The first motif is based around the amino acid sequence SYHYY. Database analysis revealed that a similar sequence is present in the cytoplasmic tail of all transmembrane micronemal proteins identified so far in different apicomplexan species. The second signal consists of a stretch of acidic residues, EIEYE. The creation of an artificial tail containing only the two motifs SYHYY and EIEYE in a preserved spacing configuration is sufficient to target the surface protein SAG1 to the micronemes of T. gondii. These findings shed new light on the molecular mechanisms that control the formation of the microneme content and the functional relationship that links these organelles with the endoplasmic reticulum of the parasite.

Temperature-responsive culture dishes allow nonenzymatic harvest of differentiated Madin-Darby canine kidney (MDCK) cell sheets

We have developed a temperature-responsive culture dish grafted with a poly(N-isopropylacrylamide) (PIPAAm). Various types of cells adhere, spread, and proliferate on the grafted dishes in the presence of serum at 37 degrees C. By reducing only temperature, these cells can be harvested noninvasively from the dishes according to rapid hydration of the grafted polymer. Because the harvest does not need enzymatic digestion, differentiated cell phenotypes are retained. In the present study, a renal epithelial cell line, Madin-Darby canine kidney (MDCK) cell, was cultured on the dishes, and cell behavior was examined. MDCK cells showed differentiated phenotypes such as dome formation during long-term culture, similar to on ungrafted dishes. After 1-week culture at 37 degrees C, trypsin digestion disrupted cell-cell junctions but failed to liberate cells from both ungrafted and grafted dishes. However, short-term incubation at 20 degrees C released confluent MDCK cells as a single contiguous cell sheet only from the polymer-grafted dishes because of selective disruption of the cell-surface binding. Immunocytochemistry with anti-beta-catenin antibody revealed that functional cell-cell junctions were organized even in the recovered cell sheets. Intriguingly, incubation time at 20 degrees C required for cell sheet detachment gradually shortened during long-term culture before reducing temperature. The acceleration of cell detachment was correlated to the decrease of a single cell area by means of cell contractile force. These findings suggest that cell sheet detachment from PIPAAm-grafted dishes should be accomplished by both PIPAAm hydration and cellular metabolic activity such as cell contraction.

Membrane traffic in polarized epithelial cells

Epithelial cells contain apical and basolateral surfaces with distinct compositions. Sorting of certain proteins to the basolateral surface involves the epithelial-specific mu 1b clathrin adaptor subunit. Recent results have shown that targeting to the basolateral surface utilizes the exocyst, whereas traffic to the apical surface uses syntaxin 3. Endocytosis at the apical surface is regulated by ARF6. Transcytosis of IgA is regulated by the p62Yes tyrosine kinase.

Membrane trafficking and processing in Paramecium

Cellular membranes are made in a cell's biosynthetic pathway and are composed of similar biochemical constituents. Nevertheless, they become differentiated as membrane components are sorted into different membrane-limited compartments. We summarize the morphological and immunological similarities and differences seen in the membranes of the various interacting compartments in the single-celled organism, Paramecium. Besides the biosynthetic pathway, membranes of the regulated secretory pathway, endocytic pathway, and phagocytic pathway are highlighted. Paramecium is a multipolarized cell in the sense that several different pools of membrane-limited compartments are targeted for exocytosis at very specific sites at the cell surface. Thus, the method used by this cell to sort and package its membrane subunits into different compartments, the processes used to transport these compartments to specific locations at the plasma membrane and to other intracellular fusion sites, the processes of membrane retrieval, and the processes of membrane docking and fusion are reviewed. Paramecium has provided an excellent model for studying the complexities of membrane trafficking in one cell using both morphological and immunocytochemical techniques. This cell also promises to be a useful model for studying aspects of the molecular biology of membrane sorting, retrieval, transport, and fusion.

Characterization of a phosphoinositide-mediated odor transduction pathway reveals plasma membrane localization of an inositol 1,4, 5-trisphosphate receptor in lobster olfactory receptor neurons

The role of phosphoinositide signaling in olfactory transduction is still being resolved. Compelling functional evidence for the transduction of odor signals via phosphoinositide pathways in olfactory transduction comes from invertebrate olfactory systems, in particular lobster olfactory receptor neurons. We now provide molecular evidence for two components of the phosphoinositide signaling pathway in lobster olfactory receptor neurons, a G protein alpha subunit of the G(q) family and an inositol 1,4, 5-trisphosphate-gated channel or an inositol 1,4,5-trisphosphate (IP(3)) receptor. Both proteins localize to the site of olfactory transduction, the outer dendrite of the olfactory receptor neurons. Furthermore, the IP(3) receptor localizes to membranes in the ciliary transduction compartment of these cells at both the light microscopic and electron microscopic levels. Given the absence of intracellular organelles in the sub-micron diameter olfactory cilia, this finding indicates that the IP(3) receptor is associated with the plasma membrane and provides the first definitive evidence for plasma membrane localization of an IP(3)R in neurons. The association of the IP(3) receptor with the plasma membrane may be a novel mechanism for regulating intracellular cations in restricted cellular compartments of neurons.

Interactions between the exocytic and endocytic pathways in polarized Madin-Darby canine kidney cells

The compartments involved in polarized exocytosis of membrane proteins are not well defined. In this study we hypothesized that newly synthesized polymeric immunoglobulin receptors are targeted from the trans-Golgi network to endosomes prior to their appearance on the basolateral cell surface of polarized Madin-Darby canine kidney cells. To examine this hypothesis, we have used an assay designed to measure the meeting of newly synthesized receptors with a selective population of apical or basolateral endosomes loaded with horseradish peroxidase. We found that in the course of basolateral exocytosis, the wild-type polymeric immunoglobulin receptor is targeted from the trans-Golgi network to apical and basolateral endosomes. Phosphorylation of a Ser residue in the cytoplasmic tail of the receptor is implicated in this process. The biosynthetic pathway of apically sorted polymeric immunoglobulin receptor mutants similarly traversed apical endosomes, raising the possibility that apical receptors are segregated from basolateral receptors in apical endosomes. The post-endocytic pathway of transcytosing and recycling receptors also passed through apical endosomes. Together, these observations are consistent with the possibility that the biosynthetic and endocytic routes merge into endosomes and justify a model suggesting that endosomal recycling processes govern polarized trafficking of proteins traveling in both pathways.

Polarized distribution of glycine transporter isoforms in epithelial and neuronal cells

Asymmetrical distribution of Na(+)- and Cl(-)-dependent neurotransmitter transporters on the cell surface of polarized cells seems to be a generalized feature in this gene family. In the present study we analyzed the subcellular distribution of the various isoforms of the glycine transporters GLYT1 and GLYT2 after heterologous expression in polarized MDCK cells and in hippocampal neurons. Our results indicate that glycine transporters are asymmetrically distributed in an isoform- and cell-type-specific manner. GLYT1b is localized in the basolateral and somatodendritic domains of MDCK cells and neurons, respectively. However, GLYT1a is somatodendritic in neurons but is predominantly expressed in the apical surface of MDCK cells. The two isoforms of GLYT2 (GLYT2a and GLYT2b) are found at the apical surface in epithelial cells but are uniformly distributed in neurons. By using site-directed mutagenesis we have been able to identify signals for basolateral/somatodendritic localization in the amino-terminal region of GLYT1 and in two dileucine motifs located in the carboxyl tail of this protein. These results contribute to defining the mechanisms of asymmetrical distribution of transporters on the cell surface of polarized cells.

Syntaxin 2 splice variants exhibit differential expression patterns, biochemical properties and subcellular localizations

The syntaxins are a large protein family implicated in the targeting and fusion of intracellular transport vesicles. A subset of proteins of this family are the four syntaxin 2 splice variants, syntaxins 2A (2), 2B (2′), 2C (2″) and 2D. Each syntaxin 2 variant contains an identical, or nearly identical, amino-terminal cytoplasmic domain followed by a distinct hydrophobic (syntaxins 2A and 2B) or hydrophilic (syntaxins 2C and 2D) carboxyl-terminal domain. To investigate whether the difference among the syntaxin 2 variants is functionally important, we have examined comparatively their RNA transcript and protein expression patterns, membrane associations, protein-protein interactions and intracellular localizations. Analysis of the RNA transcript and protein expression patterns demonstrated that syntaxins 2A, 2B and 2C are broadly, but not uniformly, expressed while syntaxin 2D expression is restricted to the brain. Subcellular fractionation studies showed that syntaxins 2A and 2B behave as integral membrane proteins while syntaxin 2C is only partially associated with membranes. In vitro biochemical assays demonstrated that the syntaxin 2 variants exhibit similar yet distinct interactions with other proteins implicated in vesicular trafficking, including SNAP-25, SNAP-23, VAMP-2 and n-sec1. In a variety of nonpolarized cell types, syntaxins 2A and 2B localized to both the plasma membrane and endosomal membranes. However, in two polarized epithelial cell lines, MDCK and Caco-2, syntaxin 2A localized predominantly to the apical plasma membrane while syntaxin 2B was associated with both the apical and the basolateral membranes. These observations indicate that the distinct carboxyl-terminal domains of the syntaxin 2 variants influence their biochemical and localization properties and may therefore confer upon these variants different functional roles in the regulation of intracellular membrane trafficking.

A PDZ-interacting domain in CFTR is an apical membrane polarization signal

Polarization of the cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-activated chloride channel, to the apical plasma membrane of epithelial cells is critical for vectorial transport of chloride in a variety of epithelia, including the airway, pancreas, intestine, and kidney. However, the motifs that localize CFTR to the apical membrane are unknown. We report that the last 3 amino acids in the COOH-terminus of CFTR (T-R-L) comprise a PDZ-interacting domain that is required for the polarization of CFTR to the apical plasma membrane in human airway and kidney epithelial cells. In addition, the CFTR mutant, S1455X, which lacks the 26 COOH-terminal amino acids, including the PDZ-interacting domain, is mispolarized to the lateral membrane. We also demonstrate that CFTR binds to ezrin-radixin-moesin-binding phosphoprotein 50 (EBP50), an apical membrane PDZ domain-containing protein. We propose that COOH-terminal deletions of CFTR, which represent about 10% of CFTR mutations, result in defective vectorial chloride transport, partly by altering the polarized distribution of CFTR in epithelial cells. Moreover, our data demonstrate that PDZ-interacting domains and PDZ domain-containing proteins play a key role in the apical polarization of ion channels in epithelial cells.

Overexpression of proteins containing tyrosine- or leucine-based sorting signals affects transferrin receptor trafficking

Targeting of many transmembrane proteins to post-Golgi compartments is dependent on cytoplasmically exposed sorting signals. The most widely used signals conform to the tyrosine- or the leucine-based motifs. Both types of signals have been implicated in protein localization to the same intracellular compartments, but previous results from both cell-free experiments and studies of transfected cell lines have indicated that the two types of signals interact with separate components of the sorting machinery. We have overexpressed several transmembrane proteins in stably transfected Madin-Darby canine kidney cells using an inducible promoter system. Overexpression of proteins containing tyrosine- or leucine-based sorting signals resulted in reduced internalization of the transferrin receptor, whereas recycling and polarized distribution was not influenced. Our results indicate that proteins with tyrosine- and leucine-based sorting signals can be transported along common saturable pathways.

Interactions of the AP-1 Golgi adaptor with the polymeric immunoglobulin receptor and their possible role in mediating brefeldin A-sensitive basolateral targeting from the trans-Golgi network

We provide morphological, biochemical, and functional evidence suggesting that the AP-1 clathrin adaptor complex of the trans-Golgi network interacts with the polymeric immunoglobulin receptor in transfected Madin-Darby canine kidney cells. Our results indicate that immunofluorescently labeled gamma-adaptin subunit of the adaptor complex and the polymeric immunoglobulin receptor partially co-localize in polarized and semi-polarized cells. gamma-Adaptin is co-immunoisolated with membranes expressing the wild-type receptor. The entire AP-1 adaptor complex could be chemically cross-linked to the receptor in filter-grown cells. gamma-Adaptin could be co-immunoprecipitated with the wild-type receptor, with reduced efficiency with receptor mutant whose basolateral sorting motif has been deleted, and not with receptor lacking its cytoplasmic tail. Co-immunoprecipitation of gamma-adaptin was inhibited by brefeldin A. Mutation of cytoplasmic serine 726 inhibited receptor interactions with AP-1 but did not abrogate the fidelity of its basolateral targeting from the trans-Golgi network. However, the kinetics of receptor delivery to the basolateral cell surface were slowed by the mutation. Although surface delivery of the wild-type receptor was inhibited by brefeldin A, the delivery of the mutant receptor was insensitive to the drug. Our results are consistent with a working model in which phosphorylated cytoplasmic serine modulates the recruitment of the polymeric immunoglobulin receptor into AP-1/clathrin-coated areas in the trans-Golgi network. This process may regulate the efficiency of receptor targeting from the trans-Golgi network.