Sorting of MHC class II molecules and the associated invariant chain (Ii) in polarized MDCK cells

Basolateral sorting of chloride channel 2 is mediated by interactions between a dileucine motif and the clathrin adaptor AP-1

ClC-2 is a ubiquitous chloride channel that regulates cell volume, ion transport, and acid-base balance. Mice knocked out for ClC-2 are blind and sterile. Basolateral localization of ClC-2 in epithelia is mediated by the interaction of a dileucine motif with a highly conserved pocket in the γ1-σ1A hemicomplex of AP-1.

In spite of the many key cellular functions of chloride channels, the mechanisms that mediate their subcellular localization are largely unknown. ClC-2 is a ubiquitous chloride channel usually localized to the basolateral domain of epithelia that regulates cell volume, ion transport, and acid–base balance; mice knocked out for ClC-2 are blind and sterile. Previous work suggested that CLC-2 is sorted basolaterally by TIFS⁸¹²LL, a dileucine motif in CLC-2's C-terminal domain. However, our in silico modeling of ClC-2 suggested that this motif was buried within the channel's dimerization interface and identified two cytoplasmically exposed dileucine motifs, ESMI⁶²³LL and QVVA⁶³⁵LL, as candidate sorting signals. Alanine mutagenesis and trafficking assays support a scenario in which ESMI⁶²³LL acts as the authentic basolateral signal of ClC-2. Silencing experiments and yeast three-hybrid assays demonstrated that both ubiquitous (AP-1A) and epithelium-specific (AP-1B) forms of the tetrameric clathrin adaptor AP-1 are capable of carrying out basolateral sorting of ClC-2 through interactions of ESMI⁶²³LL with a highly conserved pocket in their γ1-σ1A hemicomplex.

Invariant chain increases the half-life of MHC II by delaying endosomal maturation

Mounting adaptive immune responses requires the cell surface expression of major histocompatibility class II molecules (MHC II) loaded with antigenic peptide. However, in the absence of antigenic stimuli, the surface population of MHC II is highly dynamic and exhibits a high turnover. Several studies have focused on the regulation of MHC II, and it is now recognized that ubiquitination is one key mechanism operating in the turnover of MHC II in B cells and dendritic cells. Here, we describe how the invariant chain (Ii) can prolong the half-life of MHC II through its action on the endocytic pathway. We find that in cells expressing intermediate-to-high levels of Ii, the half-life of MHC II is increased, with MHC II accumulating in slowly-maturing endosomes. The accumulation in endosomes is not due to retention of new MHC II directed from the endoplasmatic reticulum, as also mature, not Ii associated, MHC II is preserved. We suggest that this alternative endocytic pathway induced by Ii would serve to enhance the rate, quantity and diversity of MHC II antigen presentation by concentrating MHC II into specialized compartments and reducing the need for new MHC II synthesis upon antigen encounter.

Alternative Ii-independent antigen-processing pathway in leukemic blasts involves TAP-dependent peptide loading of HLA class II complexes

During HLA class II synthesis in antigen-presenting cells, the invariant chain (Ii) not only stabilizes HLA class II complexes in the endoplasmic reticulum, but also mediates their transport to specialized lysosomal antigen-loading compartments termed MIICs. This study explores an alternative HLA class II presentation pathway in leukemic blasts that involves proteasome and transporter associated with antigen processing (TAP)-dependent peptide loading. Although HLA-DR did associate with Ii, Ii silencing in the human class II-associated invariant chain peptide (CLIP)-negative KG-1 myeloid leukemic cell line did not affect total and plasma membrane expression levels of HLA-DR, as determined by western blotting and flow cytometry. Since HLA-DR expression does require peptide binding, we examined the role of endogenous antigen-processing machinery in HLA-DR presentation by CLIP(-) leukemic blasts. The suppression of proteasome and TAP function using various inhibitors resulted in decreased HLA-DR levels in both CLIP(-) KG-1 and ME-1 blasts. Simultaneous inhibition of TAP and Ii completely down-modulated the expression of HLA-DR, demonstrating that together these molecules form the key mediators of HLA class II antigen presentation in leukemic blasts. By the use of a proteasome- and TAP-dependent pathway for HLA class II antigen presentation, CLIP(-) leukemic blasts might be able to present a broad range of endogenous leukemia-associated peptides via HLA class II to activate leukemia-specific CD4(+) T cells.

Major histocompatibility complex class II-peptide complexes internalize using a clathrin- and dynamin-independent endocytosis pathway

Major histocompatibility complex (MHC) class II molecules (MHC-II) function by binding antigenic peptides and displaying these peptides on the surface of antigen presenting cells (APCs) for recognition by peptide-MHC-II (pMHC-II)-specific CD4 T cells. It is known that cell surface MHC-II can internalize, exchange antigenic peptides in endosomes, and rapidly recycle back to the plasma membrane; however, the molecular machinery and trafficking pathways utilized by internalizing/recycling MHC-II have not been identified. We now demonstrate that unlike newly synthesized invariant chain-associated MHC-II, mature cell surface pMHC-II complexes internalize following clathrin-, AP-2-, and dynamin-independent endocytosis pathways. Immunofluorescence microscopy of MHC-II expressing HeLa-CIITA cells, human B cells, and human DCs revealed that pMHC enters Arf6(+)Rab35(+)EHD1(+) tubular endosomes following endocytosis. These data contrast the internalization pathways followed by newly synthesized and peptide-loaded MHC-II molecules and demonstrates that cell surface pMHC-II internalize and rapidly recycle from early endocytic compartments in tubular endosomes.

T cell-induced secretion of MHC class II-peptide complexes on B cell exosomes

Antigen-specific interactions between B cells and T cells are essential for the generation of an efficient immune response. Since this requires peptide-MHC class II complexes (pMHC-II) on the B cell to interact with TCR on antigen-specific T cells, we have examined the mechanisms regulating the persistence, loss, and secretion of specific pMHC-II complexes on activated B cells. Using a mAb that recognizes specific pMHC-II, we found that activated B cells degrade approximately 50% of pMHC-II every day and release 12% of these pMHC-II from the cell on small membrane vesicles termed exosomes. These exosomes directly stimulate primed, but not naïve, CD4 T cells. Interestingly, engagement of antigen-loaded B cells with specific CD4 T cells stimulates exosome release in a manner that can be mimicked by pMHC-II crosslinking. Biochemical studies revealed that the pMHC-II released on exosomes was previously expressed on the plasma membrane of the B cells, suggesting that regulated exosome release from activated B cells is a mechanism to allow pMHC-II to escape intracellular degradation and decorate secondary lymphoid organs with membrane-associated pMHC-II complexes.

Molecular determinants for differential membrane trafficking of PMCA1 and PMCA2 in mammalian hair cells

The plasma membrane Ca2+-ATPase-2 (PMCA2) is expressed in stereocilia of hair cells of the inner ear, whereas PMCA1 is expressed in the basolateral plasma membrane of hair cells. Both extrude excess Ca2+ from the cytosol. They are predicted to contain ten membrane-spanning segments, two large cytoplasmic loops as well as cytosolic N- and C-termini. Several isoform variants are generated for both PMCA1 and PMCA2 by alternative splicing, affecting their first cytosolic loop (A-site) and their C-terminal tail. To understand how these isoforms are differentially targeted in hair cells, we investigated their targeting regions and expression in hair cells. Our results show that a Leu-Ile motif in 'b'-tail splice variants promotes PMCA1b and PMCA2b basolateral sorting in hair cells. Moreover, apical targeting of PMCA2 depends on the size of the A-site-spliced insert, suggesting that the conformation of the cytoplasmic loop plays a role in apical targeting.

Identification of basolateral membrane targeting signal of human sodium-dependent dicarboxylate transporter 3

Sodium-dependent dicarboxylate transporters (NaDC) include low-affinity NaDC1 and high-affinity NaDC3. Despite high similarities structurally and functionally, both are localized to opposite surfaces of renal tubular cells. The molecular mechanisms and localization signals leading to this polarized distribution remain unknown. In this study, distribution of NaDC3 in human kidney tissue was firstly observed by immunohistochemistry and immunofluorescence. Then, EGFP-fused wild-type, NH2- and COOH-terminal deletion and point mutants of NaDC3, and chimera between NaDC3 and NaDC1, were generated and transfected into polarized renal cells lines, LLC-PK1 and MDCK. Their subcellular localizations were analyzed by laser confocal microscopy. Immunolocalization results revealed that NaDC3 was expressed at basolateral membrane of human renal proximal tubular epithelia. Confocal examinations showed that wild-type NaDC3 was targeted to the basolateral membrane of MDCK and LLC-PK1. Deletion mutations indicated that the basolateral targeting signal of NaDC3 located within a short sequence AKKVWSARR of its amino-terminal cytoplasmic domain. Addition of this sequence could redirect apical NaDC1 to the basolateral membrane of LLC-PK1. Point mutagenesis revealed that mutation of either of two hydrophobic amino acids V and W in this short sequence largely redirected NaDC3 to both apical and basolateral surfaces of LLC-PK, indicating that the two hydrophobic amino acids are critical for the basolateral targeting of NaDC3. Our studies provide direct evidence of the localization of NaDC3 at the basolateral membrane of human renal proximal tubule cells and identify a di-hydrophobic amino acid motif VW as basolateral localization signal in the N-terminal cytoplasmic domain of NaDC3.

Secretion of proteases in serglycin transfected Madin-Darby canine kidney cells

Madin-Darby canine kidney (MDCK) cells, which do not normally express the proteoglycan (PG) serglycin, were stably transfected with cDNA for human serglycin fused to a polyhistidine tag (His-tag). Clones with different levels of serglycin mRNA expression were generated. One clone with lower and one with higher serglycin mRNA expression were selected for this study. 35S-labelled serglycin in cell fractions and conditioned media was isolated using HisTrap affinity chromatography. Serglycin could also be detected in conditioned media using western blotting. To investigate the possible importance of serglycin linked to protease secretion, enzyme activities using chromogenic substrates and zymography were measured in cell fractions and serum-free conditioned media of the different clones. Cells were cultured in both the absence and presence of phorbol 12-myristate 13-acetate (PMA). In general, enzyme secretion was strongly enhanced by treatment with PMA. Our analyses revealed that the clone with the highest serglycin mRNA expression, level of HisTrap isolated 35S-labelled serglycin, and amount of serglycin core protein as detected by western blotting, also showed the highest secretion of proteases. Transfection of serglycin into MDCK cells clearly leads to changes in secretion levels of secreted endogenous proteases, and could provide further insight into the biosynthesis and secretion of serglycin and potential partner molecules.

Phenotypic and functional characterization of intestinal epithelial exosomes

Intestinal epithelial cells (IEC) are located at a strategic position between the external environment and the most extended lymphoid tissue in the body. Besides their central role in the absorption of nutrients, IEC also provide antigenic information to the immune system and are involved in the balance tolerance/allergy to food antigens. Like professional antigen presenting cells, IEC have been shown to secrete 30- to 90-nm diameter vesicles named exosomes, in a polarized way, either from their apical or basolateral side. These vesicles carry molecules involved in adhesion and antigen presentation, comprising major histocompatibility complex (MHC) class I and class II molecules, tetraspan proteins, CD26/dipeptidyl-peptidase IV, and A33 antigen, a molecule essentially restricted to the intestinal epithelium. Invariant chain, transferrin receptor, and Na-K-ATPase are not expressed on epithelial exosomes. In vivo, in mice, epithelial exosomes carrying MHC/ovalbumin peptide complexes induce specific immune responses when injected intraperitoneally. A33 antigen, an Ig-like molecule highly specific for intestinal epithelial cells and enriched in epithelial exosomes, is found at the surface of cells entering mesenteric lymph nodes suggesting exosome migration from the epithelial layer to the gut associated lymphoid system. Taken together, intestinal epithelial exosomes released at the basolateral surface of enterocytes could be antigen-carrying structures constituting a link between luminal antigens and the local immune system and acting as sensors of the antigenic information present in the intestinal lumen.

Cathepsin S controls MHC class II-mediated antigen presentation by epithelial cells in vivo

Epithelial cells at environmental interfaces provide protection from potentially harmful agents, including pathogens. In addition to serving as a physical barrier and producing soluble mediators of immunity, such as cytokines or antimicrobial peptides, these cells are thought to function as nonprofessional APCs. In this regard, intestinal epithelial cells are particularly prominent because they express MHC class II molecules at the site of massive antigenic exposure. However, unlike bone marrow-derived professional APC, such as dendritic cells or B cells, little is known about the mechanisms of MHC class II presentation by the nonprofessional APC in vivo. The former use the lysosomal cysteine protease cathepsin S (Cat S), whereas thymic cortical epithelial cells use cathepsin L (Cat L) for invariant chain degradation and MHC class II maturation. Unexpectedly, we found that murine Cat S plays a critical role in invariant chain degradation in intestinal epithelial cells. Furthermore, we report that nonprofessional APC present a class II-bound endogenous peptide to naive CD4 T cells in vivo in a Cat S-dependent fashion. These results suggest that in vivo, both professional and nonprofessional MHC class II-expressing APC use Cat S, but not Cat L, for MHC class II-mediated Ag presentation.

The basolateral targeting signal of CD147 (EMMPRIN) consists of a single leucine and is not recognized by retinal pigment epithelium

CD147, a type I integral membrane protein of the immunoglobulin superfamily, exhibits reversed polarity in retinal pigment epithelium (RPE). CD147 is apical in RPE in contrast to its basolateral localization in extraocular epithelia. This elicited our interest in understanding the basolateral sorting signals of CD147 in prototypic Madin-Darby canine kidney (MDCK) cells. The cytoplasmic domain of CD147 has basolateral sorting information but is devoid of well-characterized basolateral signals, such as tyrosine and di-leucine motifs. Hence, we carried out systematic site-directed mutagenesis to delineate basolateral targeting information in CD147. Our detailed analysis identified a single leucine (252) as the basolateral targeting motif in the cytoplasmic tail of CD147. Four amino acids (243-246) N-terminal to leucine 252 are also critical basolateral determinants of CD147, because deletion of these amino acids leads to mistargeting of CD147 to the apical membranes. We ruled out the involvement of adaptor complex 1B (AP1B) in the basolateral trafficking of CD147, because LLC-PK1 cells lacking AP1B, target CD147 basolaterally. At variance with MDCK cells, the human RPE cell line ARPE-19 does not distinguish between CD147 (WT) and CD147 with leucine 252 mutated to alanine and targets both proteins apically. Thus, our study identifies an atypical basolateral motif of CD147, which comprises a single leucine and is not recognized by RPE cells. This unusual basolateral sorting signal will be useful in unraveling the specialized sorting machinery of RPE cells.

Delineation of a CD1d-restricted antigen presentation pathway associated with human and mouse intestinal epithelial cells

BACKGROUND & AIMS

CD1d, a major histocompatibility complex (MHC) class I-related molecule that is responsible for the presentation of glycolipid antigens to subsets of natural killer T (NK-T) cells, is expressed by intestinal epithelial cells (IECs). However, CD1d-restricted antigen presentation has not yet been examined on IECs.

METHODS

A mouse intestinal epithelial cell line (MODE-K), a human epithelial cell line (T84), T84 cells transfected with CD1d and/or MHC class II, and freshly isolated human IECs were examined for their ability to present model glycolipid antigens to NK-T cells as defined by interleukin (IL)-2 or IL-4 secretion.

RESULTS

MODE-K and freshly isolated human IECs exhibited dose-dependent, CD1d-restricted presentation of the functional glycolipid antigen, alpha-galactosylceramide (alpha GalCer), to the mouse NK-T cell hybridoma, DN32.D3. The human IEC line, T84, mainly presented alpha GalCer when transfected with human CD1d. Presentation of alpha GalCer by CD1d-transfected T84 cells (T84d) to DN32.D3 cells was greater along the basal surface in comparison with the apical surface. Induction of the MHC class II antigen presentation machinery by cotransfecting T84d with the MHC class I transactivator (CIITA) did not alter this polarity of presentation. Neither MODE-K nor T84 cells transfected with CD1d, CD1d plus CIITA, or CD1d plus HLA-DR were able to present glycolipid antigens requiring intracellular processing. The MODE-K cell line could also present alpha GalCer to primary mouse NK-T cells.

CONCLUSIONS

CD1d is expressed functionally on IECs with a polarity of presentation (basal > apical) predicting a role in presentation of mucosal glycolipid antigens to local CD1d-restricted T cells.

The epithelial cell cytoskeleton and intracellular trafficking. V. Polarized compartmentalization of antigen processing and Toll-like receptor signaling in intestinal epithelial cells

The intestinal epithelial cell (IEC) is exposed at the apical surface to a high concentration of foreign antigen and bacterial products capable of triggering inflammatory responses. Complex intracellular pathways of antigen trafficking and the polarized expression of immunologically active receptors provide additional means to regulate the inflammatory pathways in these cells. In the case of human leukocyte antigen (HLA) class II heterodimers, surface expression is highly restricted to the basolateral surface, and this also appears to be the case for Toll-like receptor 5 (TLR5) on polarized T84 human colon cancer cells. Processing of soluble antigen via HLA class II in IEC can occur following internalization from the apical surface but is highly inefficient. In addition, certain bacteria can facilitate the transport of flagellin (the ligand for TLR5) across an intact epithelium. Disruption of the tight junctions between IECs, allowing direct access of antigen and flagellin to the basolateral surface of the cell, dramatically affects the functional outcome HLA class II and TLR5 pathways.

The epithelial cell cytoskeleton and intracellular trafficking. II. Intestinal epithelial cell exosomes: perspectives on their structure and function

Intestinal epithelial cells (IEC) are located at the strategic interface between the external environment and the most extensive lymphoid compartment in the body. Besides their central role in the absorption of nutrients, they also provide sample information to the immune system on soluble or particulate antigens present in the intestinal lumen. Like professional antigen-presenting cells, IEC have recently been shown to secrete 30- to 90-nm diameter vesicles named exosomes from their apical and basolateral surfaces. These vesicles carry molecules that are implicated in adhesion and antigen presentation, such as major histocompatibility complex (MHC) class I molecules, MHC class II molecules, CD63, CD26/dipeptidyl-peptidase IV, tetraspan proteins, and A33 antigen. IEC exosomes therefore, constitute a link by which IEC may influence antigen presentation in the mucosal or systemic immune system independent of direct cellular contact with effector cells.

Mechanism of interaction between leucine-based sorting signals from the invariant chain and clathrin-associated adaptor protein complexes AP1 and AP2

The cytoplasmic tail of the invariant chain contains two leucine-based sorting signals, and each of those seems sufficient to route the invariant chain to its intracellular destination in either normal or polarized cells. It is believed that the intracellular routing of the invariant chain is mediated by its interactions with the clathrin-associated adaptor protein complexes AP1 and AP2. We () have previously demonstrated the in vitro interactions between the cytoplasmic tail of the invariant chain and AP1/AP2 complexes. These interactions were specific and depended on the critical leucine residues in the invariant chain's sorting signals. In the present study, we decided to investigate the molecular mechanism of these interactions. To this end, we constructed a set of glutathione S-transferase fusion proteins that contained the intact cytoplasmic tail of the invariant chain and its various mutants to define residues important for its interactions with AP1 and AP-2. Our results demonstrated the importance of several residues other than the critical leucine residues for such interactions. A strong correlation between in vitro binding of AP2 to the invariant chain and in vivo internalization of the invariant chain was observed, confirming the primary role of AP2 in recognition of endocytic signals. In addition, we demonstrated different requirements for AP1 and AP2 binding to cytoplasmic tail of the invariant chain, which may reflect that the different sorting pathways mediated by AP1 and AP2 involve their recognition of the primary structure of the sorting signal.

Basolateral sorting signal of the 300-kDa mannose 6-phosphate receptor

In polarized cells, the delivery of numerous membrane proteins from the trans-Golgi network to the basolateral surface depends on specific sequences located in their cytoplasmic domain. We have previously shown that the insulin-like growth factor-II/mannose 6-phosphate receptor (IGF-II/MPR) exhibits a polarized cell surface distribution in the human colon adenocarcinoma (Caco-2) cell line in which there is a threefold enrichment on the basolateral surface. To investigate the role of residues in the cytoplasmic region of the receptor that facilitates its entry into the basolateral sorting pathway, we generated stably transfected Caco-2 cell lines expressing various mutant bovine IGF-II/MPRs. The steady-state surface distribution of mutant receptors was analyzed by subjecting filter-grown cell monolayers to incubation with iodinated IGF-II/MPR-specific antibody or to indirect immunofluorescence and visualization by confocal microscopy. Together, these results demonstrate that the sorting of the IGF-II/MPR to the basolateral cell surface depends on recognition of sequences located in its cytoplasmic region that are distinct from the Tyr-based internalization and dileucine-dependent endosomal trafficking motifs.

Basolateral sorting of human poliovirus receptor alpha involves an interaction with the mu1B subunit of the clathrin adaptor complex in polarized epithelial cells

Poliovirus receptor (hPVR/CD155) is a cell surface glycoprotein that belongs to the immunoglobulin superfamily but its natural function remains unknown. Two membrane-bound isoforms, hPVRalpha and hPVRdelta, are known to date, and they differ only in the amino acid sequence of their cytoplasmic domains. To gain an insight into the possible function of the cytoplasmic domains, we examined the localization of introduced hPVRalpha and hPVRdelta in polarized epithelial cells deficient of native hPVRs. Basolateral sorting of hPVRalpha was observed in Madine-Darby canine kidney cells expressing mu1B, but not in LLC-PK1 porcine kidney cells deficient in mu1B. Distribution of hPVRdelta, however, occurred both on the apical and basolateral plasma membranes of these two cell lines. Basolateral sorting of hPVRalpha was also seen in LLC-PK1 cells that expressed an intact exogenous mu1B, but not in the cells that expressed a mutant mu1B lacking binding ability to tyrosine-containing signals. These results indicate that mu1B is involved in the distribution of hPVRalpha to the basolateral membrane. Comparative distribution analysis of hPVRalpha using a series of mutants with truncations and substitutions in the cytoplasmic tail demonstrated that determinant for the basolateral sorting resided in the tyrosine-containing motif of the cytoplasmic tail. Furthermore, yeast two hybrid analysis strongly suggested that the tyrosine motif directly interacted with mu1B protein. Thus, basolateral sorting of hPVRalpha appears to involve the interaction with mu1B through a tyrosine motif existing in the cytoplasmic domain.

Intestinal epithelial cells secrete exosome-like vesicles

BACKGROUND & AIMS

Given the observations that intestinal epithelial cells (IECs) can present antigens to CD4(+) T lymphocytes and that professional antigen-presenting cells secrete exosomes (antigen-presenting vesicles), we hypothesized that IECs may secrete exosomes carrying molecules implicated in antigen presentation, which may be able to cross the basement membrane and convey immune information to noncontiguous immune cells.

METHODS

Human IEC lines HT29-19A and T84-DRB1*0401/CIITA were grown on microporous filters. Release of exosomes under basal or inflammatory conditions was evaluated in conditioned apical and basolateral media after differential ultracentrifugations. Morphologic and biochemical characterization of exosomes was performed using immunoelectron microscopy, Western blotting, and matrix-assisted laser desorption ionization-time of flight mass spectrometry.

RESULTS

The intestinal cell lines released 30-90-nm-diameter vesicles from the apical and basolateral sides, and this release was significantly increased in the presence of interferon gamma. MHC class I, MHC class II, CD63, CD26/dipeptidyl-peptidase IV, and A33 antigen were present in epithelial-derived exosomes. CONCLUSIONS; Human IEC lines secrete exosomes bearing accessory molecules that may be involved in antigen presentation. These data are consistent with a model in which IECs may influence antigen presentation in the mucosal or systemic immune system independent of direct cellular contact with effector cells.

Stem cell factor presentation to c-Kit. Identification of a basolateral targeting domain

Stem cell factor (also known as mast cell growth factor and kit-ligand) is a transmembrane growth factor with a highly conserved cytoplasmic domain. Basolateral membrane expression in epithelia and persistent cell surface exposure of stem cell factor are required for complete biological activity in pigmentation, fertility, learning, and hematopoiesis. Here we show by site-directed mutagenesis that the cytoplasmic domain of stem cell factor contains a monomeric leucine-dependent basolateral targeting signal. N-terminal to this motif, a cluster of acidic amino acids serves to increase the efficiency of basolateral sorting mediated by the leucine residue. Hence, basolateral targeting of stem cell factor requires a mono-leucine determinant assisted by a cluster of acidic amino acids. This mono-leucine determinant is functionally conserved in colony-stimulating factor-1, a transmembrane growth factor related to stem cell factor. Furthermore, this leucine motif is not capable of inducing endocytosis, allowing for persistent cell surface expression of stem cell factor. In contrast, the mutated cytoplasmic tail found in the stem cell factor mutant Mgf(Sl17H) induces constitutive endocytosis by a motif that is related to signals for endocytosis and lysosomal targeting. Our findings therefore present mono-leucines as a novel type of protein sorting motif for transmembrane growth factors.

The cytoplasmic tail of CD1d contains two overlapping basolateral sorting signals

CD1d is a member of the CD1 polypeptide family that represents a new arm of host defense against invading pathogens. In our previous work (Rodionov, D. G., Nordeng, T. W., Pedersen, K., Balk, S. P., and Bakke, O. (1999) J. Immunol. 162, 1488-1495) we have shown that CD1d contained a classic tyrosine-based internalization signal (YQGV) in its short cytoplasmic tail. CD1d is expressed in polarized epithelial cells, and we found that the cytoplasmic tail of CD1d also contained information for basolateral sorting. Interestingly, a mutation of the critical tyrosine residue of the endosomal sorting signal did not result in the loss of basolateral targeting of the mutant CD1d. To search for a basolateral sorting signal we have constructed a full set of alanine mutants, but no single alanine substitution inactivated the signal. However, deletions or mutations of either the C-terminal valine/leucine pair or the critical tyrosine residue from the internalization signal and either residue from the C-terminal valine/leucine pair inactivated basolateral sorting. Our data thus suggest that the cytoplasmic tail contains two overlapping basolateral signals, one tyrosine- and the other leucine-based, each being sufficient to direct CD1d to the basolateral membrane of polarized Madin-Darby canine kidney cells.

The leucine-based sorting motifs in the cytoplasmic domain of the invariant chain are recognized by the clathrin adaptors AP1 and AP2 and their medium chains

Recognition of sorting signals within the cytoplasmic tail of membrane proteins by adaptor protein complexes is a crucial step in membrane protein sorting. The three known adaptor complexes, AP1, AP2, and AP3, have all been shown to recognize tyrosine- and leucine-based sorting signals, which are the most common sorting signals within membrane protein cytoplasmic tails. Although tyrosine-based signals are recognized by the micro-chains of adaptor complexes, the subunit recognizing leucine-based sorting signals is less clear. In this report we show by surface plasmon resonance that the two leucine-based sorting signals within the cytoplasmic tail of the invariant chain bind independently from each other to AP1 and AP2 but not to AP3. We also show that both motifs can be recognized by the micro-chains of AP1 and AP2. Moreover, by using monomeric as well as trimeric invariant chain constructs, we show that adaptor binding does not require trimerization of the invariant chain.

Intracellular traffic to compartments for MHC class II peptide loading: signals for endosomal and polarized sorting

In this review we focus on the traffic of MHC class II and endocytosed antigens to intracellular compartments where antigenic peptides are loaded. We also discuss briefly the nature of the peptide loading compartment and the sorting signals known to direct antigen receptors and MHC class II and associated molecules to this location. MHC class II molecules are expressed on a variety of polarized epithelial and endothelial cells, and polarized cells are thus potentially important for antigen presentation. Here we review some cell biological aspects of polarized sorting of MHC class II and the associated invariant chain and the signals that are involved in the sorting process to the basolateral domain. The molecules involved in sorting and loading of peptide may modulate antigen presentation, and in particular we discuss how invariant chain may change the cellular phenotype and the kinetics of the endosomal pathway.

A Role for the Transmembrane Domain in the Trimerization of the MHC Class II-Associated Invariant Chain

MHC class II and invariant chain (Ii) associate early in biosynthesis to form a nonameric complex. Ii first assembles into a trimer and then associates with three class II αβ heterodimers. Although the membrane-proximal region of the Ii luminal domain is structurally disordered, the C-terminal segment of the luminal domain is largely α-helical and contains a major interaction site for the Ii trimer. In this study, we show that the Ii transmembrane domain plays an important role in the formation of Ii trimers. The Ii transmembrane domain contains an unusual patch of hydrophilic residues near the luminal interface. Substitution of these polar residues with nonpolar amino acids resulted in a decrease in the efficiency of Ii trimerization and subsequent class II association. Moreover, N-terminal fragments of Ii were found to trimerize independently of the luminal α-helical domain. Progressive C-terminal truncations mapped a homotypic association site to the first 80 aa of Ii. Together, these results implicate the Ii transmembrane domain as a site of trimer interaction that can play an important role in the initiation of trimer formation.

Polarized Transport of MHC Class II Molecules in Madin-Darby Canine Kidney Cells Is Directed by a Leucine-Based Signal in the Cytoplasmic Tail of the β-Chain

MHC class II molecules are found on the basolateral plasma membrane domain of polarized epithelial cells, where they can present Ag to intraepithelial lymphocytes in the vascular space. We have analyzed the sorting information required for efficient intracellular localization and polarized distribution of MHC class II molecules in stably transfected Madin-Darby canine kidney cells. These cells were able to present influenza virus particles to HLA-DR1-restricted T cell clones. Wild-type MHC class II molecules were located on the basolateral plasma membrane domain, in basolateral early endosomes, and in late multivesicular endosomes, the latter also containing the MHC class II-associated invariant chain and an HLA-DM fusion protein. A phenylalanine-leucine residue within the cytoplasmic tail of the β-chain was required for basolateral distribution, efficient internalization, and localization of the MHC class II molecules to basolateral early endosomes. However, distribution to apically located, late multivesicular endosomes did not depend on signals in the class II cytoplasmic tails as both wild-type class II molecules and mutant molecules lacking the phenylalanine-leucine motif were found in these compartments. Our results demonstrate that sorting information in the tails of class II dimers is an absolute requirement for their basolateral surface distribution and intracellular localization.

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.

Mu1B, a novel adaptor medium chain expressed in polarized epithelial cells

The apical and basolateral plasma membrane domains of polarized epithelial cells contain distinct sets of integral membrane proteins. Biosynthetic targeting of proteins to the basolateral plasma membrane is mediated by cytosolic tail determinants, many of which resemble signals involved in the rapid endocytosis or lysosomal targeting. Since these signals are recognized by adaptor proteins, we hypothesized that there could be epithelial-specific adaptors involved in polarized sorting. Here, we report the identification of a novel member of the adaptor medium chain family, named mu1B, which is closely related to the previously described mu1A (79% amino acid sequence identity). Northern blotting and in situ hybridization analyses reveal the specific expression of mu1B mRNA in a subset of polarized epithelial and exocrine cells. Yeast two-hybrid analyses show that mu1B is capable of interacting with generic tyrosine-based sorting signals. These observations suggest that mu1B may be involved in protein sorting events specific to polarized cells.

Basolateral sorting of furin in MDCK cells requires a phenylalanine-isoleucine motif together with an acidic amino acid cluster

Furin is a subtilisin-related endoprotease which processes a wide range of bioactive proteins. Furin is concentrated in the trans-Golgi network (TGN), where proteolytic activation of many precursor proteins takes place. A significant fraction of furin, however, cycles among the TGN, the plasma membrane, and endosomes, indicating that the accumulation in the TGN reflects a dynamic localization process. The cytosolic domain of furin is necessary and sufficient for TGN localization, and two signals are responsible for retrieval of furin to the TGN. A tyrosine-based (YKGL) motif mediates internalization of furin from the cell surface into endosomes. An acidic cluster that is part of two casein kinase II phosphorylation sites (SDSEEDE) is then responsible for retrieval of furin from endosomes to the TGN. In addition, the acidic EEDE sequence also mediates endocytic activity. Here, we analyzed the sorting of furin in polarized epithelial cells. We show that furin is delivered to the basolateral surface of MDCK cells, from where a significant fraction of the protein can return to the TGN. A phenylalanine-isoleucine motif together with the acidic EEDE cluster is required for basolateral sorting and constitutes a novel signal regulating intracellular traffic of furin.

Surface-expressed invariant chain (CD74) is required for internalization of human leucocyte antigen-DR molecules to early endosomal compartments

Transport of major histocompatibility complex (MHC) class II molecules to the endocytic route is directed by the associated invariant chain (Ii). In the endocytic pathway, Ii is proteolytically cleaved and, upon removal of residual Ii fragments, class II alpha beta dimers are charged with antigenic peptide and recognized by CD4+ T cells. Although distinct peptide-loading compartments such as MIIC (MHC class II loading compartment) and CIIV (MHC class II vesicles) have been characterized in different cells, there is growing evidence of a multitude of subcellular compartments in which antigenic peptide loading takes place. We employed a physiological cellular system in which surface Ii (CD74) and surface human leucocyte antigen (HLA)-DR were induced either alone or in combination. This was achieved by transient exposure of HT-29 cells to recombinant interferon-gamma (rIFN-gamma). Using distinct cellular variants, we showed that: (i) the majority of Ii molecules physically associate on the cell membrane with class II dimers to form DR alpha beta:Ii complexes; (ii) the presence of surface Ii is a prerequisite for the rapid uptake of HLA-DR-specific monoclonal antibodies into early endosomes because only the surface DR+/Ii+ phenotype, and not the DR+/Ii- variant, efficiently internalizes; and (iii) the HLA-DR:Ii complexes are targeted to early endosomes, as indicated by co-localization with the GTPase, Rab5, and endocytosed bovine serum albumin. Internalization of HLA-DR:Ii complexes, accommodation of peptides by DR alphabeta heterodimers in early endosomes and recycling to the cell surface may be a mechanism used to increase the peptide repertoire that antigen-presenting cells display to MHC class II-restricted T cells.

A Critical Tyrosine Residue in the Cytoplasmic Tail Is Important for CD1d Internalization But Not for Its Basolateral Sorting in MDCK Cells

The CD1 family of polypeptides is divided into two groups, the CD1b and CD1d group. Both groups are involved in stimulation of T cell response. Molecules of the CD1b group can present Ag derived from bacterial cell walls to T cells; the process of Ag acquisition is thought to take place in endosomes. Little is known about Ag presentation by CD1d. We therefore studied the intracellular trafficking of human CD1d in Madin-Darby canine kidney (MDCK) and COS cells. CD1d was found in endosomal compartments after its internalization from the plasma membrane. It is therefore possible that CD1d acquires its yet unidentified exogenous ligand in the same compartments as the MHC class II and CD1b molecules. CD1d contains a tyrosine-based sorting signal in its cytoplasmic tail that is necessary for internalization. Furthermore, the cytoplasmic tail of CD1d also contains a signal for basolateral sorting that is, however, different from the internalization signal.

Structural requirements for major histocompatibility complex class II invariant chain endocytosis and lysosomal targeting

The invariant chain (Ii) targets newly synthesized major histocompatibility complex class II complexes to a lysosome-like compartment. Previously, we demonstrated that both the cytoplasmic tail (CT) and transmembrane (TM) domains of Ii were sufficient for this targeting and that the CT contains two di-leucine signals, 3DQRDLI8 and 12EQLPML17 (Odorizzi, C. G., Trowbridge, I. S., Xue, L., Hopkins, C. R., Davis, C. D., and Collawn, J. F. (1994) J. Cell Biol. 126, 317-330). In the present study, we examined the relationship between signals required for endocytosis and those required for lysosomal targeting by analyzing Ii-transferrin receptor chimeras in quantitative transport assays. Analysis of the Ii CT signals indicates that although 3DQRDLI8 is necessary and sufficient for endocytosis, either di-leucine signal is sufficient for lysosomal targeting. Deletions between the two signals reduced endocytosis without affecting lysosomal targeting. Transplantation of the DQRDLI sequence in place of the EQLPML signal produced a chimera that trafficked normally, suggesting that this di-leucine sequence coded for an independent structural motif. Structure-function analysis of the Ii TM region showed that when Ii TM residues 11-19 and 20-29 were individually substituted for the corresponding regions in the wild-type transferrin receptor, lysosomal targeting was dramatically enhanced, whereas endocytosis remained unchanged. Our results therefore demonstrate that the structural requirements for Ii endocytosis and lysosomal targeting are different.

The leucine-based motif DDQxxLI is recognized both for internalization and basolateral sorting of invariant chain in MDCK cells

The major histocompatibility complex (MHC) class II-associated invariant chain (Ii) contains signals for transport to endocytic compartments where the class II molecules bind antigenic peptides for presentation to CD4+ T cells. Two leucine-based signals in the Ii cytoplasmic tail can be independently recognized for endosomal sorting of Ii, and we have recently shown that each signal is sufficient for basolateral sorting and internalization of Ii in polarized Madine Darby Canine Kidney (MDCK) II cells. The recognition motif for endosomal sorting is complex and consists of two critical leucine-like residues as well as surrounding amino acids. Here, we have analyzed the importance of residues surrounding the membrane-distal leucine-based signal in basolateral sorting and internalization of Ii in MDCK II cells. We find that the DDQxxLI motif is involved in both sorting events indicating the presence of similar signal recognition components both at the TGN and at the plasma membrane. The identical motif is required for endosomal localization and internalization of Ii also in simian COS cells and the human HeLa and M1 cells.