MHC class II-associated invariant chain-induced enlarged endosomal structures: a morphological study

De novo formation of early endosomes during Rab5-to-Rab7a transition

Rab5 and Rab7a are the main determinants of early and late endosomes and are important regulators of endosomal progression. The transport from early endosomes to late endosome seems to be regulated through an endosomal maturation switch, where Rab5 is gradually exchanged by Rab7a on the same endosome. Here, we provide new insight into the mechanism of endosomal maturation, for which we have discovered a stepwise Rab5 detachment, sequentially regulated by Rab7a. The initial detachment of Rab5 is Rab7a independent and demonstrates a diffusion-like first-phase exchange between the cytosol and the endosomal membrane, and a second phase, in which Rab5 converges into specific domains that detach as a Rab5 indigenous endosome. Consequently, we show that early endosomal maturation regulated through the Rab5-to-Rab7a switch induces the formation of new fully functional Rab5-positive early endosomes. Progression through stepwise early endosomal maturation regulates the direction of transport and, concomitantly, the homeostasis of early endosomes.

Highlighted Article: A crucial step in endosomal maturation is the exchange of Rab5 with Rab7a, and we show that this two-phase exchange is finalized by the formation of Rab5-positive early endosomes.

Invariant chain regulates endosomal fusion and maturation through an interaction with the SNARE Vti1b

The invariant chain (Ii, also known as CD74) is a multifunctional regulator of adaptive immune responses and is responsible for sorting major histocompatibility complex class I and class II (MHCI and MHCII, respectively) molecules, as well as other Ii-associated molecules, to a specific endosomal pathway. When Ii is expressed, endosomal maturation and proteolytic degradation of proteins are delayed and, in non-antigen presenting cells, the endosomal size increases, but the molecular mechanisms underlying this are not known. We identified that a SNARE, Vti1b, is essential for regulating these Ii-induced effects. Vti1b binds to Ii and is localized at the contact sites of fusing Ii-positive endosomes. Furthermore, truncated Ii lacking the cytoplasmic tail, which is not internalized from the plasma membrane, relocates Vti1b to the plasma membrane. Knockout of Ii in an antigen-presenting cell line was found to speed up endosomal maturation, whereas silencing of Vti1b inhibits the Ii-induced maturation delay. Our results suggest that Ii, by interacting with the SNARE Vti1b in antigen-presenting cells, directs specific Ii-associated SNARE-mediated fusion in the early part of the endosomal pathway that leads to a slower endosomal maturation for efficient antigen processing and MHC antigen loading.

TBC1D5 controls the GTPase cycle of Rab7b

Rab GTPases are key regulators of intracellular trafficking, and cycle between a GTP-bound active state and a GDP-bound inactive state. This cycle is regulated by guanine-nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). Several efforts have been made in connecting the correct GEFs and GAPs to their specific Rab. Here, we aimed to identify GAPs for Rab7b, the small GTPase involved in transport from late endosomes to the trans-Golgi. An siRNA screen targeting proteins containing TBC domains critical for Rab GAPs was performed and coupled to a phenotypic read-out that visualized the distribution of Rab7b. Silencing of TBC1D5 provided the strongest phenotype and this protein was subsequently validated in various in vitro and cell-based assays. TBC1D5 localizes to Rab7b-positive vesicles, interacts with Rab7b and has GAP activity towards Rab7b in vitro, which is further increased by retromer proteins. Similarly to the constitutively active mutant of Rab7b, inactivation of TBC1D5 also reduces the number of CI-MPR- and sortilin-positive vesicles. Together, the results show that TBC1D5 is a GAP for Rab7b in the control of endosomal transport to the trans-Golgi.This article has an associated First Person interview with the first author of the paper.

Endosomal binding kinetics of Eps15 and Hrs specifically regulate the degradation of RTKs

Activation of EGF-R and PDGF-R triggers autophosphorylation and the recruitment of Eps15 and Hrs. These two endosomal proteins are important for specific receptor sorting. Hrs is recruiting ubiquitinated receptors to early endosomes to further facilitate degradation through the ESCRT complex. Upon receptor activation Hrs becomes phosphorylated and is relocated to the cytosol, important for receptor degradation. In this work we have studied the endosomal binding dynamics of Eps15 and Hrs upon EGF-R and PDGF-R stimulation. By analysing the fluorescence intensity on single endosomes after ligand stimulation we measured a time-specific decrease in the endosomal fluorescence level of Eps15-GFP and Hrs-YFP. Through FRAP experiments we could further register a specific change in the endosomal-membrane to cytosol binding properties of Eps15-GFP and Hrs-YFP. This specific change in membrane fractions proved to be a redistribution of the immobile fraction, which was not shown for the phosphorylation deficient mutants. We here describe a mechanism that can explain the previously observed relocation of Hrs from the endosomes to cytosol after EGF stimulation and show that Eps15 follows a similar mechanism. Moreover, this specific redistribution of the endosomal protein binding dynamics proved to be of major importance for receptor degradation.

Misdirection of endosomal trafficking mediated by herpes simplex virus-encoded glycoprotein B

Herpes simplex virus (HSV)-encoded glycoprotein B (gB) is the most abundant protein in the viral envelope and promotes fusion of the virus with the cellular membrane. In the present study, we found that gB impacts on the major histocompatibility complex (MHC)-II pathway of antigen presentation by fostering homotypic fusion of early endosomes and trapping MHC-II molecules in these altered endosomes. By using an overexpression approach, we demonstrated that transient expression of gB induces giant vesicles of early endosomal origin, which contained Rab5, early endosomal antigen 1 (EEA1), and large amounts of MHC-II molecules [human leukocyte antigen (HLA)-DR, and HLA-DM], but no CD63. In HSV-1-infected and stably transfected cell lines that expressed lower amounts of gB, giant endosomes were not observed, but strongly increased amounts of HLA-DR and HLA-DM were found in EEA1⁺ early endosomes. We used these giant vesicles as a model system and revealed that gB interacts with Rab5 and EEA1, and that gB-induced homotypic fusion of early endosomes to giant endosomes requires phosphatidylinositol 3-phosphate, the activity of soluble N-ethylmaleimide-sensitive factor attachment protein receptors, and the cytosolic gB sequence ⁸⁸⁹YTQVPN⁸⁹⁴ We conclude that gB expression alters trafficking of molecules of the HLA-II processing pathway, which leads to increased retention of MHC-II molecules in early endosomal compartments, thereby intercepting antigen presentation.-Niazy, N., Temme, S., Bocuk, D., Giesen, C., König, A., Temme, N., Ziegfeld, A., Gregers, T. F., Bakke, O., Lang, T., Eis-Hübinger, A. M., Koch, N. Misdirection of endosomal trafficking mediated by herpes simplex virus-encoded glycoprotein B.

Substrate determinants of signal peptide peptidase-like 2a (SPPL2a)-mediated intramembrane proteolysis of the invariant chain CD74

Intramembrane proteolysis of CD74 by SPPL2a is essential for B- and dendritic cells. We show that CD74 is proteolysed in the luminal third of the transmembrane segment and identify determinants within its transmembrane and luminal membrane-proximal domain facilitating this cleavage.

Spatiotemporal Resolution of Rab9 and CI-MPR Dynamics in the Endocytic Pathway

Rab9 is a small GTPase that localizes to the trans-Golgi Network (TGN) and late endosomes. Its main function has long been connected to the recycling of mannose-6-phosphate receptors (MPRs). However, recent studies link Rab9 also to autophagy and lysosome biogenesis. In this paper, using confocal imaging, we characterize for the first time the live dynamics of the Rab9 constitutively active mutant, Rab9Q66L. We find that it localizes predominantly to late endosomes and that its expression in HeLa cells disperses TGN46 and cation-independent (CI-MPR) away from the Golgi yet, has no effect on the retrograde transport of CI-MPR. We also show that CI-MPR and Rab9 enter the endosomal pathway together at the transition stage between early, Rab5-positive, and late, Rab7a-positive, endosomes. CI-MPR localizes transiently to separate domains on these endosomes, where vesicles carrying CI-MPR attach and detach within seconds. Taken together, our results demonstrate that Rab9 mediates the delivery of CI-MPR to the endosomal pathway, entering the maturing endosome at the early-to-late transition.

Interferon-γ-inducible Rab20 regulates endosomal morphology and EGFR degradation in macrophages

IFN-γ is able to modulate endosome dynamics in myelocytic cells, but the molecular mechanisms behind this process remain to be elucidated. Rab20 is identified as part of the molecular machinery that links immune activation and control of endocytic function in macrophages.

Little is known about the molecular players that regulate changes in the endocytic pathway during immune activation. Here we investigate the role of Rab20 in the endocytic pathway during activation of macrophages. Rab20 is associated with endocytic structures, but the function of this Rab GTPase in the endocytic pathway remains poorly characterized. We find that in macrophages, Rab20 expression and endosomal association significantly increase after interferon-γ (IFN-γ) treatment. Moreover, IFN-γ and Rab20 expression induce a dramatic enlargement of endosomes. These enlarged endosomes are the result of homotypic fusion promoted by Rab20 expression. The expression of Rab20 or the dominant-negative mutant Rab20T19N does not affect transferrin or dextran 70 kDa uptake. However, knockdown of Rab20 accelerates epidermal growth factor (EGF) trafficking to LAMP-2–positive compartments and EGF receptor degradation. Thus this work defines a function for Rab20 in the endocytic pathway during immune activation of macrophages.

The human-specific invariant chain isoform Iip35 modulates Iip33 trafficking and function

The invariant chain (Ii) is a multifunctional protein, which has an essential role in the assembly and transport of major histocompatibility complex class II (MHC II) molecules. From a single gene, Ii is synthesized as four different isoforms: Iip33, Iip35, Iip41 and Iip43. Iip35 and Iip43 are specific to humans, and are formed due to an upstream alternative translation site, resulting in an N-terminal extension of 16 amino acids. This extension harbors a strong endoplasmic reticulum (ER) retention motif. Consequently, Iip35 or Iip43 expressed alone are retained in the ER, whereas Iip33 and Iip41 rapidly traffic to the endosomal pathway. Endogenously expressed, the four isoforms form mixed heterotrimers in the ER; however, mainly due to the absence of the Iip35/p43 isoforms in mice, little is known about how they influence general Ii function. In this study, we have co-expressed Iip33 and Iip35 in human cells with and without MHC II to gain a better understanding of how Iip35 isoform influences the cellular properties of Iip33. We find that Iip35 significantly affects the properties of Iip33. In the presence of Iip35, the transport of Iip33 out of the ER is delayed, its half-life is dramatically prolonged and its ability to induce enlarged endosomes and delayed endosomal maturation is abrogated.

Exposing the Specific Roles of the Invariant Chain Isoforms in Shaping the MHC Class II Peptidome

The peptide repertoire (peptidome) associated with MHC class II molecules (MHCIIs) is influenced by the polymorphic nature of the peptide binding groove but also by cell-intrinsic factors. The invariant chain (Ii) chaperones MHCIIs, affecting their folding and trafficking. Recent discoveries relating to Ii functions have provided insights as to how it edits the MHCII peptidome. In humans, the Ii gene encodes four different isoforms for which structure-function analyses have highlighted common properties but also some non-redundant roles. Another layer of complexity arises from the fact that Ii heterotrimerizes, a characteristic that has the potential to affect the maturation of associated MHCIIs in many different ways, depending on the isoform combinations. Here, we emphasize the peptide editing properties of Ii and discuss the impact of the various isoforms on the MHCII peptidome.

Metallophilic macrophages of the rodent thymus

For a very long time, we studied the metallophilic macrophages of the rodent thymus and in this review our results on morphological, histochemical, enzymehistochemical, immunohistochemical, ultrastructural and functional features of these cells, as well as the molecular regulation of their development, will be presented. Furthermore, the differences between species will also be discussed and the comparisons with similar/related cell types (metallophilic macrophages in the marginal sinus of the spleen, subcapsular sinus of the lymph nodes and germinal centers of secondary lymphoid follicles) will be made. Metallophilic macrophages are strategically positioned in the thymic cortico-medullary zone and are very likely to be involved in: (i) the metabolism, synthesis and production of bioactive lipids, most likely arachidonic acid metabolites, based on their histochemical and enzymehistochemical features, and (ii) the process of negative selection that occurs in the thymus, based on their ultrastructural features and their reactivity after the application of toxic or immunosuppressive/immunomodulatory agents. Taken together, their phenotypic and functional features strongly suggest that metallophilic macrophages play a significant role in the thymic physiology.

The fusion of early endosomes induces molecular-motor-driven tubule formation and fission

Organelles in the endocytic pathway interact and communicate through the crucial mechanisms of fusion and fission. However, any specific link between fusion and fission has not yet been determined. To study the endosomal interactions with high spatial and temporal resolution, we enlarged the endosomes by two mechanistically different methods: by expression of the MHC-class-II-associated chaperone invariant chain (Ii; or CD74) or Rab5, both of which increased the fusion rate of early endosomes and resulted in enlarged endosomes. Fast homotypic fusions were studied, and immediately after the fusion a highly active and specific tubule formation and fission was observed. These explosive tubule formations following fusion seemed to be a direct effect of fusion. The tubule formations were dependent on microtubule interactions, and specifically controlled by Kif16b and dynein. Our results show that fusion of endosomes is a rapid process that destabilizes the membrane and instantly induces molecular-motor-driven tubule formation and fission.

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.

The herpes simplex virus-1 encoded glycoprotein B diverts HLA-DR into the exosome pathway

Neutralizing Abs play an important role for immunity against HSV-1 infection. This branch of the immune response is initiated by MHC class II Ag presentation and activation of T cell help. In this study, we show that the HSV-1 encoded glycoprotein B (gB) manipulates the class II processing pathway by perturbing endosomal sorting and trafficking of HLA-DR (DR) molecules. Expression of gB in the human melanoma cell line Mel JuSo results in formation of enlarged DR(+) intracellular vesicles. Costaining of the vesicles revealed the presence of DR, gB, and the late endosomal marker CD63. The lumen of these late endosomal membranes shows a variable content, containing either gB or CD63, or both CD63 and gB. gB targets DR molecules on their biosynthetic route, after the MHC class II invariant chain is released from the DR heterodimer. gB-DR complexes were detected in a post-Golgi compartment and in exosomes, but not on the cell surface. Interestingly, increasing expression of gB strongly elevated the amount of DR and CD63 released into the exosome pathway. In conclusion, this is a previously undescribed mode of viral immune evasion involving hijacking of DR from its normal transport route to the cell surface, followed by viral-mediated release of DR into the exosome pathway.

MHC II and the endocytic pathway: regulation by invariant chain

The major histocompatibility complex (MHC) class I and II molecules perform vital functions in innate and adaptive immune responses towards invading pathogens. MHC class I molecules load peptides in the endoplasmatic reticulum (ER) and display them to the T cell receptors (TcR) on CD8(+) T lymphocytes. MHC class II molecules (MHC II) acquire their peptides in endosomes and present these to the TcR on CD4+ T lymphocytes. They are vital for the generation of humoral immune responses. MHC II assembly in the ER and trafficking to endosomes is guided by a specialized MHC II chaperone termed the invariant chain (Ii). Ii self-associates into a trimer in the ER, this provides a scaffold for the assembly of three MHC II heterodimers and blocks their peptide binding grooves, thereby avoiding premature peptide binding. Ii then transports the nascent MHC II to more or less specialized compartment where they can load peptides derived from internalized pathogens.

Cell-cycle-dependent binding kinetics for the early endosomal tethering factor EEA1

Early endosomal antigen 1 (EEA1) is a cytosolic protein that specifically binds to early endosomal membranes where it has a crucial role in the tethering process leading to homotypic endosome fusion. Green fluorescent protein-tagged EEA1 (EEA1-GFP) was bound to the endosomal membrane throughout the cell cycle, and measurements using fluorescent recovery after photobleaching showed two fractions: one rapidly exchanging with the cytosolic pool, and the other with a long half-life. The exchange consists of a release and binding process, and we have separated these two by using GFP and photoactivable GFP. The release rate was identical to the exchange rate, showing that the dissociation characteristics determine the cycling of this molecule. During mitosis, we found that the dissociation rate was markedly accelerated and, in addition, the long-lived fraction was markedly reduced. This indicates that a fusion arrest in mitosis is not the result of EEA1 not binding to early endosomes, but rather due to the marked shift in membrane-binding characteristics. This might be a general mechanism to fine-tune and control tethering and fusion of early endosomes.

Re-routing of the invariant chain to the direct sorting pathway by introduction of an AP3-binding motif from LIMP II

AP3 is a heteromeric adaptor protein complex involved in the biogenesis of late endosomal/lysosomal structures. It recognizes tyrosine- and leucine-based sorting signals present in the cytoplasmic tails or loops of a number of proteins and is thought to be responsible for the direct transport of these proteins from the Golgi network to late endosomal/lysosomal structures. We have previously reported (Rodionov, Höning, Silye, Kongsvik, von Figura, Bakke, 2002. Structural requirements for interactions between leucine-sorting signals and clathrin-associated adaptor protein complex AP3. J. Biol. Chem. 277, 47436-47443) that in vitro binding of AP3 to the leucine signals is dependent on the nature of three residues immediately upstream of the leucine signal and suggested that these three amino acids define whether the protein is sorted to endosomes via the plasma membrane (PM) or traffics directly to the late endosomes/lysosomes. In this paper, we show in vivo evidence that residues favoring AP3 binding introduced into a protein that is transported via the PM such as the invariant chain can re-route such protein into direct sorting to late endosomal/lysosomal structures.

In vivo control of endosomal architecture by class II-associated invariant chain and cathepsin S

The invariant chain (Ii) is a chaperone that regulates assembly and transport of class II MHC molecules. In the absence of the lysosomal protease cathepsin S (CatS), degradation of Ii is impaired and an Ii remnant that extends from the N terminus to about residue 110 accumulates in class II MHC-positive endosomal compartments, which are enlarged in size and lack multivesicular morphology. In primary B cells examined in vitro and in lymph nodes examined by immuno-electron microscopy, CatS controls architecture of class II-positive endosomal compartments. In a compound mutant mouse that lacks both CatS and Ii, the normal size of endosomes in class II-positive cells is restored, although internal endosomal membranes are absent. Proper degradation of Ii is thus essential for normal endosomal morphology in antigen-presenting cells in vivo.

The NH(2)-terminal transmembrane and lumenal domains of LGP85 are needed for the formation of enlarged endosomes/lysosomes

LGP85 is a lysosomal membrane protein possessing a type III topology and is also known as a member of the CD36 superfamily of proteins, such as CD36 and the scavenger-receptor BI (SR-BI). We have recently demonstrated that overexpression of LGP85 in various mammalian cell lines causes the enlargement of endosomal/lysosomal compartments (ELCs). Using chimeras and deletion mutants, we show here that the lumenal region of LGP85 is necessary, but not sufficient, for the development of ELCs. Effective formation of enlarged ELC was largely dependent on the presence of a preceding NH(2)-terminal transmembrane segment. Analyses of deletion mutants within the lumenal domain further revealed a requirement of the NH(2)-terminal transmembrane proximal lumenal region, with high sequence similarity with SR-BI for the enlargement of ELC. These results suggest that an interaction of the NH(2)-terminal transmembrane proximal lumenal domain of LGP85 with the inner leaflet of endosomal/lysosomal membranes through the connection with the transmembrane domain is an essential determinant for the regulation of endosomal/lysosomal membrane traffic. Interestingly, although the NH(2)-terminal transmembrane domain itself was not sufficient for the enlargement of ELCs, it appeared to be required for direct targeting of LGP85 from the trans-Golgi network to late endosomes/lysosomes. Taken together, these results indicate the involvement of distinct domain of LGP85 in the targeting to, and biogenesis and maintenance of, ELC.

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.

A role for the lysosomal membrane protein LGP85 in the biogenesis and maintenance of endosomal and lysosomal morphology

LGP85 (LIMP II) is a type III transmembrane glycoprotein that is located primarily in the limiting membranes of lysosomes and late endosomes. Despite being the abundant molecule of these compartments, whether LGP85 merely resides as one of the constituents of these membranes or plays a role in the regulation of endosome and lysosome biogenesis remains unclear. To elucidate these questions, we examined the effects of overexpression of LGP85 on the morphology and membrane traffic of the endosomal/lysosomal system. Here we demonstrate that overexpression of LGP85 causes an enlargement of early endosomes and late endosomes/lysosomes. Such a morphological alteration was not observed by overexpression of other lysosomal membrane proteins, LGP107 (LAMP-1) or LGP96 (LAMP-2), reflecting a LGP85-specific function. We further demonstrate that overexpression of LGP85 impairs the endocytic membrane traffic out of these enlarged compartments, which may be correlated with or account for the accumulation of cholesterol observed in these compartments. Interestingly, co-transfection of LGP85 and the dominant-negative form of Rab5b (Rab5bS34N) abolished the formation of large vacuoles, suggesting that the GTP-bound active form of Rab5b is involved in the enlargement of endosomal/lysosomal compartments induced by overexpression of LGP85. Thus, these findings provide important new insights into the role of LGP85 in the biogenesis and the maintenance of endosomes/lysosomes. We conclude that LGP85 may participate in reorganizing the endosomal/lysosomal compartments.

Invariant chain-induced B cell differentiation requires intramembrane proteolytic release of the cytosolic domain

Immature B cells differentiate in the spleen into mature B cells, a process that is essential for their participation in the immune response. Previously, we showed that the MHC class II chaperone, invariant chain (Ii), controls this differentiation to the mature stage. Ii cytosolic domain-induced B cell maturation involves activation of transcription mediated by the NF-kappaB p65/RelA homodimer and requires the B cell enriched coactivator, TAF(II)105. In this study we show that the cytosolic region of Ii is cleaved within the plane of the membrane to generate a cytosolic fragment, which is essential for NF-kappaB activation and B cell differentiation. Our results suggest that Ii functions as a membrane-bound inactive inducer of NF-kappaB transcription that is activated by intramembrane proteolytic cleavage.

The cytoplasmic tail of invariant chain regulates endosome fusion and morphology

The major histocompatibility complex class II associated invariant chain (Ii) has been shown to inhibit endocytic transport and to increase the size of endosomes. We have recently found that this property has a significant impact on antigen processing and presentation. Here, we show in a cell-free endosome fusion assay that expression of Ii can increase fusion after phosphatidylinositol 3-kinase activity is blocked by wortmannin. In live cells wortmannin was also not able to block formation of the Ii-induced enlarged endosomes. The effects of Ii on endosomal transport and morphology depend on elements within the cytoplasmic tail. Data from mutagenesis analysis and nuclear magnetic resonance-based structure calculations of the Ii cytoplasmic tail demonstrate that free negative charges that are not involved in internal salt bridges are essential for both interactions between the tails and for the formation of enlarged endosomes. This correlation indicates that it is interactions between the Ii cytoplasmic tails that are involved in endosome fusion. The combined data from live cells, cell-free assays, and molecular dynamic simulations suggest that Ii molecules on different vesicles can promote endosome docking and fusion and thereby control endosomal traffic of membrane proteins and endosomal content.

Uncoating ATPase Hsc70 is recruited by invariant chain and controls the size of endocytic compartments

Targeting of class II major histocompatibility complex molecules to endocytic compartments is mediated by their association with the invariant chain (Ii). Although the identity of certain sorting signals located in Ii's cytoplasmic tail is known, proteins that interact with Ii's cytoplasmic tail in living cells remain to be identified. Synthesis of a biotinylated trimeric Ii cytoplasmic tail allowed the retrieval of two proteins that interact with this domain. We identify one of them as the 70-kDa heat-shock cognate protein (hsc70), the uncoating ATPase of clathrin-coated vesicles, and the other as its mitochondrial homologue, the glucose-regulated protein grp75. Expression of Ii in COS cells results in the formation of large endocytic compartments. We observe extensive colocalization of hsc70 with Ii in these macrosomes. Expression of a dominant-negative (K71M) green fluorescent protein-tagged version of hsc70 counteracted the ability of Ii to modify the endocytic pathway, demonstrating an interaction in vivo of Ii with hsc70 as part of the machinery responsible for macrosome formation.

Mitotic partitioning of endosomes and lysosomes

BACKGROUND

Some of the mechanisms underlying cell division and partitioning of the cellular components into the daughter cells are well known. Within the endomembrane system, there is a general cessation of membrane traffic, including endocytosis and endosome fusion, at the onset of mitosis. However, the fate of endosomes and lysosomes during mitosis has been less well studied.

RESULTS

Using video and confocal microscopy of living cells, we show here that endosomes and lysosomes remain intact and separate during mitosis. The segregation into daughter cells takes place by coordinated movements, and during cytokinesis, these organelles accumulate in the vicinity of the microtubule organization center. However, partitioning into daughter cells is not more accurate than a calculated stochastic distribution, despite the apparent order to the process.

CONCLUSION

We conclude that partitioning of endosomes and lysosomes is an ordered, yet imprecise, process, and that the organelle copy number is maintained by the daughter cells.

The MHC class II-associated chicken invariant chain shares functional properties with its mammalian homologs

The nucleotide sequence of chicken invariant chain (Ii) was determined, and the amino acid sequence similarity with human Ii is 61%. Certain regions important for the biological function of human Ii are highly conserved between chicken and mammals. The cytoplasmic tail of chicken Ii fused to the plasma membrane reporter molecule neuraminidase relocated the protein to endosomes. Moreover, like the mammalian orthologs, the cytoplasmic tail was found to contain two independent leucine-based endosomal sorting signals. Chicken Ii was found to interact with human Ii and crosslinking studies also indicate that chicken Ii assembles as a trimer. The chicken Ii can furthermore bind the human MHC class II (HLA-DR1). Many of the functional properties between the chicken Ii and its mammalian orthologs are thus maintained in spite of their sequence differences.

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.

Cellular distribution of a mixed MHC class II heterodimer between DRalpha and a chimeric DObeta chain

Human MHC class II antigens include HLA-DR, -DQ, and -DP molecules that present antigens to CD4+ T cells, as well as the non-classical molecules HLA-DM and -DO. HLA-DM promotes peptide binding to class II molecules in endocytic compartments and HLA-DO, which is physically associated with HLA-DM in B lymphocytes, regulates HLA-DM function. Antibodies specific for the DObeta chain were obtained by immunization of mice with a heterodimer consisting of a chimeric DObeta chain (DR/DObeta), containing 18 N-terminal residues of DRbeta, paired with the DRalpha chain and isolated from transfected murine fibroblasts. The specificity of this serum for the DObeta chain and the lysosomal expression of the HLA-DO protein was confirmed using mutant human B cell lines lacking DR or DO molecules. The lysosomal localization of HLA-DO in human B cells contrasts with the cell surface expression of the mixed pair in transfected murine fibroblasts and raises questions concerning the role of the putative targeting motifs in HLA-DO. Transfection of the chimeric DR/DObeta chain along with DRalpha into human epithelial HeLa cells resulted in high levels of expression of the mixed isotypic pair at the surface of transfectants as well as in lysosomes. The same pattern was observed in HeLa cells transfected with the DObeta chimera and a DRa chain lacking the cytoplasmic tail. Taken together, these results suggest that functional sorting motifs exist in the DObeta chain but that the tight compartmentalization of HLA-DO observed inside B lymphocytes is controlled by the HLA-DOalpha chain and HLA-DM.

DR/CLIP (Class II-Associated Invariant Chain Peptides) and DR/Peptide Complexes Colocalize in Prelysosomes in Human B Lymphoblastoid Cells

In APCs, MHC class II molecules (MHC class II) bind antigenic peptides after HLA-DM mediated removal of CLIP. To characterize intracellular sites of peptide loading in human B lymphoblastoid cell lines, we conducted immunoelectron microscopy studies with Abs recognizing MHC class II associated with CLIP or bound peptide, respectively, together with Abs to HLA-DM and endocytic markers. The distribution of these molecules indicates that peptide binding occurs in compartments with characteristics of normal late endosomes, and in compartments that show characteristics of late endosomes, but are not detectably accessed by endocytosed BSA-gold. The latter compartments may represent or give rise to recycling vesicles that deliver peptide-loaded class II molecules to the cell surface. In addition, we have compared cells in which HLA-DM and HLA-DR interaction is defective with cells in which this interaction is intact, and find that DM/DR interaction is not required for the proper localization of either molecule to peptide-loading compartments.

A new triple-stranded alpha-helical bundle in solution: the assembling of the cytosolic tail of MHC-associated invariant chain

BACKGROUND

The invariant chain (li) is a transmembrane protein that associates with the major histocompatibility complex class II (MHC II) molecules in the endoplasmic reticulum. The cytosolic tail of li contains two leucine-based sorting motifs and is involved in sorting the MHC II molecules to the endosomal pathway where the peptide antigen is bound. This region of li also contributes to phenotypical changes in cells, such as the formation of large endocytic structures.

RESULTS

We report here the three-dimensional structure of a 27 amino acid peptide corresponding to the cytosolic tail of li. The structure was determined by nuclear magnetic resonance (NMR) spectroscopy using a computational strategy. At high concentration, this structure reveals a new triple-stranded alpha-helical bundle in which the helices, two parallel and one antiparallel, are almost coplanar. Trimerization is mediated by electrostatic interactions intercalated by three hydrophobic layers.

CONCLUSIONS

The new trimer fold, the first to be identified by NMR data alone, can be used to improve understanding of protein-protein interactions and to model multiple-helical transmembrane proteins and receptors. We suggest that interactions of the li cytosolic tails may form part of a mechanism that could cause the endosomal retention and enlarged endosomes induced by li.