SNX27 regulates DRA activity and mediates its direct recycling by PDZ-interaction in early endosomes at the apical pole of Caco2 cells

DRA (downregulated in adenoma, SLC26A3) and NHE3 (Na+/H+ exchanger 3, SLC9A3) together mediate intestinal electroneutral NaCl absorption. Both transporters contain PDZ (postsynaptic density 95, disc large, zonula occludens 1) binding motifs and interact with PDZ adaptor proteins regulating their activity and recycling. SNX27 (sorting nexin 27) contains a PDZ domain and is involved in the recycling of cargo proteins including NHE3. The interaction of SNX27 with DRA and its potential role for the activity and recycling of DRA have been evaluated in this study. SNX27 specifically interacts with DRA via its PDZ domain. The knockdown (KD) of SNX27 reduced DRA activity by 50% but was not accompanied by a decrease of DRA surface expression. This indicates that DRA is trafficked to specific functional domains in the plasma membrane in which DRA is particularly active. Consistently, the disruption of lipid raft integrity by methyl-β-cyclodextrin has an inhibitory effect on DRA activity that was strongly reduced after SNX27 KD. In differentiated intestinal Caco2 cells, superresolution microscopy and a novel quantitative axial approach revealed that DRA and SNX27 colocalize in rab5-positive early endosomes at the apical pole. SNX27 regulates the activity of DRA in the apical plasma membrane through binding with its PDZ domain. This interaction occurs in rab5-positive early endosomes at the apical pole of differentiated intestinal Caco2 cells. SNX27 is involved in the direct recycling of DRA to the plasma membrane where it is inserted into lipid rafts facilitating increased activity. NEW & NOTEWORTHY SNX27 has a PDZ domain and is involved in the regulation and recycling of transmembrane proteins. The role of SNX27 on the activity and recycling of the intestinal Cl−/[Formula: see text] exchanger DRA has not yet been studied. This study shows that SNX27 directly interacts with DRA in early endosomes at the apical pole of intestinal Caco2 cells and mediates its direct recycling to facilitate high activity in lipid rafts in the apical plasma membrane.

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Several adaptor proteins promote intracellular localisation of the transporter MRP4/ABCC4 in platelets and haematopoietic cells

Thrombosis and Haemostasis ◽

10.1160/th16-01-0045 ◽

2017 ◽

Vol 117 (01) ◽

pp. 105-115 ◽

Cited By ~ 6

Author(s):

Yvonne Schaletzki ◽

Marie-Luise Kromrey ◽

Susanne Bröderdorf ◽

Elke Hammer ◽

Markus Grube ◽

...

Keyword(s):

Plasma Membrane ◽

Protein Interactions ◽

Pdz Domain ◽

Adaptor Protein ◽

Adaptor Proteins ◽

Apical Plasma Membrane ◽

Dense Granules ◽

Haematopoietic Cells ◽

And Function

SummaryThe multidrug resistance protein 4 (MRP4/ABCC4) has been identified as an important transporter for signalling molecules including cyclic nucleotides and several lipid mediators in platelets and may thus represent a novel target to interfere with platelet function. Besides its localisation in the plasma membrane, MRP4 has been also detected in the membrane of dense granules in resting platelets. In polarised cells it is localised at the basolateral or apical plasma membrane. To date, the mechanism of MRP4 trafficking has not been elucidated; protein interactions may regulate both the localisation and function of this transporter. We approached this issue by searching for interacting proteins by in vitro binding assays, followed by immunoblotting and mass spectrometry, and by visualising their co-localisation in platelets and haematopoietic cells. We identified the PDZ domain containing scaffold proteins ezrin-binding protein 50 (EBP50/NHERF1), postsynaptic density protein 95 (PSD95), and sorting nexin 27 (SNX27), but also the adaptor protein complex 3 subunit β3A (AP3B1) and the heat shock protein HSP90 as putative interaction partners of MRP4. The knockdown of SNX27, PSD95, and AP3B1 by siRNA in megakaryoblastic leuk aemia cells led to a redistribution of MRP4 from intracellular structures to the plasma membrane. Inhibition of HSP90 led to a diminished expression and retention of MRP4 in the endoplasmic reticulum. These results indicate that MRP4 localisation and function are regulated by multiple protein interactions. Changes in the adaptor proteins can hence lead to altered localisation and function of the transporter.Supplementary Material to this article is available at www.thrombosis-online.com.

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Rab7 involves Vps35 to mediate AQP2 sorting and apical trafficking in collecting duct cells

AJP Renal Physiology ◽

10.1152/ajprenal.00297.2019 ◽

2020 ◽

Vol 318 (4) ◽

pp. F956-F970 ◽

Cited By ~ 4

Author(s):

Wei-Ling Wang ◽

Shih-Han Su ◽

Kit Yee Wong ◽

Chan-Wei Yang ◽

Chin-Fu Liu ◽

...

Keyword(s):

Plasma Membrane ◽

Apical Membrane ◽

Collecting Duct ◽

Water Channel ◽

Small Gtpase ◽

Apical Plasma Membrane ◽

Water Reabsorption ◽

Recycling Endosomes ◽

Early Endosomes ◽

Vacuolar Protein

Aquaporin-2 (AQP2) is a vasopressin-regulated water channel protein responsible for osmotic water reabsorption by kidney collecting ducts. In response to vasopressin, AQP2 traffics from intracellular vesicles to the apical plasma membrane of collecting duct principal cells, where it increases water permeability and, hence, water reabsorption. Despite continuing efforts, gaps remain in our knowledge of vasopressin-regulated AQP2 trafficking. Here, we studied the functions of two retromer complex proteins, small GTPase Rab7 and vacuolar protein sorting 35 (Vps35), in vasopressin-induced AQP2 trafficking in a collecting duct cell model (mpkCCD cells). We showed that upon vasopressin removal, apical AQP2 returned to Rab5-positive early endosomes before joining Rab11-positive recycling endosomes. In response to vasopressin, Rab11-associated AQP2 trafficked to the apical plasma membrane before Rab5-associated AQP2 did so. Rab7 knockdown resulted in AQP2 accumulation in early endosomes and impaired vasopressin-induced apical AQP2 trafficking. In response to vasopressin, Rab7 transiently colocalized with Rab5, indicative of a role of Rab7 in AQP2 sorting in early endosomes before trafficking to the apical membrane. Rab7-mediated apical AQP2 trafficking in response to vasopressin required GTPase activity. When Vps35 was knocked down, AQP2 accumulated in recycling endosomes under vehicle conditions and did not traffic to the apical plasma membrane in response to vasopressin. We conclude that Rab7 and Vps35 participate in AQP2 sorting in early endosomes under vehicle conditions and apical membrane trafficking in response to vasopressin.

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Aldosterone induces rapid apical translocation of ENaC in early portion of renal collecting system: possible role of SGK

AJP Renal Physiology ◽

10.1152/ajprenal.2001.280.4.f675 ◽

2001 ◽

Vol 280 (4) ◽

pp. F675-F682 ◽

Cited By ~ 239

Author(s):

Johannes Loffing ◽

Marija Zecevic ◽

Eric Féraille ◽

Brigitte Kaissling ◽

Carol Asher ◽

...

Keyword(s):

Plasma Membrane ◽

Surface Expression ◽

Apical Plasma Membrane ◽

Sodium Reabsorption ◽

Xenopus Laevis Oocytes ◽

Subunit Expression ◽

Early Portion ◽

Potassium Secretion ◽

Early Effects

Aldosterone controls sodium reabsorption and potassium secretion in the aldosterone-sensitive distal nephron (ASDN). Although clearance measurements have shown that aldosterone induces these transports within 30–60 min, no early effects have been demonstrated in vivo at the level of the apical epithelial sodium channel (ENaC), the main effector of this regulation. Here we show by real-time RT-PCR and immunofluorescence that an aldosterone injection in adrenalectomized rats induces α-ENaC subunit expression along the entire ASDN within 2 h, whereas β- and γ-ENaC are constitutively expressed. In the proximal ASDN portions only, ENaC is shifted toward the apical cellular pole and the apical plasma membrane within 2 and 4 h, respectively. To address the question of whether the early aldosterone-induced serum and glucocorticoid-regulated kinase (SGK) might mediate this apical shift of ENaC, we analyzed SGK induction in vivo. Two hours after aldosterone, SGK was highly induced in all segment-specific cells of the ASDN, and its level decreased thereafter. In Xenopus laevis oocytes, SGK induced ENaC activation and surface expression by a kinase activity-dependent mechanism. In conclusion, the rapid in vivo accumulation of SGK and α-ENaC after aldosterone injection takes place along the entire ASDN, whereas the translocation of α,β,γ-ENaC to the apical plasma membrane is restricted to its proximal portions. Results from oocyte experiments suggest the hypothesis that a localized activation of SGK may play a role in the mediation of ENaC translocation.

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TLR4 and CD14 trafficking and its influence on LPS-induced pro-inflammatory signaling

Cellular and Molecular Life Sciences ◽

10.1007/s00018-020-03656-y ◽

2020 ◽

Author(s):

Anna Ciesielska ◽

Marta Matyjek ◽

Katarzyna Kwiatkowska

Keyword(s):

Plasma Membrane ◽

Inflammatory Responses ◽

Adaptor Proteins ◽

Human Diseases ◽

Recycling Endosomes ◽

Lysosomal Compartment ◽

E Coli ◽

Early Endosomes ◽

Endosome Maturation

Abstract Toll-like receptor (TLR) 4 belongs to the TLR family of receptors inducing pro-inflammatory responses to invading pathogens. TLR4 is activated by lipopolysaccharide (LPS, endotoxin) of Gram-negative bacteria and sequentially triggers two signaling cascades: the first one involving TIRAP and MyD88 adaptor proteins is induced in the plasma membrane, whereas the second engaging adaptor proteins TRAM and TRIF begins in early endosomes after endocytosis of the receptor. The LPS-induced internalization of TLR4 and hence also the activation of the TRIF-dependent pathway is governed by a GPI-anchored protein, CD14. The endocytosis of TLR4 terminates the MyD88-dependent signaling, while the following endosome maturation and lysosomal degradation of TLR4 determine the duration and magnitude of the TRIF-dependent one. Alternatively, TLR4 may return to the plasma membrane, which process is still poorly understood. Therefore, the course of the LPS-induced pro-inflammatory responses depends strictly on the rates of TLR4 endocytosis and trafficking through the endo-lysosomal compartment. Notably, prolonged activation of TLR4 is linked with several hereditary human diseases, neurodegeneration and also with autoimmune diseases and cancer. Recent studies have provided ample data on the role of diverse proteins regulating the functions of early, late, and recycling endosomes in the TLR4-induced inflammation caused by LPS or phagocytosis of E. coli. In this review, we focus on the mechanisms of the internalization and intracellular trafficking of TLR4 and CD14, and also of LPS, in immune cells and discuss how dysregulation of the endo-lysosomal compartment contributes to the development of diverse human diseases.

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Apical endosomes isolated from kidney collecting duct principal cells lack subunits of the proton pumping ATPase.

The Journal of Cell Biology ◽

10.1083/jcb.119.1.111 ◽

1992 ◽

Vol 119 (1) ◽

pp. 111-122 ◽

Cited By ~ 22

Author(s):

I Sabolic ◽

F Wuarin ◽

L B Shi ◽

A S Verkman ◽

D A Ausiello ◽

...

Keyword(s):

Plasma Membrane ◽

Proton Pump ◽

Transmembrane Domain ◽

Collecting Duct ◽

Water Channels ◽

Proton Pumping ◽

Apical Plasma Membrane ◽

Principal Cells ◽

Kidney Collecting Duct ◽

Early Endosomes

Endocytic vesicles that are involved in the vasopressin-stimulated recycling of water channels to and from the apical membrane of kidney collecting duct principal cells were isolated from rat renal papilla by differential and Percoll density gradient centrifugation. Fluorescence quenching measurements showed that the isolated vesicles maintained a high, HgCl2-sensitive water permeability, consistent with the presence of vasopressin-sensitive water channels. They did not, however, exhibit ATP-dependent luminal acidification, nor any N-ethylmaleimide-sensitive ATPase activity, properties that are characteristic of most acidic endosomal compartments. Western blotting with specific antibodies showed that the 31- and 70-kD cytoplasmically oriented subunits of the vacuolar proton pump were not detectable in these apical endosomes from the papilla, whereas they were present in endosomes prepared in parallel from the cortex. In contrast, the 56-kD subunit of the proton pump was abundant in papillary endosomes, and was localized at the apical pole of principal cells by immunocytochemistry. Finally, an antibody that recognizes the 16-kD transmembrane subunit of oat tonoplast ATPase cross-reacted with a distinct 16-kD band in cortical endosomes, but no 16-kD band was detectable in endosomes from the papilla. This antibody also recognized a 16-kD band in affinity-purified H+ ATPase preparations from bovine kidney medulla. Therefore, early endosomes derived from the apical plasma membrane of collecting duct principal cells fail to acidify because they lack functionally important subunits of a vacuolar-type proton pumping ATPase, including the 16-kD transmembrane domain that serves as the proton-conducting channel, and the 70-kD cytoplasmic subunit that contains the ATPase catalytic site. This specialized, non-acidic early endosomal compartment appears to be involved primarily in the hormonally induced recycling of water channels to and from the apical plasma membrane of vasopressin-sensitive cells in the kidney collecting duct.

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GLUT8 Subcellular Localization and Absence of Translocation to the Plasma Membrane in PC12 Cells and Hippocampal Neurons

Endocrinology ◽

10.1210/en.2005-0668 ◽

2005 ◽

Vol 146 (11) ◽

pp. 4727-4736 ◽

Cited By ~ 20

Author(s):

Mathieu Widmer ◽

Marc Uldry ◽

Bernard Thorens

Keyword(s):

Plasma Membrane ◽

Protein Kinase ◽

Cell Surface ◽

Pc12 Cells ◽

Hippocampal Neurons ◽

Osmotic Shock ◽

Surface Expression ◽

Cell Surface Expression ◽

Intracellular Compartment ◽

Early Endosomes

GLUT8 is a high-affinity glucose transporter present mostly in testes and a subset of brain neurons. At the cellular level, it is found in a poorly defined intracellular compartment in which it is retained by an N-terminal dileucine motif. Here we assessed GLUT8 colocalization with markers for different cellular compartments and searched for signals, which could trigger its cell surface expression. We showed that when expressed in PC12 cells, GLUT8 was located in a perinuclear compartment in which it showed partial colocalization with markers for the endoplasmic reticulum but not with markers for the trans-Golgi network, early endosomes, lysosomes, and synaptic-like vesicles. To evaluate its presence at the plasma membrane, we generated a recombinant adenovirus for the expression of GLUT8 containing an extracellular myc epitope. Cell surface expression was evaluated by immunofluorescence microscopy of transduced PC12 cells or primary hippocampal neurons exposed to different stimuli. Those included substances inducing depolarization, activation of protein kinase A and C, activation or inhibition of tyrosine kinase-linked signaling pathways, glucose deprivation, AMP-activated protein kinase stimulation, and osmotic shock. None of these stimuli-induced GLUT8 cell surface translocation. Furthermore, when GLUT8myc was cotransduced with a dominant-negative form of dynamin or GLUT8myc-expressing PC-12 cells or neurons were incubated with an anti-myc antibody, no evidence for constitutive recycling of the transporter through the cell surface could be obtained. Thus, in cells normally expressing it, GLUT8 was associated with a specific intracellular compartment in which it may play an as-yet-uncharacterized role.

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AtRBOHC/RHD2 vesicular delivery to the apical plasma membrane domain during root hair development

10.1101/2021.09.13.460100 ◽

2021 ◽

Author(s):

Lenka Kuběnová ◽

Michaela Tichá ◽

Jozef Šamaj ◽

Miroslav Ovečka

Keyword(s):

Plasma Membrane ◽

Quantitative Analysis ◽

Root Hair ◽

Tip Growth ◽

Root Hairs ◽

Apical Plasma Membrane ◽

Loss Of Function ◽

Short Root ◽

Early Endosomes ◽

Root Hair Initiation

AbstractArabidopsis root hairs develop as long tubular extensions from the rootward pole of trichoblasts and exert polarized tip growth. The establishment and maintenance of root hair polarity is a complex process involving the local apical production of reactive oxygen species (ROS) generated by NADPH oxidase RESPIRATORY BURST OXIDASE HOMOLOG PROTEIN C/ROOT HAIR DEFECTIVE 2 (AtRBOHC/RHD2). It has been shown that loss-of-function rhd2 mutants have short root hairs that are unable to elongate by tip growth, and this phenotype was fully complemented by GFP-RHD2 expressed under the RHD2 promoter. However, the spatiotemporal mechanism of AtRBOHC/RHD2 subcellular redistribution and delivery to the plasma membrane (PM) during root hair initiation and tip growth are still unclear. Here, we used advanced microscopy for detailed qualitative and quantitative analysis of vesicular compartments containing GFP-RHD2 and characterization of their movements in developing bulges and growing root hairs. These compartments, identified by an independent marker such as the trans-Golgi network (TGN), deliver GFP-RHD2 to the apical PM domain, the extent of which correlates with the stage of root hair formation. Movements of TGN/early endosomes, but not late endosomes, were affected in the bulging domains of the rhd2-1 mutant. Finally, we reveal that accumulation in the growing tip, docking, and incorporation of TGN compartments containing GFP-RHD2 to the apical PM of root hairs requires structural sterols. These results help clarify the mechanism of polarized AtRBOHC/RHD2 targeting, maintenance, and recycling at the apical PM domain, coordinated with different developmental stages of root hair initiation and growth.One-sentence summaryAdvanced microscopy and quantitative analysis of vesicular TGN compartments revealed that delivering GFP-RHD2 to the apical plasma membrane domains of developing bulges and growing root hairs requires structural sterols.

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Immunomorphological characterization and effects of 12-(S)-HETE on a dynamic intracellular pool of the alpha IIb beta 3-integrin in melanoma cells

Journal of Cell Science ◽

10.1242/jcs.108.6.2175 ◽

1995 ◽

Vol 108 (6) ◽

pp. 2175-2186 ◽

Cited By ~ 1

Author(s):

J. Timar ◽

R. Bazaz ◽

V. Kimler ◽

M. Haddad ◽

D.G. Tang ◽

...

Keyword(s):

Plasma Membrane ◽

Cell Surface ◽

Intermediate Filaments ◽

Brefeldin A ◽

Surface Expression ◽

Melanoma Cells ◽

Apical Plasma Membrane ◽

Intracellular Pool ◽

Soluble Ligand ◽

Surface Receptor

In metastatic B16a murine melanoma cells, alpha IIb beta 3 integrin was shown to be one of the key adhesion molecules responsible for matrix adhesion and spreading. Upon stimulation, alpha IIb beta 3 can be upregulated at the cell surface due to translocation of the receptor to the plasma membrane from an intracellular pool. Here we have characterized this integrin pool as a tubulovesicular structure (TVS) corresponding to endosomes. TVS was found to be associated temporarily with microtubules and intermediate filaments especially after protein kinase C (PKC) stimulation with a lipoxygenase metabolite of arachidonic acid, 12-(S)-hydroxyeicosatetraenoic acid [12-(S)-HETE]. After PKC stimulation, the predominantly vesicular TVS became elongated and alpha IIb beta 3 appeared at the apical plasma membrane and microvilli. Disruption of either the microtubules or intermediate filaments prevented the 12-(S)-HETE effect both on vesicular to tubular transition of TVS as well as on surface expression of this integrin. The connection with the Golgi system of the integrin-containing TVS was proved by a Golgi-inhibitor (brefeldin A) pretreatment, which prevented the PKC-stimulation-induced TVS elongation and subsequent receptor-upregulation at the cell surface. After a soluble ligand binding (mAb to the alpha IIb beta 3 complex) the surface receptor endocytosed back to the TVS indicating the presence of a dynamic, cytoskeleton associated integrin pool in melanoma cells.

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ANG II and calmodulin/CaMKII regulate surface expression and functional activity of NBCe1 via separate means

AJP Renal Physiology ◽

10.1152/ajprenal.00454.2006 ◽

2007 ◽

Vol 293 (1) ◽

pp. F68-F77 ◽

Cited By ~ 16

Author(s):

Clint Perry ◽

Hong Le ◽

Irina I. Grichtchenko

Keyword(s):

Fluorescent Protein ◽

Surface Expression ◽

Inhibitory Effect ◽

Additive Effect ◽

Xenopus Laevis Oocytes ◽

Ang Ii ◽

Early Endosomes ◽

Green Fluorescent ◽

Induced Changes

We recently reported that ANG II inhibits NBCe1 current and surface expression in Xenopus laevis oocytes (Perry C, Blaine J, Le H, and Grichtchenko II. Am J Physiol Renal Physiol 290: F417–F427, 2006). Here, we investigated mechanisms of ANG II-induced changes in NBCe1 surface expression. We showed that the PKC inhibitor GF109203X blocks and EGTA reduces surface cotransporter loss in ANG II-treated oocytes, suggesting roles for PKC and Ca2+. Using the endosomal marker FM 4-64 and enhanced green fluorescent protein (EGFP)-tagged NBCe1, we showed that ANG II stimulates endocytosis of NBCe1. To eliminate the possibility that ANG II inhibits NBCe1 recycling, we demonstrated that the recycling inhibitor monensin decreases surface expression, accumulates NBCe1-EGFP in endosomes, and inhibits NBCe1 current. Monensin and ANG II applied together produce greater inhibition of NBCe1 current than either did alone. This additive effect of monensin and ANG II suggests that ANG II stimulates internalization of NBCe1. We used the calmodulin (CaM) antagonist W13, which controls recycling by blocking the exit of the endocytosed cargo from early endosomes, to determine the role of CaM in NBCe1 trafficking. We demonstrated that W13 decreases surface expression of NBCe1, accumulates NBCe1-EGFP in endosomal-like formations, and inhibits NBCe1 current. W13 and ANG II applied together produce greater inhibition of NBCe1 current than either does alone, while W13 and monensin applied together do not. The additive effect of ANG II and W13 and lack of additive effect of monensin and W13 suggest that CaM is not involved in ANG II stimulation of internalization but controls recycling of endocytosed NBCe1. The CaM-activated enzyme CaM kinase II (CaMKII) applied with ANG II also gives an additive inhibitory effect, suggesting a role for CaMKII in NBCe1 recycling.

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Intracellular Retention of Glycosylphosphatidyl Inositol-Linked Proteins in Caveolin-Deficient Cells

Molecular and Cellular Biology ◽

10.1128/mcb.22.11.3905-3926.2002 ◽

2002 ◽

Vol 22 (11) ◽

pp. 3905-3926 ◽

Cited By ~ 66

Author(s):

Federica Sotgia ◽

Babak Razani ◽

Gloria Bonuccelli ◽

William Schubert ◽

Michela Battista ◽

...

Keyword(s):

Plasma Membrane ◽

Cell Surface ◽

Lipid Rafts ◽

Recombinant Expression ◽

Surface Expression ◽

Cell Surface Expression ◽

Null Mouse ◽

Tissue Samples ◽

Caveolin 1 ◽

Glycosylphosphatidyl Inositol

ABSTRACT The relationship between glycosylphosphatidyl inositol (GPI)-linked proteins and caveolins remains controversial. Here, we derived fibroblasts from Cav-1 null mouse embryos to study the behavior of GPI-linked proteins in the absence of caveolins. These cells lack morphological caveolae, do not express caveolin-1, and show a ∼95% down-regulation in caveolin-2 expression; these cells also do not express caveolin-3, a muscle-specific caveolin family member. As such, these caveolin-deficient cells represent an ideal tool to study the role of caveolins in GPI-linked protein sorting. We show that in Cav-1 null cells GPI-linked proteins are preferentially retained in an intracellular compartment that we identify as the Golgi complex. This intracellular pool of GPI-linked proteins is not degraded and remains associated with intracellular lipid rafts as judged by its Triton insolubility. In contrast, GPI-linked proteins are transported to the plasma membrane in wild-type cells, as expected. Furthermore, recombinant expression of caveolin-1 or caveolin-3, but not caveolin-2, in Cav-1 null cells complements this phenotype and restores the cell surface expression of GPI-linked proteins. This is perhaps surprising, as GPI-linked proteins are confined to the exoplasmic leaflet of the membrane, while caveolins are cytoplasmically oriented membrane proteins. As caveolin-1 normally undergoes palmitoylation on three cysteine residues (133, 143, and 156), we speculated that palmitoylation might mechanistically couple caveolin-1 to GPI-linked proteins. In support of this hypothesis, we show that palmitoylation of caveolin-1 on residues 143 and 156, but not residue 133, is required to restore cell surface expression of GPI-linked proteins in this complementation assay. We also show that another lipid raft-associated protein, c-Src, is retained intracellularly in Cav-1 null cells. Thus, Golgi-associated caveolins and caveola-like vesicles could represent part of the transport machinery that is necessary for efficiently moving lipid rafts and their associated proteins from the trans-Golgi to the plasma membrane. In further support of these findings, GPI-linked proteins were also retained intracellularly in tissue samples derived from Cav-1 null mice (i.e., lung endothelial and renal epithelial cells) and Cav-3 null mice (skeletal muscle fibers).

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