TGN38-green fluorescent protein hybrid proteins expressed in stably transfected eukaryotic cells provide a tool for the real-time, in vivo study of membrane traffic pathways and suggest a possible role for ratTGN38

The CLN3 gene and protein: What we know

Background

One of the most important steps taken by Beyond Batten Disease Foundation in our quest to cure juvenile Batten (CLN3) disease is to understand the State of the Science. We believe that a strong understanding of where we are in our experimental understanding of the CLN3 gene, its regulation, gene product, protein structure, tissue distribution, biomarker use, and pathological responses to its deficiency, lays the groundwork for determining therapeutic action plans.

Objectives

To present an unbiased comprehensive reference tool of the experimental understanding of the CLN3 gene and gene product of the same name.

Methods

BBDF compiled all of the available CLN3 gene and protein data from biological databases, repositories of federally and privately funded projects, patent and trademark offices, science and technology journals, industrial drug and pipeline reports as well as clinical trial reports and with painstaking precision, validated the information together with experts in Batten disease, lysosomal storage disease, lysosome/endosome biology.

Results

The finished product is an indexed review of the CLN3 gene and protein which is not limited in page size or number of references, references all available primary experiments, and does not draw conclusions for the reader.

Conclusions

Revisiting the experimental history of a target gene and its product ensures that inaccuracies and contradictions come to light, long‐held beliefs and assumptions continue to be challenged, and information that was previously deemed inconsequential gets a second look. Compiling the information into one manuscript with all appropriate primary references provides quick clues to which studies have been completed under which conditions and what information has been reported. This compendium does not seek to replace original articles or subtopic reviews but provides an historical roadmap to completed works.

The Salmonella effector SseJ disrupts microtubule dynamics when ectopically expressed in normal rat kidney cells

Salmonella effector protein SseJ is secreted by Salmonella into the host cell cytoplasm where it can then modify host cell processes. Whilst host cell small GTPase RhoA has previously been shown to activate the acyl-transferase activity of SseJ we show here an un-described effect of SseJ protein production upon microtubule dynamism. SseJ prevents microtubule collapse and this is independent of SseJ's acyl-transferase activity. We speculate that the effects of SseJ on microtubules would be mediated via its known interactions with the small GTPases of the Rho family.

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.

Microtubules depolymerization caused by the CK1 inhibitor IC261 may be not mediated by CK1 blockage

The ubiquitously expressed serine/threonine specific casein kinase 1 (CK1) family plays important roles in the regulation of various physiological processes. Small-molecule inhibitors, such as the CK1δ/ε selectively inhibitor IC261, have been used to antagonize CK1 phosphorylation events in cells in many studies. Here we present data to show that, similarly to the microtubule destabilizing agent nocodazole, IC261 depolymerizes microtubules in interphase cells. IC261 treatment of interphase cells affects the morphology of the TGN and Golgi apparatus as well as the localization of CK1δ, which co-localizes with COPI positive membranes. IC261-induced depolymerization of microtubules is rapid, reversible and can be antagonized by pre-treatment of cells with taxol. At lower concentrations of IC261, mitotic spindle microtubule dynamics are affected; this leads to cell cycle arrest and, depending on the cellular background, to apoptosis in a dose-dependent manner. In addition, FACS analysis revealed that IC261 could induce apoptosis independent of cell cycle arrest. In summary this study provides additional and valuable information about various IC261-induced effects that could be caused by microtubule depolymerization rather than by inhibition of CK1. Data from studies that have used IC261 as an inhibitor of CK1 should be interpreted in light of these observations.

A murine retrovirus co-Opts YB-1, a translational regulator and stress granule-associated protein, to facilitate virus assembly

The Gag protein of the murine retrovirus mouse mammary tumor virus (MMTV) orchestrates the assembly of immature virus particles in the cytoplasm which are subsequently transported to the plasma membrane for release from the cell. The morphogenetic pathway of MMTV assembly is similar to that of Saccharomyces cerevisiae retrotransposons Ty1 and Ty3, which assemble virus-like particles (VLPs) in intracytoplasmic ribonucleoprotein (RNP) complexes. Assembly of Ty1 and Ty3 VLPs depends upon cellular mRNA processing factors, prompting us to examine whether MMTV utilizes a similar set of host proteins to facilitate viral capsid assembly. Our data revealed that MMTV Gag colocalized with YB-1, a translational regulator found in stress granules and P bodies, in intracytoplasmic foci. The association of MMTV Gag and YB-1 in cytoplasmic granules was not disrupted by cycloheximide treatment, suggesting that these sites were not typical stress granules. However, the association of MMTV Gag and YB-1 was RNA dependent, and an MMTV RNA reporter construct colocalized with Gag and YB-1 in cytoplasmic RNP complexes. Knockdown of YB-1 resulted in a significant decrease in MMTV particle production, indicating that YB-1 plays a role in MMTV capsid formation. Analysis by live-cell imaging with fluorescence recovery after photobleaching (FRAP) revealed that the population of Gag proteins localized within YB-1 complexes was relatively immobile, suggesting that Gag forms stable complexes in association with YB-1. Together, our data imply that the formation of intracytoplasmic Gag-RNA complexes is facilitated by YB-1, which promotes MMTV virus assembly.

IMPORTANCE

Cellular mRNA processing factors regulate the posttranscriptional fates of mRNAs, affecting localization and utilization of mRNAs under normal conditions and in response to stress. RNA viruses such as retroviruses interact with cellular mRNA processing factors that accumulate in ribonucleoprotein complexes known as P bodies and stress granules. This report shows for the first time that mouse mammary tumor virus (MMTV), a mammalian retrovirus that assembles intracytoplasmic virus particles, commandeers the cellular factor YB-1, a key regulator of translation involved in the cellular stress response. YB-1 is essential for the efficient production of MMTV particles, a process directed by the viral Gag protein. We found that Gag and YB-1 localize together in cytoplasmic granules. Functional studies of Gag/YB-1 granules suggest that they may be sites where virus particles assemble. These studies provide significant insights into the interplay between mRNA processing factors and retroviruses.

Transgenic Xenopus laevis for live imaging in cell and developmental biology

The stable transgenesis of genes encoding functional or spatially localized proteins, fused to fluorescent proteins such as green fluorescent protein (GFP) or red fluorescent protein (RFP), is an extremely important research tool in cell and developmental biology. Transgenic organisms constructed with fluorescent labels for cell membranes, subcellular organelles, and functional proteins have been used to investigate cell cycles, lineages, shapes, and polarity, in live animals and in cells or tissues derived from these animals. Genes of interest have been integrated and maintained in generations of transgenic animals, which have become a valuable resource for the cell and developmental biology communities. Although the use of Xenopus laevis as a transgenic model organism has been hampered by its relatively long reproduction time (compared to Drosophila melanogaster and Caenorhabditis elegans), its large embryonic cells and the ease of manipulation in early embryos have made it a historically valuable preparation that continues to have tremendous research potential. Here, we report on the Xenopus laevis transgenic lines our lab has generated and discuss their potential use in biological imaging.

Mutation conferring apical-targeting motif on AE1 exchanger causes autosomal dominant distal RTA

Mutations in SLC4A1 that mislocalize its product, the chloride/bicarbonate exchanger AE1, away from its normal position on the basolateral membrane of the α-intercalated cell cause autosomal dominant distal renal tubular acidosis (dRTA). We studied a family exhibiting dominant inheritance and defined a mutation (AE1-M909T) that affects the C terminus of AE1, a region rich in potential targeting motifs that are incompletely characterized. Expression of AE1-M909T in Xenopus oocytes confirmed preservation of its anion exchange function. Wild-type GFP-tagged AE1 localized to the basolateral membrane of polarized MDCK cells, but AE1-M909T localized to both the apical and basolateral membranes. Wild-type AE1 trafficked directly to the basolateral membrane without apical passage, whereas AE1-M909T trafficked to both cell surfaces, implying the gain of an apical-targeting signal. We found that AE1-M909T acquired class 1 PDZ ligand activity that the wild type did not possess. In summary, the AE1-M909T mutation illustrates the role of abnormal targeting in dRTA and provides insight into C-terminal motifs that govern normal trafficking of AE1.

Endoplasmic reticulum localization of DHHC palmitoyltransferases mediated by lysine-based sorting signals

Intracellular palmitoylation dynamics are regulated by a family of 24 DHHC (aspartate-histidine-histidine-cysteine) palmitoyltransferases, which are localized in a compartment-specific manner. The majority of DHHC proteins localize to endoplasmic reticulum (ER) and Golgi membranes, and a small number target to post-Golgi membranes. To date, there are no reports of the fine mapping of sorting signals in mammalian DHHC proteins; thus, it is unclear how spatial distribution of the DHHC family is achieved. Here, we have identified and characterized lysine-based sorting signals that determine the restricted localization of DHHC4 and DHHC6 to ER membranes. The ER targeting signal in DHHC6 conforms to a KKXX motif, whereas the signal in DHHC4 is a distinct KXX motif. The identified dilysine signals are sufficient to specify ER localization as adding the C-terminal pentapeptide sequences from DHHC4 or DHHC6, which contain these KXX and KKXX motifs, to the C terminus of DHHC3, redistributes this palmitoyltransferase from Golgi to ER membranes. Recent work proposed that palmitoylation of newly synthesized peripheral membrane proteins occurs predominantly at the Golgi. Indeed, previous analyses of the peripheral membrane proteins, SNAP25 and cysteine string protein, are fully consistent with their initial palmitoylation being mediated by Golgi-localized DHHC proteins. Interestingly, ER-localized DHHC3 is able to palmitoylate SNAP25 and cysteine string protein to a similar level as wild-type Golgi-localized DHHC3 in co-expression studies. These results suggest that targeting of intrinsically active DHHC proteins to defined membrane compartments is an important factor contributing to spatially restricted patterns of substrate palmitoylation.

Glyceraldehyde 3-phosphate dehydrogenase is required for band 3 (anion exchanger 1) membrane residency in the mammalian kidney

The mammalian kidney isoform of the essential chloride-bicarbonate exchanger AE1 differs from its erythrocyte counterpart, being shorter at its N terminus. It has previously been reported that the glycolytic enzyme GAPDH interacts only with erythrocyte AE1, by binding to the portion not found in the kidney isoform. (Chu H, Low PS. Biochem J 400:143–151, 2006). We have identified GAPDH as a candidate binding partner for the C terminus of both AE1 and AE2. We show that full-length AE1 and GAPDH coimmunoprecipitated from both human and rat kidney as well as from Madin-Darby canine kidney (MDCK) cells stably expressing kidney AE1, while in human liver, AE2 coprecipitated with GAPDH. ELISA and glutathione S-transferase (GST) pull-down assays using GST-tagged C-terminal AE1 fusion protein confirmed that the interaction is direct; fluorescence titration revealed saturable binding kinetics with K_d 2.3 ± 0.2 μM. Further GST precipitation assays demonstrated that the D⁹⁰²EY residues in the D⁹⁰²EYDE motif located within the C terminus of AE1 are important for GAPDH binding. In vitro GAPDH activity was unaffected by C-terminal AE1 binding, unlike in erythrocytes. Also, differently from red cell N-terminal binding, GAPDH-AE1 C-terminal binding was not disrupted by phosphorylation of AE1 in kidney AE1-expressing MDCK cells. Importantly, small interfering RNA knockdown of GAPDH in these cells resulted in significant intracellular retention of AE1, with a concomitant reduction in AE1 at the cell membrane. These results indicate differences between kidney and erythrocyte AE1/GAPDH behavior and show that in the kidney, GAPDH is required for kidney AE1 to achieve stable basolateral residency.

γ-secretase-dependent cleavage initiates notch signaling from the plasma membrane

Notch signaling is critical to animal development, and its dysregulation leads to human maladies ranging from birth defects to cancer. Although endocytosis is currently thought to promote signal activation by delivering activated Notch to endosome-localized gamma-secretase, the data are controversial and the mechanisms that control Notch endocytosis remain poorly defined. Here, we investigated the relationship between Notch internalization and signaling. siRNA-mediated depletion studies reveal that Notch endocytosis is clathrin-dependent and requires epsin1, the adaptor protein complex (AP2) and Nedd4. Moreover, we show that epsin1 interaction with Notch is ubiquitin-dependent. Contrary to the current model, we show that internalization defects lead to elevated gamma-secretase-mediated Notch processing and downstream signaling. These results indicate that signal activation occurs independently of Notch endocytosis and that gamma-secretase cleaves Notch at the plasma membrane. These observations support a model where endocytosis serves to downregulate Notch in signal-receiving cells.

Expression of plasmid DNA in the salivary gland epithelium: novel approaches to study dynamic cellular processes in live animals

The ability to dynamically image cellular and subcellular structures in a live animal and to target genes to a specific cell population in a living tissue provides a unique tool to address many biological questions in the proper physiological context. Here, we describe a powerful approach that is based on the use of rat submandibular salivary glands, which offer the possibility to easily perform intravital imaging and deliver molecules from the oral cavity, and plasmid DNA, which offers the advantage of rapid manipulations. We show that, under different experimental conditions, a reporter molecule can be rapidly expressed in specific compartments of the glands: 1) in the intercalated ducts, when plasmid DNA is administered alone, and 2) in granular ducts, striated ducts, and, to a lesser extent, acini, when plasmid DNA is mixed with replication-deficient adenovirus subtype 5 particles. Remarkably, we also found that gene expression can be directed to acinar cells when plasmid DNA is administered during isoproterenol-stimulated exocytosis, suggesting a novel mechanism of plasmid internalization regulated by compensatory endocytosis. Finally, as a practical application of these strategies, we show how the expression of fluorescently tagged molecules enables the study of the dynamics of various organelles in live animals at a resolution comparable to that achieved in cell cultures.

Clathrin-mediated endocytosis of a lipid-raft-associated protein is mediated through a dual tyrosine motif

We have previously shown that the integral membrane protein CD317 has both a conventional transmembrane domain near its N-terminus and a C-terminal glycosyl-phosphatidylinositol (GPI) anchor. With the possible exception of a minor topological variant of the prion protein, there remain no other convincing examples of a mammalian protein with such a topology. CD317 is localised to cholesterol-rich lipid microdomains (`lipid rafts') in the plasma membrane and is internalised from the cell surface for delivery to a juxta-nuclear compartment (most probably the TGN). We have now investigated the mechanism by which CD317 is internalised and find that this raft-associated integral membrane protein is internalised through a clathrin-dependent pathway, internalisation is dependent upon a novel dual-tyrosine-based motif in the cytosolic domain of CD317, the cytosolic domain of CD317 can interact with the μ subunits of the AP2 and AP1 adaptor complexes, interaction with AP1 is required for delivery of CD317 back to the TGN, and removal of the GPI anchor from CD317 reduces the efficiency of CD317 internalisation. Collectively, these data indicate that CD317 is internalised and delivered back to the TGN by the sequential action of AP2 and AP1 adaptor complexes and that, surprisingly, the clathrin-mediated internalisation of CD317 occurs more efficiently if CD317 is localised to lipid rafts.

Mucolipin-1 is a lysosomal membrane protein required for intracellular lactosylceramide traffic

Mucolipin-1 is a membrane protein encoded by the gene MCOLN1, mutations in which result in the lysosomal storage disorder mucolipidosis type IV (MLIV). Efficient lysosomal targeting of mucolipin-1 requires di-leucine motifs in both the N-terminal and the C-terminal cytosolic tails. We have shown that aberrant lactosylceramide trafficking in MLIV cells may be rescued by wild-type mucolipin-1 expression but not by mucolipin-1 mistargeted to the plasma membrane or by lysosome-localized mucolipin-1 mutated in its predicted ion pore-selectivity region. Our data demonstrate that the correct localization of mucolipin-1 and the integrity of its ion pore are essential for its physiological function in the late endocytic pathway.

Casein kinase 1 delta (CK1delta) interacts with the SNARE associated protein snapin

In this study we identified snapin as an interaction partner of the CK1 isoform delta (CK1delta) in the yeast two-hybrid system and localized the interacting domains of both proteins. The interaction of CK1delta with snapin was confirmed by co-immunoprecipitation. Snapin was phosphorylated by CK1delta in vitro. Both proteins localized in close proximity in the perinuclear region, wherein snapin was found to associate with membranes of the Golgi apparatus. The identification of snapin as a new substrate of CK1delta points towards a possible function for CK1delta in modulating snapin specific functions.

Targeting of transmembrane protein shrew-1 to adherens junctions is controlled by cytoplasmic sorting motifs

We recently identified transmembrane protein shrew-1 and showed that it is able to target to adherens junctions in polarized epithelial cells. This suggested shrew-1 possesses specific basolateral sorting motifs, which we analyzed by mutational analysis. Systematic mutation of amino acids in putative sorting signals in the cytoplasmic domain of shrew-1 revealed three tyrosines and a dileucine motif necessary for basolateral sorting. Substitution of these amino acids leads to apical localization of shrew-1. By applying tannic acid to either the apical or basolateral part of polarized epithelial cells, thereby blocking vesicle fusion with the plasma membrane, we obtained evidence that the apically localized mutants were primarily targeted to the basolateral membrane and were then redistributed to the apical domain. Further support for a postendocytic sorting mechanism of shrew-1 was obtained by demonstrating that mu1B, a subunit of the epithelial cell-specific adaptor complex AP-1B, interacts with shrew-1. In conclusion, our data provide evidence for a scenario where shrew-1 is primarily delivered to the basolateral membrane by a so far unknown mechanism. Once there, adaptor protein complex AP-1B is involved in retaining shrew-1 at the basolateral membrane by postendocytic sorting mechanisms.

Multiphenotypic Whole-Cell Sensor for Viability Screening

Here, we describe the fabrication of whole mammalian cell biosensors for the optical monitoring of cell viability. Three phenotypes were examined for their response to the addition of two model chemotoxins: sodium hypochlorite and sodium azide, and one model biotoxin, concanavalin A. Two sensing platforms containing cells, hydrogel microspheres, or hydrogel arrays, were also explored. Step changes in viability in response to small doses of sodium hypochlorite were seen nearly instantaneously in all cell lines, in solution, microspheres, and microarrays. Linear detection of sodium azide by entrapped hepatocytes was 0-10 microM, whereas the linear detection range for macrophages and endothelial cells was 0-75 microM. Macrophages and hepatocytes have a greater sensitivity, as indicated by a 40% change in fluorescence over the linear range, whereas endothelial cells show only a 15% change in fluorescence over the linear range. Using photoreaction injection molding, we were also able to generate a multiphenotype sensor that enables the measurement of the toxic effect of 100 microg/mL concanavalin A on macrophages and hepatocytes, but not on endothelial cells.

Translation of an integral membrane protein in distal dendrites of hippocampal neurons

Maintenance of synaptic plasticity requires protein translation. Because changes in synaptic strength are regulated at the level of individual synapses, a mechanism is required for newly translated proteins to specifically and persistently modify only a subset of synapses. Evidence suggests this may be accomplished through local translation of proteins at or near synapses in response to plasticity-inducing patterns of activity. A number of proteins important for synaptic function are integral membrane proteins, which require a specialized group of organelles, proteins and enzymatic activities for proper synthesis. Dendrites appear to contain machinery necessary for the proper production of these proteins, and mRNAs for integral membrane proteins have been found localized to dendrites. Experiments are described that investigate the local translation of membrane proteins in the dendrites of cultured rat hippocampal neurons, using fluorescence recovery after photobleaching. Neurons were transfected with cDNAs encoding a fluorescently labeled transmembrane protein, TGN-38. Under conditions where the transport of this reporter construct was inhibited, the appearance of newly synthesized protein was observed via fluorescent microscopy. The dendritic translation of this protein required activation of glutamate receptors. The results demonstrate a functional capacity for activity-dependent synthesis of integral membrane proteins for distal dendrites in hippocampal neurons.

AP-1 and AP-3 facilitate lysosomal targeting of Batten disease protein CLN3 via its dileucine motif

CLN3 is a transmembrane protein with a predominant localization in lysosomes in non-neuronal cells but is also found in endosomes and the synaptic region in neuronal cells. Mutations in the CLN3 gene result in juvenile neuronal ceroid lipofuscinosis or Batten disease, which currently is the most common cause of childhood dementia. We have recently reported that the lysosomal targeting of CLN3 is facilitated by two targeting motifs: a dileucine-type motif in a cytoplasmic loop domain and an unusual motif in the carboxyl-terminal cytoplasmic tail comprising a methionine and a glycine separated by nine amino acids (Kyttala, A., Ihrke, G., Vesa, J., Schell, M. J., and Luzio, J. P. (2004) Mol. Biol. Cell 15, 1313-1323). In the present study, we investigated the pathways and mechanisms of CLN3 sorting using biochemical binding assays and immunofluorescence methods. The dileucine motif of CLN3 bound both AP-1 and AP-3 in vitro, and expression of mutated CLN3 in AP-1- or AP-3-deficient mouse fibroblasts showed that both adaptor complexes are required for sequential sorting of CLN3 via this motif. Our data indicate the involvement of complex sorting machinery in the trafficking of CLN3 and emphasize the diversity of parallel and sequential sorting pathways in the trafficking of membrane proteins.

Differential Use of Two AP-3-mediated Pathways by Lysosomal Membrane Proteins

The adaptor protein complex AP-3 is involved in the sorting of lysosomal membrane proteins to late endosomes/lysosomes. It is unclear whether AP-3-containing vesicles form at the trans-Golgi network (TGN) or early endosomes. We have compared the trafficking routes of endolyn/CD164 and 'typical' lysosomal membrane glycoproteins (lgp120/lamp-1 and CD63/lamp-3) containing cytosolic YXXPhi-targeting motifs preceded by asparagine and glycine, respectively. Endolyn, which has a NYHTL-motif, is concentrated in lysosomes, but also occurs in endosomes and at the cell surface. We observed predominant interaction of the NYHTL-motif with the mu-subunits of AP-3 in the yeast two-hybrid system. Endolyn was mislocalized to the cell surface in AP-3-deficient pearl cells, confirming a major role of AP-3 in endolyn traffic. However, lysosomal delivery of endolyn (or a NYHTL-reporter), but not GYXXPhi-containing proteins, was practically abolished when AP-2-mediated endocytosis or traffic from early to late endosomes was inhibited in NRK and 3T3 cells. This indicates that endolyn is mostly transported along the indirect lysosomal pathway (via the cell surface), rather than directly from the TGN to late endosomes/lysosomes. Our results suggest that AP-3 mediates lysosomal sorting of some membrane proteins in early endosomes in addition to sorting of proteins with intrinsically strong AP-3-interacting lysosomal targeting motifs at the TGN.

Rapid dendritic transport of TGN38, a putative cargo receptor

Protein transport to and from the postsynaptic plasma membrane is thought to be of central importance for synaptic plasticity. However, the molecular details of such processes are poorly understood. One mechanism by which membrane and secretory proteins may be transported to and from postsynaptic membranes is via cargo receptors. We studied the dendritic transport of TGN38, a putative cargo receptor thought to mediate protein transport between the trans-Golgi network (TGN), endosomes, and the plasma membrane. With fluorescence time-lapse imaging of neurons expressing a TGN38-green fluorescent protein fusion protein (GFP-TGN38), we observed rapid bidirectional dynamics of the protein in dendritic shafts. In addition, the protein was present on the surface and on intracellular membranes of dendrites and dendritic spines. Finally, GFP-TGN38 was found to cycle rapidly between the plasma membrane and intracellular membranes within dendrites, including those of spines. Together, our results suggest a role for TGN38 in facilitating rapid changes in the protein composition of postsynaptic membranes.

Visualization of retroviral replication in living cells reveals budding into multivesicular bodies

Retroviral assembly and budding is driven by the Gag polyprotein and requires the host-derived vacuolar protein sorting (vps) machinery. With the exception of human immunodeficiency virus (HIV)-infected macrophages, current models predict that the vps machinery is recruited by Gag to viral budding sites at the cell surface. However, here we demonstrate that HIV Gag and murine leukemia virus (MLV) Gag also drive assembly intracellularly in cell types including 293 and HeLa cells, previously believed to exclusively support budding from the plasma membrane. Using live confocal microscopy in conjunction with electron microscopy of cells generating fluorescently labeled virions or virus-like particles, we observed that these retroviruses utilize late endosomal membranes/multivesicular bodies as assembly sites, implying an endosome-based pathway for viral egress. These data suggest that retroviruses can interact with the vps sorting machinery in a more traditional sense, directly linked to the mechanism by which cellular proteins are sorted into multivesicular endosomes.

Two motifs target Batten disease protein CLN3 to lysosomes in transfected nonneuronal and neuronal cells

Batten disease is a neurodegenerative disorder resulting from mutations in CLN3, a polytopic membrane protein, whose predominant intracellular destination in nonneuronal cells is the lysosome. The topology of CLN3 protein, its lysosomal targeting mechanism, and the development of Batten disease are poorly understood. We provide experimental evidence that both the N and C termini and one large loop domain of CLN3 face the cytoplasm. We have identified two lysosomal targeting motifs that mediate the sorting of CLN3 in transfected nonneuronal and neuronal cells: an unconventional motif in the long C-terminal cytosolic tail consisting of a methionine and a glycine separated by nine amino acids [M(X)9G], and a more conventional dileucine motif, located in the large cytosolic loop domain and preceded by an acidic patch. Each motif on its own was sufficient to mediate lysosomal targeting, but optimal efficiency required both. Interestingly, in primary neurons, CLN3 was prominently seen both in lysosomes in the cell body and in endosomes, containing early endosomal antigen-1 along neuronal processes. Because there are few lysosomes in axons and peripheral parts of dendrites, the presence of CLN3 in endosomes of neurons may be functionally important. Endosomal association of the protein was independent of the two lysosomal targeting motifs.

Fluorogenic Peptide Sequences − Transformation of Short Peptides into Fluorophores under Ambient Photooxidative Conditions

Long-lived proteins are susceptible to nonenzymatic chemical reactions and the evolution of fluorescence; however, little is known about the sequence-dependence of fluorogenesis. We synthesized a library of over half a million octapeptides and exposed it to light and air in pH 7.4 buffer to identify fluorogenic peptides that evolve under mild oxidative conditions. The bead-based peptide library was composed of the general sequence H(2)N-Ala-(Xxx)(6)-Ala-resin, where Xxx was one of nine representative amino acids: Asp, Gly, His, Leu, Lys, Pro, Ser, Trp, and Tyr. Next, we selected five highly fluorescent beads from the library and subjected them to microsequencing, revealing the sequence of the unreacted peptide. All five of the fluorogenic sequences were ionic; lacked Tyr, His, and Leu; and most of the sequences contained only one Trp. We then synthesized the five soluble peptides corresponding to the fluorogenic peptide sequences and exposed them to photooxidative conditions. In general, the soluble peptides reacted slowly, generating nonfluorescent monooxygenated and dioxygenated products. However, one peptide (H(2)N-AlaLysProTrpGlyGlyAspAla-CONH(2)) evolved into a highly fluorescent photoproduct as well as a nonfluorescent monooxygenated photoproduct. The fluorescent photoproduct consisted of a 2-carboxy-quinolin-4-yl moiety fused to the N-terminus of GlyGlyAspAla. The formation of this photoproduct requires cleavage of the peptide backbone and a dramatic reorganization of tryptophan. This work demonstrates that sequencing unreacted peptide on beads can reveal sequences with unique nonenzymatic reactivity. The study also confirms that peptide fluorogenesis is dependent on sequence and not merely on the presence of tryptophan. The potential importance of fluorogenic peptide sequences is two-fold. First, fluorogenic sequences that arise through mutation could prove to be hot spots for human aging. Second, fluorogenic sequences, particularly those compatible with intracellular conditions, may serve as fluorescent tags for proteins or as fluorescent biomaterials.

Bst-2/HM1.24 is a raft-associated apical membrane protein with an unusual topology

An expression screen of a rat cDNA library for sequences encoding Golgi-localized integral membrane proteins identified a protein with an apparent novel topology, i.e. with both an N-terminal transmembrane domain and a C-terminal glycosyl-phosphatidylinositol (GPI) anchor. Our data are consistent with this. Thus, the protein would have a topology that, in mammalian cells, is shared only by a minor, but pathologically important, topological isoform of the prion protein (PrP). The human orthologue of this protein has been described previously (BST-2 or HM1.24 antigen) as a cell surface molecule that appears to be involved in early pre-B-cell development and which is present at elevated levels at the surface of myeloma cells. We show that rat BST-2/HM1.24 has both a cell surface and an intracellular (juxtanuclear) location and is efficiently internalized from the cell surface. We also show that the cell surface pool of BST-2/HM1.24 is predominantly present in the apical plasma membrane of polarized cells. The fact that rat BST-2/HM1.24 apparently possesses a GPI anchor led us to speculate that it might exist in cholesterol-rich lipid microdomains (lipid rafts) at the plasma membrane. Data from several experiments are consistent with this localization. We present a model in which BST-2/HM1.24 serves to link adjacent lipid rafts within the plasma membrane.

Intracellular sorting and transport of proteins

The secretory and endocytic pathways of eukaryotic organelles consist of multiple compartments, each with a unique set of proteins and lipids. Specific transport mechanisms are required to direct molecules to defined locations and to ensure that the identity, and hence function, of individual compartments are maintained. The localisation of proteins to specific membranes is complex and involves multiple interactions. The recent dramatic advances in understanding the molecular mechanisms of membrane transport has been due to the application of a multi-disciplinary approach, integrating membrane biology, genetics, imaging, protein and lipid biochemistry and structural biology. The aim of this review is to summarise the general principles of protein sorting in the secretory and endocytic pathways and to highlight the dynamic nature of these processes. The molecular mechanisms involved in this transport along the secretory and endocytic pathways are discussed along with the signals responsible for targeting proteins to different intracellular locations.

Quantitative identification of the protonation state of histidines in vitro and in vivo

The C[bond]N coupling constants centered at the C(epsilon 1) and C(delta 2) carbons in histidine residues depend on the protonation state and tautomeric form of the imidazole ring, making them excellent indicators of pH or pK(a), and the ratio of the tautomeric states. In this paper, we demonstrate that the intensity ratios for the C(epsilon 1)-H and C(delta 2)-H cross-peaks measured with a constant time HSQC experiment without and with J(C[bond]N) amplitude modulation are determined by the ratios of the protonated and deprotonated forms and tautomeric states. This allows one to investigate the tautomeric state of histidines as well as their pK(a) in situations where changing the pH value by titration is difficult, for example, for in-cell NMR experiments. We apply this technique to the investigation of the bacterial protein NmerA and determine that the intracellular pH in the Escherichia coli cytoplasm is 7.1 +/- 0.1.

Targeting of the human immunodeficiency virus type 1 envelope to the trans-Golgi network through binding to TIP47 is required for env incorporation into virions and infectivity

Here, we report that human immunodeficiency virus type 1 (HIV-1) Env glycoprotein is located mainly in the trans-Golgi network (TGN) due to determinants present in the cytoplasmic domain of the transmembrane gp41 glycoprotein (TMgp41). Internalization assays demonstrated that Env present at the cell surface returns to the TGN. We found that the cytoplasmic domain of TMgp41 binds to TIP47, a protein required for the transport of mannose-6-phosphate receptors from endosomes to the TGN. Overexpression of a mutant of TIP47 affected the transport of Env from endosomes to the TGN. Retrograde transport of Env to the TGN requires a Y(802)W(803) diaromatic motif present in the TMgp41 cytoplasmic domain. Mutation of this motif abolished both targeting to the TGN as well as interaction with TIP47. These data support the view that binding of TIP47 to HIV-1 Env facilitates its delivery to the TGN. Lastly, we show that virus mutated in the Y(802)W(803) motif is poorly infectious and presents a defect in Env incorporation, supporting a model in which retrograde transport of Env is implicated in the optimization of fully infectious HIV-1 production.

Determinant for endoplasmic reticulum retention in the luminal domain of the human cytomegalovirus US3 glycoprotein

US3 of human cytomegalovirus is an endoplasmic reticulum resident transmembrane glycoprotein that binds to major histocompatibility complex class I molecules and prevents their departure. The endoplasmic reticulum retention signal of the US3 protein is contained in the luminal domain of the protein. To define the endoplasmic reticulum retention sequence in more detail, we have generated a series of deletion and point mutants of the US3 protein. By analyzing the rate of intracellular transport and immunolocalization of the mutants, we have identified Ser58, Glu63, and Lys64 as crucial for retention, suggesting that the retention signal of the US3 protein has a complex spatial arrangement and does not comprise a contiguous sequence of amino acids. We also show that a modified US3 protein with a mutation in any of these amino acids maintains its ability to bind class I molecules; however, such mutated proteins are no longer retained in the endoplasmic reticulum and are not able to block the cell surface expression of class I molecules. These findings indicate that the properties that allow the US3 glycoprotein to be localized in the endoplasmic reticulum and bind major histocompatibility complex class I molecules are located in different parts of the molecule and that the ability of US3 to block antigen presentation is due solely to its ability to retain class I molecules in the endoplasmic reticulum.

Neural cell adhesion molecule promotes accumulation of TGN organelles at sites of neuron-to-neuron contacts

Transformation of a contact between axon and dendrite into a synapse is accompanied by accumulation of the synaptic machinery at this site, being delivered in intracellular organelles mainly of TGN origin. Here, we report that in cultured hippocampal neurons, TGN organelles are linked via spectrin to clusters of the neural cell adhesion molecule (NCAM) in the plasma membrane. These complexes are translocated along neurites and trapped at sites of initial neurite-to-neurite contacts within several minutes after initial contact formation. The accumulation of TGN organelles at contacts with NCAM-deficient neurons is reduced when compared with wild-type cells, suggesting that NCAM mediates the anchoring of intracellular organelles in nascent synapses.

Characterisation of the lumenal domain of TGN38 and effects of elevated expression of TGN38 on glycoprotein secretion

The TGN-localised, type I integral membrane protein TGN38 has previously been suggested to play a role as a cargo transporter within mammalian cells. We have undertaken a series of experiments designed to address this hypothesis, and, in so doing, have partially characterised the glycosylation status of the lumenal domain of TGN38. We find that elevated expression of different regions of the lumenal domain of TGN38 has no reproducible effect on secretion from stably transfected NRK cells expressing the different lumenal domain constructs; neither does it affect the gross morphology of organelles of the secretory and endocytic pathways. However, we observed that, whilst elevated expression of full-length TGN38 in stably transfected NRK cells does not have any significant effect on the morphology of organelles of the secretory and endocytic pathways, it does lead to a change in the pattern of protein secretion from these cells. In particular, elevated expression of full-length TGN38 led to increased secretion of a 48-kDa glycoprotein identified as plasminogen activator inhibitor-1.

The epithelial-specific adaptor AP1B mediates post-endocytic recycling to the basolateral membrane

To perform vectorial secretory and transport functions that are critical for the survival of the organism, epithelial cells sort plasma membrane proteins into polarized apical and basolateral domains. Sorting occurs post-synthetically, in the trans Golgi network (TGN) or after internalization from the cell surface in recycling endosomes, and is mediated by apical and basolateral sorting signals embedded in the protein structure. Basolateral sorting signals include tyrosine motifs in the cytoplasmic domain that are structurally similar to signals involved in receptor internalization by clathrin-coated pits. Recently, an epithelial-specific adaptor protein complex, AP1B, was identified. AP-1B recognizes a subset of basolateral tyrosine motifs through its mu 1B subunit. Here, we characterized the post-synthetic and post-endocytic sorting of the fast recycling low density lipoprotein receptor (LDLR) and transferrin receptor (TfR) in LLC-PK1 cells, which lack mu 1B and mis-sort both receptors to the apical surface. Targeting and recycling assays in LLC-PK1 cells, before and after transfection with mu 1B, and in MDCK cells, which express mu 1B constitutively, suggest that AP1B sorts basolateral proteins post-endocytically.

Stable expression of Anthozoa fluorescent proteins in mammalian cells

BACKGROUND

Fluorescent proteins have become invaluable reporters in many areas of cellular and developmental biology. An enhanced version of the Aequorea victoria green fluorescent protein (AvEGFP) is the most widely used fluorescent protein. For a variety of reasons, it is useful to have alternative fluorescent proteins to AvEGFP.

METHODS

The cDNA sequences for enhanced variants of the Anemonia cyan fluorescent protein (AmCyan1), as well as the Zoanthus green (ZsGreen1) and yellow (ZsYellow1) fluorescent proteins, were cloned downstream of a constitutive cytomegalovirus (CMV) promoter within a retroviral expression vector. NIH3T3, HEK293, SW620, and WM35 cells were transduced with recombinant retroviruses at a low multiplicity of infection (MOI) to bias for single-copy integration. Both unselected and stably selected cells transduced with the retroviral expression constructs were characterized. Expression of each fluorescent protein in cells was detected using flow cytometry and fluorescence microscopy with filter sets typically used for AvEGFP/fluorescein isothiocyanate (FITC) detection and was compared with the expression of AvEGFP. In addition, a fluorescence plate reader with several excitation and emission filter sets was used for detection.

RESULTS

Expression of each protein was observable by fluorescence microscopy. Under given conditions of flow cytometry, the ZsGreen1 mean fluorescence was approximately 3-fold, 10-fold, and 50-fold greater than that of AvEGFP, ZsYellow1, and AmCyan1, respectively. AmCyan1, ZsGreen1, and AvEGFP were detected by a fluorescence plate reader.

CONCLUSION

We determined that fluorescent proteins from Anthozoa species are detectable using a standard flow cytometer and fluorescence microscope. All of the mammalian cell lines tested expressed detectable levels of fluorescent proteins from stable integrated provirus. In cell lines where the AvEGFP protein is toxic or poorly expressed, these Anthozoa fluorescent proteins may serve as alternative fluorescent reporters.

Role of adaptor complex AP-3 in targeting wild-type and mutated CD63 to lysosomes

CD63 is a lysosomal membrane protein that belongs to the tetraspanin family. Its carboxyterminal cytoplasmic tail sequence contains the lysosomal targeting motif GYEVM. Strong, tyrosine-dependent interaction of the wild-type carboxyterminal tail of CD63 with the AP-3 adaptor subunit mu 3 was observed using a yeast two-hybrid system. The strength of interaction of mutated tail sequences with mu 3 correlated with the degree of lysosomal localization of similarly mutated human CD63 molecules in stably transfected normal rat kidney cells. Mutated CD63 containing the cytosolic tail sequence GYEVI, which interacted strongly with mu 3 but not at all with mu 2 in the yeast two-hybrid system, localized to lysosomes in transfected normal rat kidney and NIH-3T3 cells. In contrast, it localized to the cell surface in transfected cells of pearl and mocha mice, which have genetic defects in genes encoding subunits of AP-3, but to lysosomes in functionally rescued mocha cells expressing the delta subunit of AP-3. Thus, AP-3 is absolutely required for the delivery of this mutated CD63 to lysosomes. Using this AP-3-dependent mutant of CD63, we have shown that AP-3 functions in membrane traffic from the trans-Golgi network to lysosomes via an intracellular route that appears to bypass early endosomes.

Assembly and function of AP-3 complexes in cells expressing mutant subunits

The mouse mutants mocha and pearl are deficient in the AP-3 delta and beta3A subunits, respectively. We have used cells from these mice to investigate both the assembly of AP-3 complexes and AP-3 function. In mocha cells, the beta3 and mu3 subunits coassemble into a heterodimer, whereas the sigma3 subunit remains monomeric. In pearl cells, the delta and sigma3 subunits coassemble into a heterodimer, whereas mu3 gets destroyed. The yeast two hybrid system was used to confirm these interactions, and also to demonstrate that the A (ubiquitous) and B (neuronal-specific) isoforms of beta3 and mu3 can interact with each other. Pearl cell lines were generated that express beta3A, beta3B, a beta3Abeta2 chimera, two beta3A deletion mutants, and a beta3A point mutant lacking a functional clathrin binding site. All six constructs assembled into complexes and were recruited onto membranes. However, only beta3A, beta3B, and the point mutant gave full functional rescue, as assayed by LAMP-1 sorting. The beta3Abeta2 chimera and the beta3A short deletion mutant gave partial functional rescue, whereas the beta3A truncation mutant gave no functional rescue. These results indicate that the hinge and/or ear domains of beta3 are important for function, but the clathrin binding site is not needed.

Subcellular localization analysis of the closely related Fps/Fes and Fer protein-tyrosine kinases suggests a distinct role for Fps/Fes in vesicular trafficking

The subcellular localizations of the Fps/Fes and closely related Fer cytoplasmic tyrosine kinases were studied using green fluorescent protein (GFP) fusions and confocal fluorescence microscopy. In contrast to previous reports, neither kinase localized to the nucleus. Fer was diffusely cytoplasmic throughout the cell cycle. Fps/Fes also displayed a diffuse cytoplasmic localization, but in addition it showed distinct accumulations in cytoplasmic vesicles as well as in a perinuclear region consistent with the Golgi. This localization was very similar to that of TGN38, a known marker of the trans Golgi. The localization of Fps/Fes and TGN38 were both perturbed by brefeldin A, a fungal metabolite that disrupts the Golgi apparatus. Fps/Fes was also found to colocalize to various extents with several Rab proteins, which are members of the monomeric G-protein superfamily involved in vesicular transport between specific subcellular compartments. Using Rabs that are involved in endocytosis (Rab5B and Rab7) or exocytosis (Rab1A and Rab3A), we showed that Fps/Fes is localized in both pathways. These results suggest that Fps/Fes may play a general role in the regulation of vesicular trafficking.

Illuminating the secretory pathway: when do we need vesicles?

Recent studies using GFP-tagged markers and time-lapse microscopy have allowed direct visualisation of membrane traffic in the secretory pathway in living mammalian cells. This work shows that larger membrane structures, 300–500 nm in size, are the vehicles responsible for long distance, microtubule-dependent ER-to-Golgi and trans-Golgi to plasma membrane transport of secretory markers. At least two retrograde transport pathways from the Golgi to the ER exist, both of which are proposed to involve a further class of long, tubular membrane carrier that forms from the Golgi and fuses with the ER. Together, this has challenged established transport models, raising the question of whether larger pleiomorphic structures, rather than small 60–80 nm transport vesicles, mediate long-range transport between the ER and Golgi and between the Golgi and plasma membrane. http://www.biologists.com/JCS/movies/jcs2220.html

Multicolour imaging of post-Golgi sorting and trafficking in live cells

The biogenesis and maintenance of asymmetry is crucial to many cellular functions including absorption and secretion, signalling, development and morphogenesis. Here we have directly visualized the segregation and trafficking of apical (glycosyl phosphatidyl inositol-anchored) and basolateral (vesicular stomatitis virus glycoprotein) cargo in living cells using multicolour imaging of green fluorescent protein variants. Apical and basolateral cargo segregate progressively into large domains in Golgi/trans-Golgi network structures, exclude resident proteins, and exit in separate transport containers. These remain distinct and do not merge with endocytic structures suggesting that lateral segregation in the trans-Golgi network is the primary sorting event. Fusion with the plasma membrane was detected by total internal reflection microscopy and reveals differences between apical and basolateral carriers as well as new 'hot spots' for exocytosis.

In vivo imaging of gene expression:

With the ability to readily engineer genes, create knock-in and knock-out models of human disease, and replace and insert genes in clinical trials of gene therapy, it has become clear that imaging will play a critical role in these fields. Imaging is particularly helpful in recording temporal and spatial resolution of gene expression in vivo, determining vector distribution, and, ultimately, understanding endogenous gene expression during disease development. While endeavors are under way to image targets ranging from DNA to entire phenotypes in vivo, this short review focuses on in vivo imaging of gene expression with magnetic resonance and optical techniques.

Loss of cation-independent mannose 6-phosphate receptor expression promotes the accumulation of lysobisphosphatidic acid in multilamellar bodies

A number of recent studies have highlighted the importance of lipid domains within endocytic organelles in the sorting and movement of integral membrane proteins. In particular, considerable attention has become focussed upon the role of the unusual phospholipid lysobisphosphatidic acid (LBPA). This lipid appears to be directly involved in the trafficking of cholesterol and glycosphingolipids, and accumulates in a number of lysosomal storage disorders. Antibody-mediated disruption of LBPA function also leads to mis-sorting of cation-independent mannose 6-phosphate receptors. We now report that the converse is also true, and that spontaneous loss of cation-independent mannose 6-phosphate receptors from a rat fibroblast cell line led to the formation of aberrant late endocytic structures enriched in LBPA. Accumulation of LBPA was directly dependent upon the loss of the receptors, and could be reversed by expression of bovine cation-independent mannose 6-phosphate receptors in the mutant cell line. Ultrastructural analysis indicated that the abnormal organelles were electron-dense, had a multi-lamellar structure, accumulated endocytosed probes, and were distinct from dense-core lysosomes present within the same cells. The late endocytic structures present at steady state within any particular cell likely reflect the balance of membrane traffic through the endocytic pathway of that cell, and the rate of maturation of individual endocytic organelles. Moreover, there is considerable evidence which suggests that cargo receptors also play a direct mechanistic role in membrane trafficking events. Therefore, loss of such a protein may disturb the overall equilibrium of the pathway, and hence cause the accumulation of aberrant organelles. We propose that this mechanism underlies the phenotype of the mutant cell line, and that the formation of inclusion bodies in many lysosomal storage diseases is also due to an imbalance in membrane trafficking within the endocytic pathway.

The luminal domain of TGN38 interacts with integrin beta 1 and is involved in its trafficking

TGN38 luminal domain (TGN38LD) was expressed in Cos-7 cells to identify potential binding partners. The luminal domain was secreted but, surprisingly, a significant portion bound to the plasma membrane. Cells overexpressing TGN38LD or the full-length molecule detached from the substratum and left footprints positive for TGN38. Unexpectedly, in these cells, TGN38 colocalizes with integrin alpha 5 beta 1 at the Golgi, the cell surface or in the footprints and an increased amount of both integrin subunits on the plasma membrane was observed. Under physiological conditions when TGN38 is not overexpressed, it interacts with integrin beta 1. This was demonstrated by reciprocal co-immunoprecipitation of integrin beta 1 and TGN38. Functional analysis reveals that modification of the trafficking of TGN38 results in a parallel change in the distribution of integrin alpha 5 beta 1, leading to the conclusion that TGN38 is involved in the trafficking of integrin beta 1.

The role of virion membrane protein endocytosis in the herpesvirus life cycle

Endocytosis of cellular surface membrane proteins is a well-characterized, common occurrence. Internalization of cell surface receptors, often with bound ligands, aid in global events, such as cellular metabolism, as well as in specific, directed functions, such as the induction of signal transduction cascades or immune function. Some, but not all, herpesvirus membrane proteins are internalized from the plasma membrane by a process similar to receptor-mediated endocytosis. No known functions, however, have been ascribed to endocytosis of these proteins. In this review, we consider the function of herpesvirus membrane protein endocytosis. We compare and contrast the endocytosis and intracellular trafficking of two pseudorabies virus membrane proteins, the type I glycoprotein, gE, and the type II, tail-anchored membrane protein, Us9, and discuss the possible function of their internalization during the virus life cycle.

Selective targeting of avidin/mannose 6-phosphate receptor chimeras to early or late endosomes

In this study we have used the Semliki forest virus expression system to transiently express chimeric proteins that contain transmembrane and cytoplasmic domains of the cation-independent mannose 6-phosphate receptor (CI-MPR) fused to chicken avidin. Immunofluorescence and electron microscopy studies showed that the chimeric protein with the entire cytoplasmic domain of CI-MPR was transported to late endosomes, where it accumulated. We made use of the biotin-binding capacity of lumenal avidin, and found that, in agreement with this distribution, the chimeric protein could be labelled with biotinylated HRP endocytosed for a long, but not a brief, period of time. However, truncation of the C-terminal tail distal to the rapid endocytosis motif (YKYSKV), caused the truncated chimera to be transported to, and accumulated within, early endosomes. This truncated chimera did not reach recycling early endosomes labelled with internalised transferrin, to any significant extent, but was accessible to biotinylated HRP internalised for 5 min (or for longer periods at 19 degrees C). Coinfection of these chimeras showed that they follow the same route from the TGN to the early endosomes. We conclude that the sequence distal to the endocytosis motif contains the signals which are required for efficient transport to late endosomes. Our results also suggest that the YKYSKV sequence close to the CI-MPR transmembrane segment is sufficient for targeting to sorting early endosomes.

Guidelines for membrane protein engineering derived from de novo designed model peptides

Notwithstanding great advances in the engineering and structural analysis of globular proteins, relatively limited success has been achieved with membrane proteins--due largely to their intrinsic high insolubility and the concomitant difficulty in obtaining crystals. Progress with de novo synthesis of model membrane-interactive peptides presents an opportunity to construct simpler peptides with definable structures, and permits one to approach an understanding of the properties of the membrane proteins themselves. In the present article, we review how our laboratory and others have used peptide approaches to assess the detailed interactions of peptides with membranes, and primary folding at membrane surfaces and in membranes. Structural studies of model peptides identified the existence of a "threshold hydrophobicity," which controls spontaneous peptide insertion into membranes. Related studies of the relative helicity of peptides in organic media such as n-butanol indicate that the helical propensity of individual residues--not simply their hydrophobicity--may dictate the conformations of peptides in membranes. The overall experimental results provide fundamental guidelines for membrane protein engineering.

Effective formation of major histocompatibility complex class II-peptide complexes from endogenous antigen by thyroid epithelial cells

In autoimmune thyroid disease, thyroid epithelial cells (TEC) express major histocompatibility complex (MHC) class II molecules, potentially enabling them to present thyroid self-antigens to CD4-positive T cells. However, despite this, TEC may fail to present endogenous antigen as a result of limited processing or MHC class II loading capacity, or inadequate MHC class II levels. We addressed these issues using the cloned rat TEC line, Fischer rat thyroid cell line (FRTL5), which was transfected using an adenoviral expression vector that expressed ovalbumin (OVA) as an integral membrane protein. OVA-expressing FRTL5 cells very efficiently activated a panel of OVA-specific, class II-restricted T-cell hybridomas. This response was dependent on induction of MHC class II molecules by interferon-gamma (IFN-gamma) and was blocked by anti-MHC class II antibodies. Poor responses were seen to exogenously added OVA or OVA peptides. These results provide the most direct evidence to date that TEC can form MHC class II-peptide complexes derived from self-antigen in sufficient quantities to activate T cells.

Functional roles for fatty acylated amino-terminal domains in subcellular localization

Several membrane-associating signals, including covalently linked fatty acids, are found in various combinations at the N termini of signaling proteins. The function of these combinations was investigated by appending fatty acylated N-terminal sequences to green fluorescent protein (GFP). Myristoylated plus mono/dipalmitoylated GFP chimeras and a GFP chimera containing a myristoylated plus a polybasic domain were localized similarly to the plasma membrane and endosomal vesicles, but not to the nucleus. Myristoylated, nonpalmitoylated mutant chimeric GFPs were localized to intracellular membranes, including endosomes and the endoplasmic reticulum, and were absent from the plasma membrane, the Golgi, and the nucleus. Dually palmitoylated GFP was localized to the plasma membrane and the Golgi region, but it was not detected in endosomes. Nonacylated GFP chimeras, as well as GFP, showed cytosolic and nuclear distribution. Our results demonstrate that myristoylation is sufficient to exclude GFP from the nucleus and associate with intracellular membranes, but plasma membrane localization requires a second signal, namely palmitoylation or a polybasic domain. The similarity in localization conferred by the various myristoylated and palmitoylated/polybasic sequences suggests that biophysical properties of acylated sequences and biological membranes are key determinants in proper membrane selection. However, dual palmitoylation in the absence of myristoylation conferred significant differences in localization, suggesting that multiple palmitoylation sites and/or enzymes may exist.

Targeting of green fluorescent protein expression to the cell surface

We have previously reported on GPI-anchored fusion proteins that bind radioactive isotopes. We targeted their expression to the cell surface to obtain a marker protein detectable by nuclear and optical imaging (1, 2). Here we suggest a novel approach for targeting a model protein (GFP) to the exoplasmic surface of the plasma membrane. An expression vector (pcPEP-GFP) was constructed containing GFP cDNA fused with the fragment encoding the N-terminal cytoplasmic domain and signal peptide/membrane anchoring domain of the rabbit neutral endopeptidase (PEP-GFP). Flow cytometry showed green fluorescence in 45% of cells transfected with GFP and in 34% of cells transfected with PEP-GFP (24 h after transfection). Fluorescence microscopy of fixed cells stained with rhodaminated anti-GFP antibodies showed positive reaction only in the case of PEP-GFP-transfected cells indicating cell-surface expression. The PEP-GFP fusion protein was identified as a component of the light microsomal and Golgi fractions by immunoblotting.

Characterization of a fourth adaptor-related protein complex

Adaptor protein complexes (APs) function as vesicle coat components in different membrane traffic pathways; however, there are a number of pathways for which there is still no candidate coat. To find novel coat components related to AP complexes, we have searched the expressed sequence tag database and have identified, cloned, and sequenced a new member of each of the four AP subunit families. We have shown by a combination of coimmunoprecipitation and yeast two-hybrid analysis that these four proteins (epsilon, beta4, mu4, and sigma4) are components of a novel adaptor-like heterotetrameric complex, which we are calling AP-4. Immunofluorescence reveals that AP-4 is localized to approximately 10-20 discrete dots in the perinuclear region of the cell. This pattern is disrupted by treating the cells with brefeldin A, indicating that, like other coat proteins, the association of AP-4 with membranes is regulated by the small GTPase ARF. Immunogold electron microscopy indicates that AP-4 is associated with nonclathrin-coated vesicles in the region of the trans-Golgi network. The mu4 subunit of the complex specifically interacts with a tyrosine-based sorting signal, indicating that, like the other three AP complexes, AP-4 is involved in the recognition and sorting of cargo proteins with tyrosine-based motifs. AP-4 is of relatively low abundance, but it is expressed ubiquitously, suggesting that it participates in a specialized trafficking pathway but one that is required in all cell types.

Topological and functional analysis of the human reduced folate carrier by hemagglutinin epitope insertion

The membrane topology of the human reduced folate carrier protein (591 amino acids) was assessed by single insertions of the hemagglutinin epitope into nine sites of the protein. Reduced folate carrier-deficient Chinese hamster ovary cells expressing each of these constructs were probed with anti-hemagglutinin epitope monoclonal antibodies to assess whether the insertion was exposed to the external environment or to the cytoplasm. The results are consistent with the 12-transmembrane topology predicted for this protein. The hemagglutinin epitope insertion mutants were also tested for their effects on the function of the reduced folate carrier. For these studies, each of the constructs had a carboxyl-terminal fusion of the enhanced green fluorescent protein to monitor and quantitate expression. Insertions into the external loop between transmembrane regions 7 and 8 (Pro-297), the cytoplasmic loop between transmembrane regions 6 and 7 (Ser-225), and near the cytoplasmic amino and carboxyl termini (Pro-20 and Gly-492, respectively) had minor effects on methotrexate binding and uptake. The insertion into the cytoplasmic loop between transmembrane regions 10 and 11 (Gln-385) greatly reduced both binding and uptake of methotrexate, whereas the insertion into the external loop between transmembrane regions 11 and 12 (Pro-427) selectively interfered with uptake but not binding.