A high-molecular-weight complex of membrane proteins BAP29/BAP31 is involved in the retention of membrane-bound IgD in the endoplasmic reticulum

Legionella uses host Rab GTPases and BAP31 to create a unique ER niche

Upon entry into host cells, the facultative intracellular bacterium Legionella pneumophila ( L.p .) uses its type IV secretion system, Dot/Icm, to secrete ~330 bacterial effector proteins into the host cell. Some of these effectors hijack endoplasmic reticulum (ER)-derived vesicles to form the Legionella -containing vacuole (LCV). Despite extensive investigation over decades, the fundamental question persists: Is the LCV membrane distinct from or contiguous with the host ER network? Here, we employ advanced photobleaching techniques, revealing a temporal acquisition of both smooth and rough ER (sER and rER) markers on the LCV. In the early stages of infection, the sER intimately associates with the LCV. Remarkably, as the infection progresses, the LCV evolves into a distinct niche comprising an rER membrane that is independent of the host ER network. We discover that the L.p. effector LidA binds to and recruits two host proteins of the Rab superfamily, Rab10, and Rab4, that play significant roles in acquiring sER and rER membranes, respectively. Additionally, we identify the pivotal role of a host ER-resident protein, BAP31, in orchestrating the transition from sER to rER. While previously recognized for shuttling between sER and rER, we demonstrate BAP31's role as a Rab effector, mediating communication between these ER sub-compartments. Furthermore, using genomic deletion strains, we uncover a novel L.p. effector, Lpg1152, essential for recruiting BAP31 to the LCV and facilitating its transition from sER to rER. Depletion of BAP31 or infection with an isogenic L.p. strain lacking Lpg1152 results in a growth defect. Collectively, our findings illuminate the intricate interplay between molecular players from both host and pathogen, elucidating how L.p. orchestrates the transformation of its residing vacuole membrane from a host-associated sER to a distinct rER membrane that is not contiguous with the host ER network.

Transmembrane Helices 7 and 8 Confer Aggregation Sensitivity to the Cystic Fibrosis Transmembrane Conductance Regulator

The Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) is a large multi-spanning membrane protein that is susceptible to misfolding and aggregation. We have identified here the region responsible for this instability. Temperature-induced aggregation of C-terminally truncated versions of CFTR demonstrated that all truncations up to the second transmembrane domain (TMD2), including the R region, largely resisted aggregation. Limited proteolysis identified a folded structure that was prone to aggregation and consisted of TMD2 and at least part of the Regulatory Region R. Only when both TM7 (TransMembrane helix 7) and TM8 were present, TMD2 fragments became as aggregation-sensitive as wild-type CFTR, in line with increased thermo-instability of late CFTR nascent chains and in silico prediction of aggregation propensity. In accord, isolated TMD2 was degraded faster in cells than isolated TMD1. We conclude that TMD2 extended at its N-terminus with part of the R region forms a protease-resistant structure that induces heat instability in CFTR and may be responsible for its limited intracellular stability.

BAP31 Knockout in Macrophages Affects CD4+T Cell Activation through Upregulation of MHC Class II Molecule

The differentiation of CD4+T cells is a crucial component of the immune response. The spleen and thymus, as immune organs, are closely associated with the differentiation and development of T cells. Previous studies have suggested that BAP31 may play a role in modulating T cell activation, but the specific impact of BAP31 on T cells through macrophages remains uncertain. In this study, we present evidence that BAP31 macrophage conditional knockout (BAP31-MCKO) mice display an enlarged spleen and thymus, accompanied by activated clustering and disrupted differentiation of CD4+T cells. In vitro co-culture studies were conducted to investigate the impact of BAP31-MCKO on the activation and differentiation of CD4+T cells. The examination of costimulatory molecule expression in BMDMs and RAW 264.7 cells, based on the endoplasmic reticulum function of BAP31, revealed an increase in the expression of antigen presenting molecules, particularly MHC-II molecule, in the absence of BAP31 in BMDMs or RAW264.7 cells. These findings suggest that BAP31 plays a role in the activation and differentiation of CD4+T cells by regulating the MHC class II molecule on macrophages. These results provide further support for the importance of BAP31 in developing interaction between macrophages and CD4+T cells.

Mitochondria-associated membranes: A hub for neurodegenerative diseases

In eukaryotic cells, organelles could coordinate complex mechanisms of signaling transduction metabolism and gene expression through their functional interactions. The functional domain between ER and mitochondria, called mitochondria-associated membranes (MAM), is closely associated with various physiological functions including intracellular lipid transport, Ca²⁺ transfer, mitochondria function maintenance, and autophagosome formation. In addition, more evidence suggests that MAM modulate cellular functions in health and disease. Studies have also demonstrated the association of MAM with numerous diseases, including neurodegenerative diseases, cancer, viral infection, obesity, and diabetes. In fact, recent evidence revealed a close relationship of MAM with Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), amyotrophic lateral sclerosis (ALS), and other neurodegenerative diseases. In this view, elucidating the role of MAM in neurodegenerative diseases is particularly important. This review will focus the main tethering protein complexes of MAM and functions of MAM. Besides, the role of MAM in the regulation of neurodegenerative diseases and the potential molecular mechanisms is introduced to provide a new understanding of the pathogenesis of these diseases.

Characterization of interactions within the Igα/Igβ transmembrane domains of the human B-cell receptor provides insights into receptor assembly

The B-cell receptor (BCR), a complex comprised of a membrane-associated immunoglobulin and the Igα/β heterodimer, is one of the most important immune receptors in humans and controls B-cell development, activity, selection, and death. BCR signaling plays key roles in autoimmune diseases and lymphoproliferative disorders, yet, despite the clinical significance of this protein complex, key regions (i.e., the transmembrane domains) have yet to be structurally characterized. The mechanism for BCR signaling also remains unclear and has been variously described by the mutually exclusive cross-linking and dissociation activation models. Common to these models is the significance of local plasma membrane composition, which implies that interactions between BCR transmembrane domains (TMDs) play a role in receptor functionality. Here we used an in vivo assay of TMD oligomerization called GALLEX alongside spectroscopic and computational methods to characterize the structures and interactions of human Igα and Igβ TMDs in detergent micelles and natural membranes. We observed weak self-association of the Igβ TMD and strong self-association of the Igα TMD, which scanning mutagenesis revealed was entirely stabilized by an E–X₁₀–P motif. We also demonstrated strong heterotypic interactions between the Igα and Igβ TMDs both in vitro and in vivo, which scanning mutagenesis and computational models suggest is multiconfigurational but can accommodate distinct interaction sites for self-interactions and heterotypic interactions of the Igα TMD. Taken together, these results demonstrate that the TMDs of the human BCR are sites of strong protein–protein interactions that may direct BCR assembly, endoplasmic reticulum retention, and immune signaling.

Maintenance of organellar protein homeostasis by ER-associated degradation and related mechanisms

Protein homeostasis mechanisms are fundamentally important to match cellular needs and to counteract stress conditions. A fundamental challenge is to understand how defective proteins are recognized and extracted from cellular organelles to be degraded in the cytoplasm. The endoplasmic reticulum (ER)-associated degradation (ERAD) pathway is the best-understood organellar protein quality control system. Here, we review new insights into the mechanism of recognition and retrotranslocation of client proteins in ERAD. In addition to the membrane-integral ERAD E3 ubiquitin ligases, we highlight one protein family that is remarkably often involved in various aspects of membrane protein quality control and protein dislocation: the rhomboid superfamily, which includes derlins and intramembrane serine proteases. Rhomboid-like proteins have been found to control protein homeostasis in the ER, but also in other eukaryotic organelles and in bacteria, pointing toward conserved principles of membrane protein quality control across organelles and evolution.

Spatiotemporally confined red light-controlled gene delivery at single-cell resolution using adeno-associated viral vectors

Methodologies for the controlled delivery of genetic information into target cells are of utmost importance for genetic engineering in both fundamental and applied research. However, available methods for efficient gene transfer into user-selected or even single cells suffer from low throughput, the need for complicated equipment, high invasiveness, or side effects by off-target viral uptake. Here, we engineer an adeno-associated viral (AAV) vector system that transfers genetic information into native target cells upon illumination with cell-compatible red light. This OptoAAV system allows adjustable and spatially resolved gene transfer down to single-cell resolution and is compatible with different cell lines and primary cells. Moreover, the sequential application of multiple OptoAAVs enables spatially resolved transduction with different transgenes. The approach presented is likely extendable to other classes of viral vectors and is expected to foster advances in basic and applied genetic research.

BAP31: Physiological functions and roles in disease

B-cell receptor-associated protein 31 (BAP31 or BCAP31) is a ubiquitously expressed transmembrane protein found mainly in the endoplasmic reticulum (ER), including in mitochondria-associated membranes (MAMs). It acts as a broad-specificity membrane protein chaperone and quality control factor, which can promote different fates for its clients, including ER retention, ER export, ER-associated degradation (ERAD), or evasion of degradation, and it also acts as a MAM tetherer and regulatory protein. It is involved in several cellular processes - it supports ER and mitochondrial homeostasis, promotes proliferation and migration, plays several roles in metabolism and the immune system, and regulates autophagy and apoptosis. Full-length BAP31 can be anti-apoptotic, but can also mediate activation of caspase-8, and itself be cleaved by caspase-8 into p20-BAP31, which promotes apoptosis by mobilizing ER calcium stores at MAMs. BAP31 loss-of-function mutations is the cause of 'deafness, dystonia, and central hypomyelination' (DDCH) syndrome, characterized by severe neurological symptoms and early death. BAP31 is furthermore implicated in a growing number of cancers and other diseases, and several viruses have been found to target it to promote their survival or life cycle progression. The purpose of this review is to provide an overview and examination of the basic properties, functions, mechanisms, and roles in disease of BAP31.

Biological roles of the B cell receptor-associated protein 31: Functional Implication in Cancer

BAP31 is a ubiquitously expressed integral membrane protein of the endoplasmic reticulum. BAP31 is involved in various biological and molecular processes, including protein transport, viral processing, apoptosis signaling, MHC 1 antigen processing and presentation, mitochondria and ER calcium regulation, and proteasomal protein degradation. We employed a BAP31 interaction search using STRING and inBioMap™ protein-protein interaction networks, and the Metabolic Atlas, which revealed molecular and metabolic interactors involved in various pathways essential for cell growth, cell survival, and disease development. BAP31, as a chaperone and resident protein of the ER, was reported in the development of some central nervous system disorders and metabolic diseases about AD, ALS, and Liver disease. In addition, BAP31 is overexpressed in many cancers. Furthermore, research around BAP31 involvement in cancer has taken up a shape, focusing on its roles in cancer cell survival, disease prognosis, and targeted treatment. Here, we address published data on the Biological roles of BAP31 in both health and disease. We present an analytical description of BAP31 expression and functional implication in some human cancers and the impact of its expression and regulation while it models as a potential target in cancer therapy. Besides, a profound understanding of BAP31 is insightful of the gap between cancer development and neurodegeneration, thus generating novel ideas surrounding the link between the two different cell phenomena.

Mechanistic Connections between Endoplasmic Reticulum (ER) Redox Control and Mitochondrial Metabolism

The past decade has seen the emergence of endoplasmic reticulum (ER) chaperones as key determinants of contact formation between mitochondria and the ER on the mitochondria-associated membrane (MAM). Despite the known roles of ER–mitochondria tethering factors like PACS-2 and mitofusin-2, it is not yet entirely clear how they mechanistically interact with the ER environment to determine mitochondrial metabolism. In this article, we review the mechanisms used to communicate ER redox and folding conditions to the mitochondria, presumably with the goal of controlling mitochondrial metabolism at the Krebs cycle and at the electron transport chain, leading to oxidative phosphorylation (OXPHOS). To achieve this goal, redox nanodomains in the ER and the interorganellar cleft influence the activities of ER chaperones and Ca²⁺-handling proteins to signal to mitochondria. This mechanism, based on ER chaperones like calnexin and ER oxidoreductases like Ero1α, controls reactive oxygen production within the ER, which can chemically modify the proteins controlling ER–mitochondria tethering, or mitochondrial membrane dynamics. It can also lead to the expression of apoptotic or metabolic transcription factors. The link between mitochondrial metabolism and ER homeostasis is evident from the specific functions of mitochondria–ER contact site (MERC)-localized Ire1 and PERK. These functions allow these two transmembrane proteins to act as mitochondria-preserving guardians, a function that is apparently unrelated to their functions in the unfolded protein response (UPR). In scenarios where ER stress cannot be resolved via the activation of mitochondrial OXPHOS, MAM-localized autophagosome formation acts to remove defective portions of the ER. ER chaperones such as calnexin are again critical regulators of this MERC readout.

BCAP31 drives TNBC development by modulating ligand-independent EGFR trafficking and spontaneous EGFR phosphorylation

Identification of novel targets for triple-negative breast cancer (TNBC) is an urgent task as targeted therapies have increased the lifespans of Oestrogen Receptor ⁺/ Progesterone Receptor ⁺ and HER²⁺ cancer patients. Methods: genes involved in protein processing in the endoplasmic reticulum, which have been reported to be key players in cancer, were used in loss-of-function screening to evaluate the oncogenic roles of these genes to identify candidate target genes in TNBC. In vitro and in vivo function assays as well as clinical prognostic analysis were used to study the oncogenic role of the gene. Molecular and cell based assays were further employed to investigate the mechanisms. Results: B Cell Receptor Associated Protein 31 (BCAP31), the expression of which is correlated with early recurrence and poor survival among patients, was identified an oncogene in our assay. In vitro studies further suggested that BCAP31 acts as a key oncogene by promoting TNBC development. We also showed that BCAP31 interacts with epidermal growth factor receptor (EGFR) and serves as an inhibitor of ligand-independent EGFR recycling, sustaining EGFR autophosphorylation and activation of downstream signalling. Conclusion: These findings reveal the functional role of BCAP31, an ER-related protein, in EGFR dysregulation and TNBC development.

Assembly-induced folding regulates interleukin 12 biogenesis and secretion

Members of the IL-12 family perform essential functions in immunoregulation by connecting innate and adaptive immunity and are emerging therapeutic targets. They are unique among other interleukins in forming heterodimers that arise from extensive subunit sharing within the family, leading to the production of at least four functionally distinct heterodimers from only five subunits. This raises important questions about how the assembly of IL-12 family members is regulated and controlled in the cell. Here, using cell-biological approaches, we have dissected basic principles that underlie the biogenesis of the founding member of the family, IL-12. Within the native IL-12 heterodimer, composed of IL-12α and IL-12β, IL-12α possesses three intramolecular and one intermolecular disulfide bridges. We show that, in isolation, IL-12α fails to form its native structure but, instead, misfolds, forming incorrect disulfide bonds. Co-expression of its β subunit inhibits misfolding and thus allows secretion of biologically active heterodimeric IL-12. On the basis of these findings, we identified the disulfide bonds in IL-12α that are critical for assembly-induced secretion and biological activity of IL-12 versus misfolding and degradation of IL-12α. Surprisingly, two of the three disulfide bridges in IL-12α are dispensable for IL-12 secretion, stability, and biological activity. Extending our findings, we show that misfolding also occurs for IL-23α, another IL-12 family protein. Our results indicate that assembly-induced folding is key in IL-12 family biogenesis and secretion. The identification of essential disulfide bonds that underlie this process lays the basis for a simplified yet functional IL-12 cytokine.

Molecular Characterization of Two Monoclonal Antibodies against the Same Epitope on B-Cell Receptor Associated Protein 31

Previously, we showed that B-cell receptor associated protein 31 (BAP31), an endoplasmic reticulum (ER) membrane chaperone, is also expressed on the cell surface by two monoclonal antibodies (MAbs) 297-D4 and 144-A8. Both MAbs recognize the same linear epitope on the C-terminal domain of BAP31, although they were independently established. Here, flow cytometric analysis showed that 144-A8 had additional binding properties to some cells, as compared to 297-D4. Quantitative antigen binding assays also showed that 144-A8 had higher antigen binding capacity than 297-D4. Affinity measurement revealed that 144-A8 had 1.54-fold higher binding affinity than 297-D4. Analysis of the heavy- and light-chain variable region sequences of two MAbs revealed that both MAbs belonged to the same heavy chain (Igh-V3660 VH3) and light chain subgroup (IGKV21) with just two amino acid differences in each framework region, indicating that both MAbs arise from the same germline origin. Seven amino acid differences were found between the complementarity determining regions (CDRs) of the two MAbs. Molecular modeling of the epitope-paratope complexes revealed that the epitope appeared to reside in closer proximity to the CDRs of 144-A8 than to those of 297-D4 with the stronger hydrogen bond interactions with the former than the latter. More interestingly, an additional hydrophobic interaction appeared to be established between the leucine residue of epitope and the paratope of 144-A8, due to the substitution of H-Tyr101 for H-Phe101 in 144-A8. Thus, the different binding specificity and affinity of 144-A8 appeared to be due to the different hydrogen bonds and hydrophobic interaction induced by the alterations of amino acids in CDRs of 144-A8. The results provide molecular insights into how the binding specificities and affinities of antibodies evolve with the same epitope in different microenvironments.

Clipping or Extracting: Two Ways to Membrane Protein Degradation

Protein degradation is a fundamentally important process that allows cells to recognize and remove damaged protein species and to regulate protein abundance according to functional need. A fundamental challenge is to understand how membrane proteins are recognized and removed from cellular organelles. While most of our understanding of this mechanism comes from studies on p97/Cdc48-mediated protein dislocation along the endoplasmic reticulum (ER)-associated degradation (ERAD) pathway, recent studies have revealed intramembrane proteolysis to be an additional mechanism that can extract transmembrane segments. Here, we review these two principles in membrane protein degradation and discuss how intramembrane proteolysis, which introduces an irreversible step in protein dislocation, is used to drive regulated protein turnover.

B cell antigen receptors of the IgM and IgD classes are clustered in different protein islands that are altered during B cell activation

The B cell antigen receptors (BCRs) play an important role in the clonal selection of B cells and their differentiation into antibody-secreting plasma cells. Mature B cells have both immunoglobulin M (IgM) and IgD types of BCRs, which have identical antigen-binding sites and are both associated with the signaling subunits Igα and Igβ, but differ in their membrane-bound heavy chain isoforms. By two-color direct stochastic optical reconstruction microscopy (dSTORM), we showed that IgM-BCRs and IgD-BCRs reside in the plasma membrane in different protein islands with average sizes of 150 and 240 nm, respectively. Upon B cell activation, the BCR protein islands became smaller and more dispersed such that the IgM-BCRs and IgD-BCRs were found in close proximity to each other. Moreover, specific stimulation of one class of BCR had minimal effects on the organization of the other. These conclusions were supported by the findings from two-marker transmission electron microscopy and proximity ligation assays. Together, these data provide evidence for a preformed multimeric organization of BCRs on the plasma membrane that is remodeled after B cell activation.

Epitope Mapping of Antibodies Suggests the Novel Membrane Topology of B-Cell Receptor Associated Protein 31 on the Cell Surface of Embryonic Stem Cells: The Novel Membrane Topology of BAP31

When located in the endoplasmic reticulum (ER) membrane, B-cell receptor associated protein 31 (BAP31) is involved in the export of secreted proteins from the ER to the plasma membrane. In a previous study, we generated two monoclonal antibodies (mAbs), 297-D4 and 144-A8, that bound to surface molecules on human embryonic stem cells (hESCs), but not to surface molecules on mouse embryonic stem cells (mESCs). Subsequent studies revealed that the mAbs recognized BAP31 on the surface of hESCs. To investigate the membrane topology of BAP31 on the cell surface, we first examined the epitope specificity of 297-D4 and 144-A8, as well as a polyclonal anti-BAP31 antibody (α-BAP31). We generated a series of GST-fused BAP31 mutant proteins in which BAP31 was serially deleted at the C- terminus. GST-fused BAP31 mutant proteins were then screened to identify the epitopes targeted by the antibodies. Both 297-D4 and 144-A8 recognized C-terminal residues 208-217, while α-BAP31 recognized C-terminal residues 165-246, of BAP31 on hESCs, suggesting that the C-terminal domain of BAP31 is exposed on the cell surface. The polyclonal antibody α-BAP31 bound to mESCs, which confirmed that the C-terminal domain of BAP31 is also exposed on the surface of these cells. Our results show for the first time the novel membrane topology of cell surface-expressed BAP31 as the extracellular exposure of the BAP31 C-terminal domain was not predicted from previous studies.

Folding and Intramembraneous BRICHOS Binding of the Prosurfactant Protein C Transmembrane Segment

Surfactant protein C (SP-C) is a novel amyloid protein found in the lung tissue of patients suffering from interstitial lung disease (ILD) due to mutations in the gene of the precursor protein pro-SP-C. SP-C is a small α-helical hydrophobic protein with an unusually high content of valine residues. SP-C is prone to convert into β-sheet aggregates, forming amyloid fibrils. Nature's way of solving this folding problem is to include a BRICHOS domain in pro-SP-C, which functions as a chaperone for SP-C during biosynthesis. Mutations in the pro-SP-C BRICHOS domain or linker region lead to amyloid formation of the SP-C protein and ILD. In this study, we used an in vitro transcription/translation system to study translocon-mediated folding of the WT pro-SP-C poly-Val and a designed poly-Leu transmembrane (TM) segment in the endoplasmic reticulum (ER) membrane. Furthermore, to understand how the pro-SP-C BRICHOS domain present in the ER lumen can interact with the TM segment of pro-SP-C, we studied the membrane insertion properties of the recombinant form of the pro-SP-C BRICHOS domain and two ILD-associated mutants. The results show that the co-translational folding of the WT pro-SP-C TM segment is inefficient, that the BRICHOS domain inserts into superficial parts of fluid membranes, and that BRICHOS membrane insertion is promoted by poly-Val peptides present in the membrane. In contrast, one BRICHOS and one non-BRICHOS ILD-associated mutant could not insert into membranes. These findings support a chaperone function of the BRICHOS domain, possibly together with the linker region, during pro-SP-C biosynthesis in the ER.

B-cell receptor-associated protein 31 regulates human embryonic stem cell adhesion, stemness, and survival via control of epithelial cell adhesion molecule

B-Cell receptor-associated protein 31 (BAP31) regulates the export of secreted membrane proteins from the endoplasmic reticulum (ER) to the downstream secretory pathway. Previously, we generated a monoclonal antibody 297-D4 against the surface molecule on undifferentiated human embryonic stem cells (hESCs). Here, we found that 297-D4 antigen was localized to pluripotent hESCs and downregulated during early differentiation of hESCs and identified that the antigen target of 297-D4 was BAP31 on the hESC-surface. To investigate the functional role of BAP31 in hESCs, BAP31 expression was knocked down by small interfering RNA. BAP31 depletion impaired hESC self-renewal and pluripotency and drove hESC differentiation into multicell lineages. BAP31 depletion hindered hESC proliferation by arresting cell cycle at G0/G1 phase and inducing caspase-independent cell death. Interestingly, BAP31 depletion reduced hESC adhesion to extracellular matrix (ECM). Analysis of cell surface molecules showed decreased expression of epithelial cell adhesion molecule (EpCAM) in BAP31-depleted hESCs, while ectopic expression of BAP31 elevated the expression of EpCAM. EpCAM depletion also reduced hESC adhesion to ECM, arrested cell cycle at G0/G1 phase and induced cell death, producing similar effects to those of BAP31 depletion. BAP31 and EpCAM were physically associated and colocalized at the ER and cell surface. Both BAP31 and EpCAM depletion decreased cyclin D1 and E expression and suppressed PI3K/Akt signaling, suggesting that BAP31 regulates hESC stemness and survival via control of EpCAM expression. These findings provide, for the first time, mechanistic insights into how BAP31 regulates hESC stemness and survival via control of EpCAM expression.

A disulfide polymerized protein crystal

A protein crystal has been grown, which uniquely, is fully cross-linked by cysteine-mediated disulfide bonds along the c-axis.

Structural and biophysical characterization of the cytoplasmic domains of human BAP29 and BAP31

Two members of the B-cell associated 31 (BAP31) family are found in humans; BAP29 and BAP31. These are ubiquitously expressed receptors residing in the endoplasmic reticulum. BAP31 functions in sorting of membrane proteins and in caspase-8 mediated apoptosis, while BAP29 appears to mainly corroborate with BAP31 in sorting. The N-terminal half of these proteins is membrane-bound while the C-terminal half is cytoplasmic. The latter include the so called variant of death effector domain (vDED), which shares weak sequence homology with DED domains. Here we present two structures of BAP31 vDED determined from a single and a twinned crystal, grown at pH 8.0 and pH 4.2, respectively. These structures show that BAP31 vDED forms a dimeric parallel coiled coil with no structural similarity to DED domains. Solution studies support this conclusion and strongly suggest that an additional α-helical domain is present in the C-terminal cytoplasmic region, probably forming a second coiled coil. The thermal stability of BAP31 vDED is quite modest at neutral pH, suggesting that it may assemble in a dynamic fashion in vivo. Surprisingly, BAP29 vDED is partially unfolded at pH 7, while a coiled coil is formed at pH 4.2 in vitro. It is however likely that folding of the domain is triggered by other factors than low pH in vivo. We found no evidence for direct interaction of the cytoplasmic domains of BAP29 and BAP31.

Sampling the membrane: function of rhomboid-family proteins

Rhomboids constitute a conserved protein superfamily that specifically binds membrane proteins and directs them into various different cellular pathways ranging from regulated secretion to endoplasmic reticulum (ER)-associated degradation (ERAD). Rhomboid proteases are known to release protein domains from membranes by a cut in their membrane anchor, whereas an emerging new class of rhomboid-family proteins lacks key catalytic residues and is not proteolytically active. Recent work has shown that these rhomboid pseudoproteases, including iRhoms and derlins, bind membrane proteins to regulate their fate, but the underlying molecular mechanism is not known. This review summarizes recent advances in the molecular understanding of rhomboid-family proteins and discusses common principles in how they recognize and bind proteins in the plane of the membrane.

Where the endoplasmic reticulum and the mitochondrion tie the knot: the mitochondria-associated membrane (MAM)

More than a billion years ago, bacterial precursors of mitochondria became endosymbionts in what we call eukaryotic cells today. The true significance of the word "endosymbiont" has only become clear to cell biologists with the discovery that the endoplasmic reticulum (ER) superorganelle dedicates a special domain for the metabolic interaction with mitochondria. This domain, identified in all eukaryotic cell systems from yeast to man and called the mitochondria-associated membrane (MAM), has a distinct proteome, specific tethers on the cytosolic face and regulatory proteins in the ER lumen of the ER. The MAM has distinct biochemical properties and appears as ER tubules closely apposed to mitochondria on electron micrographs. The functions of the MAM range from lipid metabolism and calcium signaling to inflammasome formation. Consistent with these functions, the MAM is enriched in lipid metabolism enzymes and calcium handling proteins. During cellular stress situations, like an altered cellular redox state, the MAM alters its set of regulatory proteins and thus alters MAM functions. Notably, this set prominently comprises ER chaperones and oxidoreductases that connect protein synthesis and folding inside the ER to mitochondrial metabolism. Moreover, ER membranes associated with mitochondria also accommodate parts of the machinery that determines mitochondrial membrane dynamics and connect mitochondria to the cytoskeleton. Together, these exciting findings demonstrate that the physiological interactions between the ER and mitochondria are so bilateral that we are tempted to compare their relationship to the one of a married couple: distinct, but inseparable and certainly dependent on each other. In this paradigm, the MAM stands for the intracellular location where the two organelles tie the knot. Resembling "real life", the happy marriage between the two organelles prevents the onset of diseases that are characterized by disrupted metabolism and decreased lifespan, including neurodegeneration and cancer. This article is part of a Special Issue entitled: Mitochondrial dynamics and physiology.

Mechanisms for quality control of misfolded transmembrane proteins

To prevent the accumulation of misfolded and aggregated proteins, the cell has developed a complex network of cellular quality control (QC) systems to recognize misfolded proteins and facilitate their refolding or degradation. The cell faces numerous obstacles when performing quality control on transmembrane proteins. Transmembrane proteins have domains on both sides of a membrane and QC systems in distinct compartments must coordinate to monitor the folding status of the protein. Additionally, transmembrane domains can have very complex organization and QC systems must be able to monitor the assembly of transmembrane domains in the membrane. In this review, we will discuss the QC systems involved in repair and degradation of misfolded transmembrane proteins. Also, we will elaborate on the factors that recognize folding defects of transmembrane domains and what happens when misfolded transmembrane proteins escape QC and aggregate. This article is part of a Special Issue entitled: Protein Folding in Membranes.

BAP31 and BiP are essential for dislocation of SV40 from the endoplasmic reticulum to the cytosol

How non-enveloped viruses overcome host cell membranes is poorly understood. Here, we show that after endocytosis and transport to the endoplasmic reticulum (ER), but before crossing the ER membrane to the cytosol, incoming simian virus 40 particles are structurally remodelled leading to exposure of the amino-terminal sequence of the minor viral protein VP2. These hydrophobic sequences anchor the virus to membranes. A negatively charged residue, Glu 17, in the α-helical, membrane-embedded peptide is essential for infection, most likely by introducing an 'irregularity' recognized by the ER-associated degradation (ERAD) system for membrane proteins. Using a siRNA-mediated screen, the lumenal chaperone BiP and the ER-membrane protein BAP31 (both involved in ERAD) were identified as being essential for infection. They co-localized with the virus in discrete foci and promoted its ER-to-cytosol dislocation. Virus-like particles devoid of VP2 failed to cross the membrane. The results demonstrated that ERAD-factors assist virus transport across the ER membrane.

Multiple cytosolic and transmembrane determinants are required for the trafficking of SCAMP1 via an ER-Golgi-TGN-PM pathway

How polytopic plasma membrane (PM) proteins reach their destination in plant cells remains elusive. Using transgenic tobacco BY-2 cells, we previously showed that the rice secretory carrier membrane protein 1 (SCAMP1), an integral membrane protein with four transmembrane domains (TMDs), is localized to the PM and trans-Golgi network (TGN). Here, we study the transport pathway and sorting signals of SCAMP1 by following its transient expression in tobacco BY-2 protoplasts and show that SCAMP1 reaches the PM via an endoplasmic reticulum (ER)-Golgi-TGN-PM pathway. Loss-of-function and gain-of-function analysis of various green fluorescent protein (GFP) fusions with SCAMP1 mutations further demonstrates that: (i) the cytosolic N-terminus of SCAMP1 contains an ER export signal; (ii) the transmembrane domain 2 (TMD2) and TMD3 of SCAMP1 are essential for Golgi export; (iii) SCAMP1 TMD1 is essential for TGN-to-PM targeting; (iv) the predicted topology of SCAMP1 and its various mutants remain identical as demonstrated by protease protection assay. Therefore, both the cytosolic N-terminus and TMD sequences of SCAMP1 play integral roles in mediating its transport to the PM via an ER-Golgi-TGN pathway.

Membrane-Associated RING-CH proteins associate with Bap31 and target CD81 and CD44 to lysosomes

Membrane-associated RING-CH (MARCH) proteins represent a family of transmembrane ubiquitin ligases modulating intracellular trafficking and turnover of transmembrane protein targets. While homologous proteins encoded by gamma-2 herpesviruses and leporipoxviruses have been studied extensively, limited information is available regarding the physiological targets of cellular MARCH proteins. To identify host cell proteins targeted by the human MARCH-VIII ubiquitin ligase we used stable isotope labeling of amino-acids in cell culture (SILAC) to monitor MARCH-dependent changes in the membrane proteomes of human fibroblasts. Unexpectedly, we observed that MARCH-VIII reduced the surface expression of Bap31, a chaperone that predominantly resides in the endoplasmic reticulum (ER). We demonstrate that Bap31 associates with the transmembrane domains of several MARCH proteins and controls intracellular transport of MARCH proteins. In addition, we observed that MARCH-VIII reduced the surface expression of the hyaluronic acid-receptor CD44 and both MARCH-VIII and MARCH-IV sequestered the tetraspanin CD81 in endo-lysosomal vesicles. Moreover, gene knockdown of MARCH-IV increased surface levels of endogenous CD81 suggesting a constitutive involvement of this family of ubiquitin ligases in the turnover of tetraspanins. Our data thus suggest a role of MARCH-VIII and MARCH-IV in the regulated turnover of CD81 and CD44, two ubiquitously expressed, multifunctional proteins.

Structural and functional studies of Igalphabeta and its assembly with the B cell antigen receptor

The B cell antigen receptor (BCR) plays an essential role in all phases of B cell development. Here we show that the extracellular domains of murine and human Igbeta form an I-set immunoglobulin-like structure with an interchain disulfide between cysteines on their G strands. Structural and sequence analysis suggests that Igalpha displays a similar fold as Igbeta. An Igalphabeta heterodimer model was generated based on the unique disulfide-bonded Igbeta dimer. Solution binding studies showed that the extracellular domains of Igalphabeta preferentially recognize the constant region of BCR with mu chain specificity, suggesting a role for Igalphabeta to enhance BCRmu chain signaling. Cluster mutations on Igalpha, Igbeta, and a membrane-bound form of immunoglobulin (mIgM) based on the structural model identified distinct areas of potential contacts involving charged residues on both subunits of the coreceptor and the Cmu4 domain of mIgM. These studies provide the first structural model for understanding BCR function.

Protein sorting receptors in the early secretory pathway

Estimates based on proteomic analyses indicate that a third of translated proteins in eukaryotic genomes enter the secretory pathway. After folding and assembly of nascent secretory proteins in the endoplasmic reticulum (ER), the coat protein complex II (COPII) selects folded cargo for export in membrane-bound vesicles. To accommodate the great diversity in secretory cargo, protein sorting receptors are required in a number of instances for efficient ER export. These transmembrane sorting receptors couple specific secretory cargo to COPII through interactions with both cargo and coat subunits. After incorporation into COPII transport vesicles, protein sorting receptors release bound cargo in pre-Golgi or Golgi compartments, and receptors are then recycled back to the ER for additional rounds of cargo export. Distinct types of protein sorting receptors that recognize carbohydrate and/or polypeptide signals in secretory cargo have been characterized. Our current understanding of the molecular mechanisms underlying cargo receptor function are described.

Yet1p and Yet3p, the yeast homologs of BAP29 and BAP31, interact with the endoplasmic reticulum translocation apparatus and are required for inositol prototrophy

The mammalian B-cell receptor-associated proteins of 29 and 31 kDa (BAP29 and BAP31) are conserved integral membrane proteins that have reported roles in endoplasmic reticulum (ER) quality control, ER export of secretory cargo, and programmed cell death. In this study we investigated the yeast homologs of BAP29 and BAP31, known as Yet1p and Yet3p, to gain insight on cellular function. We found that Yet1p forms a complex with Yet3p (Yet complex) and that complex assembly was important for subunit stability and proper ER localization. The Yet complex was not efficiently packaged into ER-derived COPII vesicles and therefore does not appear to act as an ER export receptor. Instead, a fraction of the Yet complex was detected in association with the ER translocation apparatus (Sec complex). Specific mutations in the Sec complex or Yet complex influenced these interactions. Moreover, associations between the Yet complex and Sec complex were increased by ER stress and diminished when protein translocation substrates were depleted. Surprisingly, yet1Delta and yet3Delta mutant strains displayed inositol starvation-related growth defects. In accord with the biochemical data, these growth defects were exacerbated by a combination of certain mutations in the Sec complex with yet1Delta or yet3Delta mutations. We propose a model for the Yet-Sec complex interaction that places Yet1p and Yet3p at the translocation pore to manage biogenesis of specific transmembrane secretory proteins.

Analysis of novel phospho-ITAM specific antibodies in a S2 reconstitution system for TCR-CD3 signalling

The T cell antigen receptor (TCR-CD3) complex contains 12 different cytoplasmic tyrosines, each of which is part of an immunoreceptor tyrosine-based activation motif and thus occurs in similar sequence context. Since phosphorylation of individual tyrosines can be correlated with the quality of the T cell response, monitoring their phosphorylation is important. We thus generated novel antibodies against phospho-tyrosines of the TCR-CD3 complex and tested the specificity in a synthetic biology approach. We utilized the Drosophila S2 reconstitution system testing several kinases and stimulation conditions that lead to optimal phosphorylation of the TCR-CD3 subunit zeta. Expressing TCR-CD3 subunits and tyrosine mutants thereof we tested the specificity of the novel antibodies in Western blot and immunopurification experiments. In particular, we generated and characterized the monoclonal antibody EM-26 that specifically recognizes phosphorylation of the membrane proximal tyrosine of zeta (phospho-zetaY1) and antisera raised against the first and the second phospho-tyrosine of CD3epsilon (phospho-epsilonY1 and phospho-epsilonY2).

The Brichos domain of prosurfactant protein C can hold and fold a transmembrane segment

Prosurfactant protein C (proSP-C) is a 197-residue integral membrane protein, in which the C-terminal domain (CTC, positions 59-197) is localized in the endoplasmic reticulum (ER) lumen and contains a Brichos domain (positions 94-197). Mature SP-C corresponds largely to the transmembrane (TM) region of proSP-C. CTC binds to SP-C, provided that it is in nonhelical conformation, and can prevent formation of intracellular amyloid-like inclusions of proSP-C that harbor mutations linked to interstitial lung disease (ILD). Herein it is shown that expression of proSP-C (1-58), that is, the N-terminal propeptide and the TM region, in HEK293 cells results in virtually no detectable protein, while coexpression of CTC in trans yields SDS-soluble monomeric proSP-C (1-58). Recombinant human (rh) CTC binds to cellulose-bound peptides derived from the nonpolar TM region, but not the polar cytosolic part, of proSP-C, and requires >/=5-residues for maximal binding. Binding of rhCTC to a nonhelical peptide derived from SP-C results in alpha-helix formation provided that it contains a long TM segment. Finally, rhCTC and rhCTC Brichos domain shows very similar substrate specificities, but rhCTC(L188Q), a mutation linked to ILD is unable to bind all peptides analyzed. These data indicate that the Brichos domain of proSP-C is a chaperone that induces alpha-helix formation of an aggregation-prone TM region.

Rabies virus glycoprotein expression in Drosophila S2 cells. I: design of expression/selection vectors, subpopulations selection and influence of sodium butyrate and culture medium on protein expression

The cDNA encoding the rabies virus glycoprotein (RVGP) gene was cloned in expression plasmids under the control of the inductive metallothionein promoter. They were designed in order to bear or not a secretion signal (i) and a cDNA coding for the selection hygromycin. These vectors were transfected into S2 cells, cell populations selected and subpopulations were then obtained by reselection with hygromycin. Cell cultures were examined for kinetics of cell growth, detection of RVGP mRNA and expression of RVGP. All cell populations were shown to express the RVGP mRNA upon induction. S2MtRVGPHy cell population, transfected with one vector that contains RGPV gene and selection gene, was shown to express higher amounts of RVGP as evaluated by flow cytometry ( approximately 52%) and ELISA (0.64 microg/10(7)cells at day 7). Subpopulation selection allowed a higher RVGP expression, specially for the S2MtRVGPHy(+) (5.5 microg/10(7)cells at day 7). NaBu treatment leading to lower cell growth and higher RVGP expression allowed an even higher RVGP synthesis by S2MtRVGPHy(+) (8.4 microg/10(7)cells at day 7). SF900II medium leading to a higher S2MtRVGPHy(+)cell growth allowed a higher final RVGP synthesis in this cell culture. RVGP synthesis may be optimized by the expression/selection vectors design, cell subpopulations selection, chromatin exposure and culture medium employed.

Interaction of Bap31 and MHC class I molecules and their traffic out of the endoplasmic reticulum

The endoplasmic reticulum (ER) protein Bap31 associates with nascent class I MHC molecules. It appears to mediate the export of class I MHC molecules from the ER and may also be involved in their quality control. In this study, we use Förster resonance energy transfer and quantitative fluorescence imaging to show that in human, HeLa cells, Bap31 clusters with MHC class I (HLA-A2) molecules in the ER, and traffics via export vesicles to the ER/Golgi intermediate compartment. Förster resonance energy transfer between Bap31 and HLA-A2 and forward traffic increases when MHC class I molecules are loaded with a pulse of peptide. The increased forward traffic is blocked by overexpression of Bap29, a partner protein for Bap31, which localizes to the ER. Thus, in HeLa cells, Bap31 is involved in the exit of peptide-loaded MHC class I from the ER, and its function is regulated by its interaction with its homologue, Bap29.

Uroplakins in urothelial biology, function, and disease

Urothelium covers the inner surfaces of the renal pelvis, ureter, bladder, and prostatic urethra. Although morphologically similar, the urothelia in these anatomic locations differ in their embryonic origin and lineages of cellular differentiation, as reflected in their different uroplakin content, expandability during micturition, and susceptibility to chemical carcinogens. Previously thought to be an inert tissue forming a passive barrier between the urine and blood, urothelia have recently been shown to have a secretory activity that actively modifies urine composition. Urothelial cells express a number of ion channels, receptors, and ligands, enabling them to receive and send signals and communicate with adjoining cells and their broader environment. The urothelial surface bears specific receptors that not only allow uropathogenic E. coli to attach to and invade the bladder mucosa, but also provide a route by which the bacteria ascend through the ureters to the kidney to cause pyelonephritis. Genetic ablation of one or more uroplakin genes in mice causes severe retrograde vesicoureteral reflux, hydronephrosis, and renal failure, conditions that mirror certain human congenital diseases. Clearly, abnormalities of the lower urinary tract can impact the upper tract, and vice versa, through the urothelial connection. In this review, we highlight recent advances in the field of urothelial biology by focusing on the uroplakins, a group of urothelium-specific and differentiation-dependent integral membrane proteins. We discuss these proteins' biochemistry, structure, assembly, intracellular trafficking, and their emerging roles in urothelial biology, function, and pathological processes. We also call attention to important areas where greater investigative efforts are warranted.

Purification of the T cell antigen receptor and analysis by blue-native PAGE

The T cell antigen receptor (TCR) is a multi-protein complex composed of six different transmembrane subunits, which form complexes of various sizes on the surface of resting T cells. The stoichiometry of the smallest form was recently determined to be alphabetagammaepsilondeltaepsilonzetazeta, whereas that of the larger forms is unknown. The roles of the different forms and their ratios are poorly defined. Biochemical analyses to address these questions must focus on the detergent and the best native conditions to maintain the integrity of the complexes. Blue-native polyacrylamide gel electrophoresis (BN-PAGE) is a high-resolution native protein separation method that relies on the dye Coomassie blue to confer negative charge for separation. Using this powerful approach, the size, subunit composition and the relative abundance of the different TCR forms can be studied. We present here four methods to isolate the TCR in a native form and details to analyse it by BN-PAGE.

Gene expression profiles of European wild boar naturally infected with Mycobacterium bovis

Global gene expression profiles were analyzed in European wild boar naturally infected with Mycobacterium bovis. Spleen RNA was extracted from 23 M. bovis-infected and 17 uninfected animals and analyzed using a Pigoligoarray representing 20,400 genes. Differentially expressed sequences (N=161) were identified affecting cellular processes such as apoptosis, cell communication and signal transduction, cell growth and/or maintenance, cytoskeleton organization and biogenesis, DNA repair, immune response, metabolism and energy pathways, protein metabolism, regulation of cell proliferation, regulation of gene expression, regulation of nucleic acid metabolism, regulation of physiological processes, and transport. Real-time RT-PCR analysis of mRNA levels was used to corroborate microarray results of selected genes. Immune response genes were among the most represented differentially expressed sequences and were selected for further discussion. Beta-defensin 129, T-cell surface glycoprotein CD8 and B-cell receptor-associated protein 29 were overexpressed in infected animals. Lower expression levels of the immune response genes galectin-1, complement component C1qB and certain HLA class I and class II histocompatibility antigens and immunoglobulin chains were found in infected animals. This study identified new mechanisms by which naturally infected European wild boar respond to M. bovis infection and how the pathogen circumvents host immune responses to establish infection. Gene expression studies in naturally infected wildlife reservoirs of bovine tuberculosis are important for functional genomics and vaccine studies to aid in disease control in wildlife.

Proteasome inhibition compromises direct retention of cytochrome P450 2C2 in the endoplasmic reticulum

To determine whether protein degradation plays a role in the endoplasmic reticulum (ER) retention of cytochromes P450, the effects of proteasomal inhibitors on the expression and distribution of green fluorescent protein chimeras of CYP2C2 and related proteins was examined. In transfected cells, expression levels of chimeras of full-length CYP2C2 and its cytosolic domain, but not its N-terminal transmembrane sequence, were increased by proteasomal inhibition. Redistribution of all three chimeras from the reticular ER into a perinuclear compartment and, in a subset of cells, also to the cell surface was observed after proteasomal inhibition. Redistribution was blocked by the microtubular inhibitor, nocodazole, suggesting that redistribution to the cell surface followed the conventional vesicular transport pathway. Similar redistributions were detected for BAP31, a CYP2C2 binding chaperone; CYP2E1 and CYP3A4, which are also degraded by the proteasomal pathway; and for cytochrome P450 reductase, which does not undergo proteasomal degradation; but not for the ER membrane proteins, sec61 and calnexin. Redistribution does not result from saturation of an ER retention "receptor" since in some cases protein levels were unaffected. Proteasomal inhibition may, therefore, alter ER retention by affecting a protein critical for ER retention, either directly, or indirectly by affecting the composition of the ER membranes.

Transmembrane domain-dependent partitioning of membrane proteins within the endoplasmic reticulum

The length and hydrophobicity of the transmembrane domain (TMD) play an important role in the sorting of membrane proteins within the secretory pathway; however, the relative contributions of protein-protein and protein-lipid interactions to this phenomenon are currently not understood. To investigate the mechanism of TMD-dependent sorting, we used the following two C tail-anchored fluorescent proteins (FPs), which differ only in TMD length: FP-17, which is anchored to the endoplasmic reticulum (ER) membrane by 17 uncharged residues, and FP-22, which is driven to the plasma membrane by its 22-residue-long TMD. Before export of FP-22, the two constructs, although freely diffusible, were seen to distribute differently between ER tubules and sheets. Analyses in temperature-blocked cells revealed that FP-17 is excluded from ER exit sites, whereas FP-22 is recruited to them, although it remains freely exchangeable with the surrounding reticulum. Thus, physicochemical features of the TMD influence sorting of membrane proteins both within the ER and at the ER-Golgi boundary by simple receptor-independent mechanisms based on partitioning.

Influence of protein-protein interactions on the cellular localization of cytochrome P450

BACKGROUND

Microsomal CYPs are integral membrane proteins that are localized in the endoplasmic reticulum (ER), which is critical for their function. CYPs are co-translationally inserted into the rough ER membrane and are then either directly retained in the smooth ER or retained by a retrieval mechanism or targeted for ER-associated degradation. Protein-protein interactions are likely to be important for proper cellular targeting of CYPs.

OBJECTIVE

Progress in understanding the mechanisms of cellular targeting and ER retention of CYPs is reviewed with emphasis on the role of protein-protein interactions. Possible mechanisms of direct retention are the incorporation of CYPs into an immobile complex in the ER membrane, homooligomerization that prevents inclusion in transport vesicles, exclusion of CYP monomers from transport vesicles or targeting of CYPs to an ER subdomain away from sites of transport vesicle formation. Degradation of CYPs occurs either by lysosomal mechanisms or by the ubiquitin-proteasomal pathway.

METHODS

The scope of this review includes studies published in the research literature that have defined the targeting of CYPs to the ER, the retention of CYPs in the ER and the degradation of CYPs.

RESULTS/CONCLUSION

Targeting of CYPs to the ER is well understood and involves signal recognition particle-mediated delivery to the sec61 complex. The mechanism of ER retention of CYPs remains unclear, but self-oligomerization or binding to large immobile networks do not underlie ER retention of CYPs. An ER retention 'receptor' remains elusive, but BAP31 is important for the proper cellular localization of CYPs and Dap1p is a CYP-binding protein that is a candidate for such a receptor. Identification of protein binding partners of CYPs will be critical to understanding the mechanism of ER retention.

The subcellular localization of TRPP2 modulates its function

TRPP2, also known as polycystin-2, is a calcium permeable nonselective cation channel that is mutated in autosomal dominant polycystic kidney disease but has also been implicated in the regulation of cardiac development, renal tubular differentiation, and left-to-right (L-R) axis determination. For obtaining further insight into how TRPP2 exerts tissue-specific functions, this study took advantage of PACS-dependent trafficking of TRPP2 in zebrafish larvae. PACS proteins recognize an acidic cluster within the carboxy-terminal domain of TRPP2 that undergoes phosphorylation and mediate retrieval of TRPP2 to the Golgi and endoplasmic reticulum (ER). The interaction of human TRPP2 with PACS proteins can be inhibited by a Ser812Ala mutation (TRPP2(S812A)), thereby allowing TRPP2 to reach other subcellular compartments, and enhanced by a Ser812Asp mutation (TRPP2(S812D)), thereby trapping TRPP2 in the ER. It was found that the TRPP2(S812A) mutant rescued cyst formation of TRPP2-deficient zebrafish larvae to the same degree as wild-type TRPP2, whereas the TRPP2(S812D) mutant was significantly more effective in normalizing the distorted body axis of TRPP2-deficient fish. Surprisingly, the TRPP2(S812D) mutant rescued the abnormalities of L-R asymmetry more effectively than either wild-type or TRPP2(S812A), suggesting that the ER localization of TRPP2 plays an important role in the development of normal L-R asymmetry. Taken together, these findings support the hypothesis that TRPP2 assumes distinct subcellular localizations to exert tissue-specific functions.

Bap31 is an itinerant protein that moves between the peripheral endoplasmic reticulum (ER) and a juxtanuclear compartment related to ER-associated Degradation

Certain endoplasmic reticulum (ER)-associated degradation (ERAD) substrates with transmembrane domains are segregated from other ER proteins and sorted into a juxtanuclear subcompartment, known as the ER quality control compartment. Bap31 is an ER protein with three transmembrane domains, and it is assumed to be a cargo receptor for ER export of some transmembrane proteins, especially those prone to ERAD. Here, we show that Bap31 is a component of the ER quality control compartment and that it moves between the peripheral ER and a juxtanuclear ER or ER-related compartment distinct from the conventional ER-Golgi intermediate compartment. The third and second transmembrane domains of Bap31 are principally responsible for the movement to and recycling from the juxtanuclear region, respectively. This cycling was blocked by depolymerization of microtubules and disruption of dynein-dynactin function. Overexpression of Sar1p and Arf1 mutants affected Bap31 cycling, suggesting that this cycling pathway is related to the conventional vesicular transport pathways.

Molecular pathogenesis of alpha-1-antitrypsin deficiency-associated liver disease: a meeting review

In recent years, we have witnessed several important paradigm shifts in understanding the molecular basis of liver disease in alpha-1-antitrypsin (AT) deficiency. These shifts have become possible as a result of a number of advances in research on the cell biology of aggregation-prone mutant proteins and in research on the pathobiological mechanisms of liver disease in general. Late-breaking research in these areas was the subject of an AASLD/Alpha-1 Foundation Single Topic Conference in Atlanta, Georgia, on January 26 to 28, 2006. The conference was titled "Alpha-1-Antitrypsin Deficiency and Other Liver Diseases Caused by Aggregated Proteins." Investigators from all over the world, representing a broad array of scientific disciplines and perspectives, discussed the pathobiology of AT deficiency, mechanisms of cell injury in diseases associated with aggregation-prone proteins, pathways by which cells respond to protein aggregation and mislocalization, and mechanisms of liver injury in general and in diseases related to AT deficiency. A session of the meeting was devoted to novel therapeutic strategies being developed for AT deficiency as well as to strategies either in development or already being applied to the class of diseases associated with mutant proteins.