Signal recognition particle-dependent membrane insertion of mouse invariant chain: a membrane-spanning protein with a cytoplasmically exposed amino terminus

Peroxin-dependent targeting of a lipid-droplet-destined membrane protein to ER subdomains

Lipid droplets (LDs) are endoplasmic reticulum (ER)-derived lipid storage organelles uniquely encapsulated by phospholipid monolayers. LD membrane proteins are embedded into the monolayer in a monotopic hairpin-topology and therefore likely have requirements for their biogenesis distinct from those inserting as bitopic and polytopic proteins into phospholipid bilayers. UBXD8 belongs to a subfamily of hairpin-proteins that localize to both the ER and LDs, and are initially inserted into the cytoplasmic leaflet of the ER bilayer before partitioning to the LD monolayer. The molecular machinery responsible for inserting hairpin-proteins into membranes, however, is unknown. Here, we report that newly synthesized UBXD8 is posttranslationally inserted into discrete ER-subdomains by a mechanism requiring cytosolic PEX19 and membrane-integrated PEX3, proteins hitherto exclusively implicated in peroxisome biogenesis. Farnesylation of PEX19 uncouples ER/LD- and peroxisome targeting, expanding the function of this peroxin to an ER targeting pathway and suggesting a coordinated biogenesis of LDs and peroxisomes.

Signal peptide peptidase (SPP) assembles with substrates and misfolded membrane proteins into distinct oligomeric complexes

SPP (signal peptide peptidase) is an aspartyl intramembrane cleaving protease, which processes a subset of signal peptides, and is linked to the quality control of ER (endoplasmic reticulum) membrane proteins. We analysed SPP interactions with signal peptides and other membrane proteins by co-immunoprecipitation assays. We found that SPP interacts specifically and tightly with a large range of newly synthesized membrane proteins, including signal peptides, preproteins and misfolded membrane proteins, but not with all co-expressed type II membrane proteins. Signal peptides are trapped by the catalytically inactive SPP mutant SPP^D/A. Preproteins and misfolded membrane proteins interact with both SPP and the SPP^D/A mutant, and are not substrates for SPP-mediated intramembrane proteolysis. Proteins interacting with SPP are found in distinct complexes of different sizes. A signal peptide is mainly trapped in a 200 kDa SPP complex, whereas a preprotein is predominantly found in a 600 kDa SPP complex. A misfolded membrane protein is detected in 200, 400 and 600 kDa SPP complexes. We conclude that SPP not only processes signal peptides, but also collects preproteins and misfolded membrane proteins that are destined for disposal.

Distinct targeting pathways for the membrane insertion of tail-anchored (TA) proteins

Tail-anchored (TA) proteins are characterised by a C-terminal transmembrane region that mediates post-translational insertion into the membrane of the endoplasmic reticulum (ER). We have investigated the requirements for membrane insertion of three TA proteins, RAMP4, Sec61beta and cytocrome b5. We show here that newly synthesised RAMP4 and Sec61beta can accumulate in a cytosolic, soluble complex with the ATPase Asna1 before insertion into ER-derived membranes. Membrane insertion of these TA proteins is stimulated by ATP, sensitive to redox conditions and blocked by alkylation of SH groups by N-ethylmaleimide (NEM). By contrast, membrane insertion of cytochrome b5 is not found to be mediated by Asna1, not stimulated by ATP and not affected by NEM or an oxidative environment. The Asna1-mediated pathway of membrane insertion of RAMP4 and Sec61beta may relate to functions of these proteins in the ER stress response.

The adenovirus E3-6.7K protein adopts diverse membrane topologies following posttranslational translocation

The E3 region of adenovirus codes for several membrane proteins, most of which are involved in immune evasion and prevention of host cell apoptosis. We explored the topology and targeting mechanisms of E3-6.7K, the most recently described member of this group, by using an in vitro translation system supplemented with microsomes. Here, we present evidence that E3-6.7K, one of the smallest signal-anchor proteins known, translocates across the membrane of the endoplasmic reticulum in a posttranslational, ribosome-independent, yet ATP-dependent manner, reminiscent of the translocation of tail-anchored proteins. Our analysis also demonstrated that E3-6.7K could achieve several distinct topological fates. In addition to the previously postulated type III orientation (N-luminal/C-cytoplasmic, termed NtmE3-6.7K), we detected a tail-anchored form adopting the opposite orientation (N-cytoplasmic/C-luminal, termed CtmE3-6.7K) as well as the possibility of a fully translocated form (N and C termini are both translocated, termed NCE3-6.7K). Due to the translocation of a positively charged domain, both the CtmE3-6.7K and NCE3-6.7K topologies of E3-6.7K constitute exceptions to the "positive inside" rule. The NtmE3-6.7K and NCE3-6.7K are the first examples of posttranslationally translocated proteins in higher eukaryotes that are not tail anchored. Distinct topological forms were also found in transfected cells, as both N and C termini of E3-6.7K were detected on the extracellular surface of transfected cells. The demonstration of unexpected topological forms and translocation mechanisms for E3-6.7K defies conventional thinking about membrane protein topogenesis and advises that both the mode of targeting and topology of signal-anchor proteins should be determined experimentally.

A role for cathepsin L and cathepsin S in peptide generation for MHC class II presentation

The enzymes that degrade proteins to peptides for presentation on MHC class II molecules are poorly understood. The cysteinal lysosomal proteases, cathepsin L (CL) and cathepsin S (CS), have been shown to process invariant chain, thereby facilitating MHC class II maturation. However, their role in Ag processing is not established. To examine this issue, we generated embryonic fibroblast lines that express CL, CS, or neither. Expression of CL or CS mediates efficient degradation of invariant chain as expected. Ag presentation was evaluated using T cell hybridoma assays as well as mass spectroscopic analysis of peptides eluted from MHC class II molecules. Interestingly, we found that the majority of peptides are presented regardless of CL or CS expression, although these proteases often alter the relative levels of the peptides. However, for a subset of Ags, epitope generation is critically regulated by CL or CS. This result suggests that these cysteinal proteases participate in Ag processing and generate qualitative and quantitative differences in the peptide repertoires displayed by MHC class II molecules.

The cytoplasmic amino-terminus of the Latent Membrane Protein-1 of Epstein-Barr Virus: relationship between transmembrane orientation and effector functions of the carboxy-terminus and transmembrane domain

The Latent Membrane Protein 1 (LMP-1) protein of Epstein-Barr virus (EBV) is localized in the plasma membrane of the infected cell. LMP-1 possesses a hydrophobic membrane spanning domain, and charged, intracellular amino- and carboxy-termini. Two models have been proposed for the contribution of the amino-terminus to LMP-1's function: (i) as an effector domain, interacting with cellular proteins, or (ii) as a structural domain dictating the correct orientation of transmembrane domains and thereby positioning LMP-1's critical effector domains (i.e. the carboxy-terminus). However, no studies to date have addressed directly the structural contributions of LMP-1's cytoplasmic amino-terminus to function. This study was designed to determine if LMP-1's cytoplasmic amino-terminus (N-terminus) encodes information required solely for maintenance of proper topological orientation. We have constructed LMP-1 chimeras in which the cytoplasmic N-terminus of LMP-1 is replaced with an unrelated domain of similar size and charge, but of different primary sequence. Retention of the charged amino-terminal (N-terminal) cytoplasmic domain and first predicted transmembrane domain was required for correct transmembrane topology. The absolute primary sequence of the cytoplasmic N-terminus was not critical for LMP-1's cytoskeletal association, turnover, plasma membrane patching, oligomerization, Tumor Necrosis Factor Receptor-associated factor (TRAF) binding, NF-kappaB activation, rodent cell transformation and cytostatic activity. Furthermore, our results point to the hydrophobic transmembrane domain, independent of the cytoplasmic domains, as the primary LMP-1 domain mediating oligomerization, patching and cytoskeletal association. The cytoplasmic amino-terminus provides the structural information whereby proper transmembrane orientation is achieved.

Theileria parva 104 kDa microneme--rhoptry protein is membrane-anchored by a non-cleaved amino-terminal signal sequence for entry into the endoplasmic reticulum

The 104 kDa microneme-rhoptry protein (p104) is the only known apical complex organelle-specific protein of Theileria parva. p104 exhibits striking structural similarities to circumsporozoite protein and sporozoite surface protein 2 of Plasmodium yoelii. Their primary sequences contain two hydrophobic segments, located at the amino-and the carboxy-terminus. The p104 amino-terminal hydrophobic region was suggested to be a signal peptide for entry into the endoplasmic reticulum and the extreme carboxy-terminal region to function as a membrane anchor. We have studied the biogenesis of p104 in a cell-free expression system and found that p104 is co-translationally transported into membranes derived from endoplasmic reticulum. The amino-terminal signal peptide is not cleaved off and anchors the protein in the membrane with the carboxy-terminal portion translocated into the lumen. We suggest that in vivo p104 is co-translationally integrated into the membrane of the endoplasmic reticulum, from where it is further transported to the microneme-rhoptry complex. Thus, p104 appears to be a suitable marker to study the development of micronemes and rhoptries in T. parva.

Class II-Associated Invariant Chain Peptide-Independent Binding of Invariant Chain to Class II MHC molecules

The class II-associated invariant chain peptide (CLIP) region of invariant chain (Ii) is believed to play a critical role in the assembly and transport of MHC class II αβIi complexes through its interaction with the class II peptide-binding site. The role of the CLIP sequence was investigated by using mutant Ii molecules with altered affinity for the DR1 peptide-binding site. Both high- and low-affinity mutants were observed to efficiently assemble with DR1 and mediate transport to endosomal compartments in COS cell transfectants. Using N- and C-terminal truncations, a region adjacent to CLIP within Ii(103–118) was identified that can complement loss of affinity for the peptide-binding site in mediating efficient assembly of αβIi. A C-terminal fragment completely lacking the CLIP region, Ii(103–216), was observed binding stably to class II molecules in immunoprecipitation studies and experiments with purified proteins. The Ii(103–118) region was required for this binding, which occurs through interactions outside of the αβ peptide-binding groove. We conclude that strong interactions involving Ii(103–118) and other regions of Ii cooperate in the assembly of functional αβIi under conditions where CLIP has little or no affinity for the class II peptide-binding site. Our results support the hypothesis that the CLIP sequence has evolved to avoid high-stability interactions with the peptide-binding sites of MHC class II molecules rather than as a promiscuous binder with moderate affinity for all class II molecules.

Distinct Peptide Loading Pathways for MHC Class II Molecules Associated with Alternative Ii Chain Isoforms

Mutant mouse strains expressing either p31 or p41 Ii chain appear equally competent with respect to their class II functional activities including Ag presentation and CD4+ T cell development. To further explore possibly divergent roles provided by alternative Ii chain isoforms, we compare class II structure and function in double mutants also carrying a null allele at the H2-DM locus. As for DM mutants expressing wild-type Ii chain, AαbAβb dimers present in DM-deficient mice expressing either Ii chain isoform appear equally occupied by class II-associated Ii chain-derived peptides (CLIP). Surprisingly, in functional assays, these novel mouse strains exhibit strikingly different phenotypes. Thus, DM-deficient mice expressing wild-type Ii chain or p31 alone are both severely compromised in their abilities to present peptides. In contrast, double mutants expressing the p41 isoform display markedly enhanced peptide-loading capabilities, approaching those observed for wild-type mice. The present data strengthen evidence for divergent class II presentation pathways and demonstrate for the first time that functionally distinct roles are mediated by alternatively spliced forms of the MHC class II-associated Ii chain in a physiologic setting.

Exon 6 is essential for invariant chain trimerization and induction of large endosomal structures

Invariant chain (Ii) is a transmembrane type II protein that forms a complex with the major histocompatibility complex (MHC) class II molecules in the endoplasmic reticulum (ER). The membrane proximal luminal region of Ii is responsible for the non-covalent association with MHC class II molecules. Chemical cross-linking in COS cells was used to study the effect of luminal and cytoplasmic deletions on trimerization of Ii. We demonstrate that trimerization of Ii is independent of the cytosolic tail of Ii, whereas residues 162-191 (the sequence encoded by exon 6) in the luminal part of Ii are essential for trimer formation. Immunofluorescence studies of the transfected luminal deletion constructs show that the amino acids encoded by exon 6 of Ii are also essential for the induction of large endosomal vesicles. The data suggest that Ii must be in a trimeric form to modify the endosomal pathway.

The Sec61 complex is essential for the insertion of proteins into the membrane of the endoplasmic reticulum

Cross-linking studies have implicated Sec61 alpha as the principal component adjacent to newly synthesised membrane proteins during insertion into the endoplasmic reticulum. Using proteoliposomes which have been reconstituted from purified components of the endoplasmic reticulum [Görlich, D and Rapoport, T.A., Cell 75 (1993) 615-630] we have found that the Sec61 complex, consisting of three subunits, is essential for the insertion of single-spanning membrane proteins. This is true for signal-anchor proteins of both orientations, and for proteins with a cleavable signal sequence. These results support the view that Sec61 alpha is a major component of the ER translocation site and promotes both the insertion of membrane proteins and the translocation of secretory proteins.

Membrane insertion of gap junction connexins: polytopic channel forming membrane proteins

Connexins, the proteins that form gap junction channels, are polytopic plasma membrane (PM) proteins that traverse the plasma membrane bilayer four times. The insertion of five different connexins into the membrane of the ER was studied by synthesizing connexins in translation-competent cell lysates supplemented with pancreatic ER-derived microsomes, and by expressing connexins in vivo in several eucaryotic cell types. In addition, the subcellular distribution of the connexins was determined. In vitro-synthesis in the presence of microsomes resulted in the signal recognition particle-dependent membrane insertion of the connexins. The membrane insertion of all connexins was accompanied by an efficient proteolytic processing that was dependent on the microsome concentration. Endogenous unprocessed connexins were detectable in the microsomes used, indicating that the pancreatic microsomes serve as a competent recipient in vivo for unprocessed full length connexins. Although oriented with their amino terminus in the cytoplasm, the analysis of the cleavage reaction indicated that an unprecedented processing by signal peptidase resulted in the removal of an amino-terminal portion of the connexins. Variable amounts of similar connexin cleavage products were also identified in the ER membranes of connexin overexpressing cells. The amount generated correlated with the level of protein expression. These results demonstrate that the connexins contain a cryptic signal peptidase cleavage site that can be processed by this enzyme in vitro and in vivo in association with their membrane insertion. Consequently, a specific factor or condition must be required to prevent this aberrant processing of connexins under normal conditions in the cell.

The invariant chain induces compact forms of class II molecules localized in late endosomal compartments

The invariant chain (Ii) binds to newly synthesized major histocompatibility complex (MHC) class II molecules and is targeted to an acidic compartment where it is degraded. To evaluate its role on the conformation and the subcellular distribution of murine MHC class II molecules we have established stable L cell transfectants expressing class II IAk heterodimers alone or in conjunction with p31 and p41 Ii chains. In these cells, class II molecules were present under three forms: alpha beta heterodimers bearing high mannose carbohydrate moieties, and fully glycosylated alpha beta heterodimers that are sensitive or resistant to sodium dodecyl sulfate dissociation at 20 degrees C. The latter class II molecules called compact heterodimers, were here highly induced in Ii-positive cells. Using in situ iodination of endosomal compartments, class II heterodimers were detected in late endosomal compartments essentially as compact forms in Ii-positive cells, and as non-compact forms in Ii-negative cells. Using confocal microscopy, IAk molecules were located in compartments distinct from early endosomes labeled with transferrin, but partially coincident with vesicles containing fluid-phase markers, and highly coincident with compartments containing large amounts of cathepsins B, D, H, and L in Ii-positive and Ii-negative cells. At the ultrastructural level, class II molecules were mostly present in multivesicular bodies, even without Ii expression. But Ii chains were needed to induce an efficient presentation of the hen egg lysozyme antigen and were sufficient to promote a major conformational change of the late endosomal, and/or lysosomal resident, class II molecules. Ii molecules are presumably playing a chaperoning function favoring the association of peptides with class II molecules in endosomal compartments.

The segment of invariant chain that is critical for association with major histocompatibility complex class II molecules contains the sequence of a peptide eluted from class II polypeptides

Major histocompatibility complex class II molecules present peptides from an extracellular source of antigens to CD4+ T lymphocytes. The class II-associated invariant chain affects this role of alpha and beta polypeptides by restriction of peptide loading to endocytic vesicles. Up to now no specific portion of the invariant chain has been defined as the class II binding site. We constructed recombinant invariant chain genes and inspected association of the mutant invariant chains with class II polypeptides. Here we demonstrate that an extracytoplasmic sequence of the invariant chain (aa 81-109) that is only 23 residues away from the transmembrane region is essential for contact with class II polypeptides, whereas the remaining C-terminal part is dispensable for binding. The sequence of invariant-chain-derived peptides that were eluted from class II molecules is contained in this segment and may define the class II binding site of the invariant chain. The membrane-proximal position of this region suggests that the invariant chain and invariant-chain-derived peptides isolated from class II molecules bind to a domain distinct from the class II pocket.

Sec61p is adjacent to nascent type I and type II signal-anchor proteins during their membrane insertion

We have identified membrane components which are adjacent to type I and type II signal-anchor proteins during their insertion into the membrane of the ER. Using two different cross-linking approaches a 37-38-kD nonglycosylated protein, previously identified as P37 (High, S., D. Görlich, M. Wiedmann, T. A. Rapoport, and B. Dobberstein. 1991. J. Cell Biol. 113:35-44), was found adjacent to all the membrane inserted nascent chains used in this study. On the basis of immunoprecipitation, this ER protein was shown to be identical to the recently identified mammalian Sec61 protein. Thus, Sec61p is the principal cross-linking partner of both type I and type II signal-anchor proteins during their membrane insertion (this work), and of secretory proteins during their translocation (Görlich, D., S. Prehn, E. Hartmann, K.-U. Kalies, and T. A. Rapoport. 1992. Cell. 71:489-503). We propose that membrane proteins of both orientations, and secretory proteins employ the same ER translocation sites, and that Sec61p is a core component of these sites.

Production and secretion in CHO cells of the extracellular domain of AMOG/beta 2, a type-II membrane protein

A hybrid gene consisting of the sequences coding for the signal peptide and N terminus of a type-I membrane protein, the neural cell adhesion molecule (N-CAM), and the extracellular domain of the adhesion molecule on glia (AMOG/beta 2), a type-II membrane protein, was constructed. The sequence was inserted into a eukaryotic expression vector containing the human cytomegalovirus promoter and the glutamine synthetase selection marker, and used to transfect Chinese hamster ovary cells. The resulting stably transformed cell lines produced large amounts of soluble recombinant AMOG/beta 2 (reAMOG/beta 2), which was secreted into the culture medium as a heavily glycosylated 40-55-kDa protein. N-terminal sequence analysis revealed that the protein is not cleaved at the natural signal peptide cleavage site of N-CAM, but two amino acids (aa) further downstream. Treatment of reAMOG/beta 2 with N-glycosidase F (GlycoF) reduced the molecular mass to 27 kDa, corresponding to the calculated mass of the unglycosylated form. In contrast to AMOG/beta 2 isolated from mouse brain, which is sensitive to endoglycosidase H, the immunoaffinity-purified re-protein is more resistant to this treatment, indicating that the sugars attached to reAMOG/beta 2 are mainly of the complex type. Our results demonstrate the feasibility of secreting the extracellular domain of a type-II membrane protein, which is usually inserted into the membrane with the C terminus facing the extracellular side.

Cell surface display of rat invariant gamma chain: detection by monoclonal antibodies directed against a C-terminal gamma chain segment

A series of 14 monoclonal antibodies (mAb) directed against the C-terminal part of the rat invariant gamma chain (amino acid 142-216) was generated using distinct fusion proteins that contain this gamma segment for immunization and hybridoma screening. Additional fusion protein were prepared carrying discrete regions of the gamma chain. Employing these reagents confirmed that the obtained mAb do indeed recognize the C-terminal portion of the invariant chain, as demonstrated by Western blot analysis. All mAb established recognize epitopes present on the native gamma chain, as revealed by immunoprecipitation analysis using nonionic detergent extracts of metabolically labeled Lewis rat splenocytes combined with two-dimensional gel electrophoresis. However, while the majority of the gamma chain-specific mAb precipitated gamma chain-containing polypeptide chain complexes in which immature, sialic acid-deficient and mature, terminally sialylated forms of the gamma chain were predominantly represented, a fraction of the antibodies preferentially precipitated the immature gamma forms. Cell surface binding of these two groups of mAb correlated with the immunoprecipitation data in that the former group of antibodies did bind to intact Lewis rat spleen cells, while essentially no binding was observed with the antibodies of the latter group. Double-fluorescence staining with the class II-specific fluorescein isothiocyanate-conjugated mAb OX3 and OX6, respectively, as well as a representative gamma chain-specific mAb visualized with phycoerythrin-coupled secondary antibody shows coexpression of class II determinants and the invariant chain at the cell surface.

Functions of signal and signal-anchor sequences are determined by the balance between the hydrophobic segment and the N-terminal charge

The signal sequence of secretory proteins and the signal-anchor sequence of type II membrane proteins initiate the translocation of the following polypeptide segments, whereas the signal-anchor sequence of cytochrome P-450-type membrane proteins mediates the membrane insertion of the polypeptide via a signal-recognition particle-dependent mechanism but does not lead to the translocation of the following C-terminal sequences. To establish the structural requirements for the function of signal and signal-anchor sequences, we constructed chimeric proteins containing artificial topogenic sequences in which the N-terminal net charge and the length of the hydrophobic segment were systematically altered. Utilizing an in vitro translation-translocation system, we found that hydrophobic segments consisting of 7-10 leucine residues functioned as signal sequences whereas segments with 12-15 leucine residues showed different topogenic functions, behaving as signal sequences or P-450-type signal-anchor sequences, depending on the N-terminal charge. From these observations, we propose that the function of N-terminal topogenic sequences depends on a balance between the N-terminal charge and the length of the following hydrophobic segment.

Class II major histocompatibility complex molecules of murine dendritic cells: synthesis, sialylation of invariant chain, and antigen processing capacity are down-regulated upon culture

Dendritic cells (DCs), such as Langerhans cells (LCs) of the epidermis and the DCs of lymphoid organs such as spleen, are potent antigen presenting cells. DCs express high levels of major histocompatibility complex (MHC) class II molecules, but, partly because of the low numbers of primary DCs in any tissue, there has been no detailed study of the biochemistry of their class II molecules. This information may be needed to help explain recent findings that DCs process native protein antigens when freshly isolated from epidermis and spleen. Processing ceases during culture, yet a strong accessory function for activating resting T cells develops. We studied immunoprecipitates of DC class II and invariant chain (Ii) molecules by two-dimensional gel electrophoresis. We found that (i) freshly isolated LCs synthesize large amounts of class II and Ii polypeptides; (ii) Ii molecules that are known to be involved in antigen processing display an unusually large number of sialic acids in fresh LCs; (iii) with culture, class II and Ii synthesis decreases dramatically and has virtually ceased at 3 days; and (iv) the turnover of class II in pulse/chase experiments is slow, being undetectable over a 12- to 32-hr culture period, whereas the turnover of Ii is rapid. We conclude that MHC class II molecules of DCs do not seem to be qualitatively unique. However, the regulation of class II and Ii expression is distinctive in that biosynthesis proceeds vigorously for a short period of time and the newly synthesized class II remains stably on the cell surface, whereas Ii turns over rapidly. This may enable DCs to process and retain antigens in the peripheral tissues such as skin and migrate to the lymphoid organs to activate T cells there.

The identification of proteins in the proximity of signal-anchor sequences during their targeting to and insertion into the membrane of the ER

Using a photocross-linking approach we have investigated the cytosolic and membrane components involved in the targeting and insertion of signal-anchor proteins into the membrane of the ER. The nascent chains of both type I and type II signal-anchor proteins can be cross-linked to the 54-kD subunit of the signal recognition particle. Upon addition of rough microsomes the type I and type II signal-anchor proteins interact with a number of components. Both types of protein interact with an integral membrane protein, the signal sequence receptor, previously identified by its proximity to preprolactin during its translocation (Wiedmann, M., T.V. Kurzchalia, E. Hartmann, and T.A. Rapoport. 1987. Nature [Lond.] 328:830-833). Three proteins, previously unidentified, were found to be cross-linked to the nascent chains of the signal-anchor proteins. Among them was a 37-kD protein that was found to be the main component interacting with the type I SA protein used. These proteins were not seen in the absence of membranes suggesting they are components of the ER. The ability of the nascent chains to be cross-linked to these identified proteins was shown to be abolished by prior treatment with agents known to disrupt translocation intermediates or ribosomes. We propose that the newly identified proteins function either in the membrane insertion of only a subset of proteins or only at a specific stage of insertion.

Requirements for the membrane insertion of signal-anchor type proteins

Proteins which are inserted and anchored in the membrane of the ER by an uncleaved signal-anchor sequence can assume two final orientations. Type I signal-anchor proteins translocate the NH2 terminus across the membrane while type II signal-anchor proteins translocate the COOH terminus. We investigated the requirements for cytosolic protein components and nucleotides for the membrane targeting and insertion of single-spanning type I signal-anchor proteins. Besides the ribosome, signal recognition particle (SRP), GTP, and rough microsomes (RMs) no other components were found to be required. The GTP analogue GMPPNP could substitute for GTP in supporting the membrane insertion of IMC-CAT. By using a photocrosslinking assay we show that for secreted, type I and type II signal-anchor proteins the presence of both GTP and RMs is required for the release of the nascent chain from the 54-kD subunit of SRP. For two of the proteins studied the release of the nascent chain from SRP54 was accompanied by a new interaction with components of the ER. We conclude that the GTP-dependent release of the nascent chain from SRP54 occurs in an identical manner for each of the proteins studied.

Topology of eukaryotic type II membrane proteins: importance of N-terminal positively charged residues flanking the hydrophobic domain

We have tested the role of different charged residues flanking the sides of the signal/anchor (S/A) domain of a eukaryotic type II (N(cyt)C(exo)) integral membrane protein in determining its topology. The removal of positively charged residues on the N-terminal side of the S/A yields proteins with an inverted topology, while the addition of positively charged residues to only the C-terminal side has very little effect on orientation. Expression of chimeric proteins composed of domains from a type II protein (HN) and the oppositely oriented membrane protein M2 indicates that the HN N-terminal domain is sufficient to confer a type II topology and that the M2 N-terminal ectodomain can direct a type II topology when modified by adding positively charged residues. These data suggest that eukaryotic membrane protein topology is governed by the presence or absence of an N-terminal signal for retention in the cytoplasm that is composed in part of positive charges.

The E2 signal sequence of rubella virus remains part of the capsid protein and confers membrane association in vitro

The capsid (C) protein of rubella virus is translated from a 24S subgenomic mRNA as the first part of a polyprotein containing all three structural proteins of the virus. It is separated from the following protein (E2) by signal peptidase, which cleaves after the E2 signal sequence. We raised an antipeptide antiserum directed against the signal sequence and used the antiserum to show that this sequence is still a part of the C protein in the mature virion. Furthermore, we also showed that, when the C protein is synthesized by in vitro transcription and translation, the resultant protein is membrane associated. This association is not seen with a variant C protein which lacks the signal sequence, and a normally soluble protein (dihydrofolate reductase) becomes membrane associated when the signal sequence is placed at its carboxy terminus.

Invariant chain trimers are sequestered in the rough endoplasmic reticulum in the absence of association with HLA class II antigens

HLA class II antigens are heterodimeric cell surface glycoproteins that interact with antigenic peptides to form complexes recognizable by CD4-positive T cells. During their biosynthesis, class II antigens are retained in a post-Golgi compartment in association with the invariant chain, which dissociates before class II cell surface expression. To address whether the invariant chain mediates this post-Golgi retention, its transport and assembly were examined in cells that do not express HLA class II antigens. Pulse-chase analysis and endoglycosidase digestions showed that very little invariant chain proceeded as far as the trans-Golgi in class II-negative cell lines. Immunofluorescence studies suggested that in these cells the invariant chain is sequestered in the RER. Gel filtration and cross-linking data showed that RER-localized invariant chain is present as trimers or aggregated trimers. Multimerization is mediated by lumenal interactions; a proteolytic fragment of the invariant chain corresponding to the lumenal domain remained trimeric as determined by cross-linking analysis. Similar transport and structural characteristics were observed for a pool of excess invariant chain in class II-positive cells, suggesting that an excess of invariant chain in the ER may be important for class II antigen function. These results have important implications for the transport of cellular proteins in general and for the role of the invariant chain in class II antigen biosynthesis.

The signal sequence of the p62 protein of Semliki Forest virus is involved in initiation but not in completing chain translocation

So far it has been demonstrated that the signal sequence of proteins which are made at the ER functions both at the level of protein targeting to the ER and in initiation of chain translocation across the ER membrane. However, its possible role in completing the process of chain transfer (see Singer, S. J., P. A. Maher, and M. P. Yaffe. Proc. Natl. Acad. Sci. USA. 1987. 84:1015-1019) has remained elusive. In this work we show that the p62 protein of Semliki Forest virus contains an uncleaved signal sequence at its NH2-terminus and that this becomes glycosylated early during synthesis and translocation of the p62 polypeptide. As the glycosylation of the signal sequence most likely occurs after its release from the ER membrane our results suggest that this region has no role in completing the transfer process.

In vitro membrane assembly of a polytopic, transmembrane protein results in an enzymatically active conformation

In vitro integration of the polytopic, transmembrane lactose permease into membrane vesicles from Escherichia coli is demonstrated. To this end the enzyme was synthesized in a homologous, cell-free transcription-translation system. In this system, synthesis occurred in an essentially membrane-free environment leading to the formation of lactose permease aggregates, which were resistant to protease digestion and detergent solubilization. However, if inverted membrane vesicles from E. coli were included in the synthesis reaction, most de novo-synthesized lactose permease could be recovered from a membrane-containing subfraction (enriched in leader [signal] peptidase activity). This membrane association of lactose permease was Na2CO3 resistant, detergent sensitive, and yielded a distinct pattern of proteolytic cleavage peptides. Moreover, membrane vesicles when present cotranslationally during synthesis of lactose permease, acquired the capability to accumulate lactose, strongly suggesting a correct in vitro assembly of the enzyme. Because of the extensive aggregation of lactose permease synthesized in the absence of membranes, only low amounts originating from the soluble enzyme pool integrated posttranslationally into the membrane vesicles. Unlike the translocation of the outer membrane protein LamB into membrane vesicles, integration of lactose permease was found to be independent of the H+-motive force.

A tripartite structure of the signals that determine protein insertion into the endoplasmic reticulum membrane

Multilineage colony stimulating factor is a secretory protein with a cleavable signal sequence that is unusually long and hydrophobic. Using molecular cloning techniques we exchanged sequences NH2- or COOH-terminally flanking the hydrophobic signal sequence. Such modified fusion proteins still inserted into the membrane but their signal sequence was not cleaved. Instead the proteins were now anchored in the membrane by the formerly cleaved signal sequence (signal-anchor sequence). They exposed the NH2 terminus on the exoplasmic and the COOH terminus on the cytoplasmic side of the membrane. We conclude from our results that hydrophilic sequences flanking the hydrophobic core of a signal sequence can determine cleavage by signal peptidase and insertion into the membrane. It appears that negatively charged amino acid residues close to the NH2 terminal side of the hydrophobic segment are compatible with translocation of this segment across the membrane. A tripartite structure is proposed for signal-anchor sequences: a hydrophobic core region that mediates targeting to and insertion into the ER membrane and flanking hydrophilic segments that determine the orientation of the protein in the membrane.

Intracellular traffic of newly synthesized proteins. Current understanding and future prospects

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Redundancy of signal and anchor functions in the NH2-terminal uncharged region of influenza virus neuraminidase, a class II membrane glycoprotein

Class II membrane glycoproteins share a common topology of the NH2 terminus inside and the COOH terminus outside the cell. Their transport to the cell surface is initiated by the function of a single hydrophobic domain near the NH2 terminus. This functional domain serves both as an uncleaved signal sequence and as a transmembrane anchor. We examined the signal and anchor functions of influenza virus neuraminidase, a prototype class II membrane glycoprotein, by deletion analysis of its long, uncharged amino-terminal region. The results presented here show that the entire stretch of 29 uncharged amino acids (7 to 35) is not required for either a signal sequence or an anchor sequence function. On the basis of translocation and membrane stability data for different mutants, we suggest that the first 20 amino acid residues (7 to 27) are likely to provide the hydrophobic core for these functions and that within this putative subdomain some sequences are more efficient than the other sequences in providing a translocation function. Finally, it appears that neuraminidase and its mutant proteins are translocated with the proper orientation, regardless of the characteristics of the flanking sequences.

Signal and membrane anchor functions overlap in the type II membrane protein I gamma CAT

I gamma CAT is a hybrid protein that inserts into the membrane of the endoplasmic reticulum as a type II membrane protein. These proteins span the membrane once and expose the NH2-terminal end on the cytoplasmic side and the COOH terminus on the exoplasmic side. I gamma CAT has a single hydrophobic segment of 30 amino acid residues that functions as a signal for membrane insertion and anchoring. The signal-anchor region in I gamma CAT was analyzed by deletion mutagenesis from its COOH-terminal end (delta C mutants). The results show that the 13 amino acid residues on the amino-terminal side of the hydrophobic segment are not sufficient for membrane insertion and translocation. Mutant proteins with at least 16 of the hydrophobic residues are inserted into the membrane, glycosylated, and partially proteolytically processed by a microsomal protease (signal peptidase). The degree of processing varies between different delta C mutants. Mutant proteins retaining 20 or more of the hydrophobic amino acid residues can span the membrane like the parent I gamma CAT protein and are not proteolytically processed. Our data suggest that in the type II membrane protein I gamma CAT, the signals for membrane insertion and anchoring are overlapping and that hydrophilic amino acid residues at the COOH-terminal end of the hydrophobic segment can influence cleavage by signal peptidase. From this and previous work, we conclude that the function of the signal-anchor sequence in I gamma CAT is determined by three segments: a positively charged NH2 terminus, a hydrophobic core of at least 16 amino acid residues, and the COOH-terminal flanking hydrophilic segment.

Identification of the glycosaminoglycan-attachment site of mouse invariant-chain proteoglycan core protein by site-directed mutagenesis

The invariant chain (Ii), a nonpolymorphic glycoprotein that associates with the immunoregulatory Ia proteins encoded by the major histocompatibility complex, has a proteoglycan form (Ii-CS) that bears a chondroitin sulfate glycosaminoglycan. In this proteoglycan form, Ii may remain associated with Ia at the cell surface. Inhibitors that prevent the addition of glycosaminoglycan to Ii have been found to depress antigen-presenting function. Ii does not have multiple candidate glycosaminoglycan-attachment sites, and we used site-directed mutagenesis to replace a candidate serine glycosaminoglycan-acceptor site with alanine at position 201 in the murine Ii protein. Transfection of the normal or altered gene into Ii-negative COS-7 cells showed that equivalent amounts of core Ii protein and its acidic, terminally glycosylated forms were synthesized, but the Ala-201 mutant Ii did not give rise to Ii-CS. The mutant protein had apparently normal transport through the Golgi compartment and associated stably with Ia molecules. Thus, this mutation directly identifies the site of glycosaminoglycan addition and shows that it can be eliminated without adversely affecting the overall biosynthesis of Ii.

Positive charges at the NH2 terminus convert the membrane-anchor signal peptide of cytochrome P-450 to a secretory signal peptide

The NH2-terminal sequences of cytochromes P-450 resemble signal peptides, but these sequences are not cleaved during the insertion of these integral membrane proteins into the microsomes. To examine whether these putative signal peptides are functionally equivalent to signal peptides of secretory proteins, cDNA coding for a fusion protein was produced, in which the signal peptide for preproparathyroid hormone was replaced with the putative signal peptide of cytochrome P450IIC2. The translational product of RNA synthesized in vitro from the cDNA was neither processed nor translocated by chicken oviduct microsomal membranes in a reticulocyte cell-free system but was resistant to extraction from the membranes by alkaline solutions. In addition, the translation of the hybrid RNA was arrested by signal recognition particle. Unlike most signal peptides, the cytochrome P450IIC2 NH2-terminal sequence does not contain basic amino acids preceding the hydrophobic core. Introduction by oligonucleotide-directed mutagenesis of lysine and arginine at the NH2 terminus resulted in a fusion protein that was partially processed by the microsomal membranes, with translocation across the membrane of both the processed and unprocessed proteins. The positive charges convert the cytochrome P450IIC2 NH2 terminus from a combination membrane insertion-halt transfer signal to a more classical secretory membrane-insertion signal, possibly by altering the orientation of the signal peptide in the membrane.

The production of recombinant HLA-DR beta and invariant chain polypeptides by cDNA expression in E. coli

In this report we describe the production of recombinant fusion proteins of the HLA-DRw6 beta chain and the murine Ia-associated invariant chain. cDNAs encoding the human HLA-DRw6 beta chain and the murine Ia-associated invariant chain were introduced into bacterial expression plasmids. These plasmids direct the synthesis of the respective molecules as fusion proteins of the bacteriophage MS-2 polymerase by E. coli. Fusion proteins purified from crude E. coli lysates were used to raise antisera in rabbits. These antisera were able to immunoprecipitate biosynthetically labelled class II and invariant chain antigens. Additionally, two anti-DR antisera were raised against single domains of the HLA-DR beta chain thus generating reagents with a defined fine specificity. The anti-murine invariant chain serum was shown to cross-react with the human invariant chain and therefore may be useful for studying invariant chain and Ia antigen expression in different species. The method described here permitted us to produce large quantities of immunologically relevant proteins, for use in the production of polyclonal and monoclonal antibodies. Soluble fragments of the fusion proteins representing certain DR domains may also be useful in functional immunological studies.

A specific transmembrane domain of a coronavirus E1 glycoprotein is required for its retention in the Golgi region

The E1 glycoprotein of the avian coronavirus infectious bronchitis virus contains a short, glycosylated amino-terminal domain, three membrane-spanning domains, and a long carboxy-terminal cytoplasmic domain. We show that E1 expressed from cDNA is targeted to the Golgi region, as it is in infected cells. E1 proteins with precise deletions of the first and second or the second and third membrane-spanning domains were glycosylated, thus suggesting that either the first or third transmembrane domain can function as an internal signal sequence. The mutant protein with only the first transmembrane domain accumulated intracellularly like the wild-type protein, but the mutant protein with only the third transmembrane domain was transported to the cell surface. This result suggests that information specifying accumulation in the Golgi region resides in the first transmembrane domain, and provides the first example of an intracellular membrane protein that is transported to the plasma membrane after deletion of a specific domain.

Membrane translocation and insertion of NH2-terminally anchored gamma-glutamyl transpeptidase require a signal recognition particle

The two subunits of the renal brush border enzyme, gamma-glutamyl transpeptidase (EC 2.3.2.2), are derived from a single-chain propeptide. The membrane-spanning domain consists of a hydrophobic sequence near its NH2-terminus and the protein is oriented with its NH2-terminus on the cytoplasmic side. The enzyme is synthesized without a cleavable signal sequence. Translocation and insertion of this enzyme have been shown to be dependent on the signal recognition particle and presumably require the same translocation machinery that other secretory and membrane proteins use for these processes.

Foreign transmembrane peptides replacing the internal signal sequence of transferrin receptor allow its translocation and membrane binding

Each subunit of the human transferrin receptor (TR) dimer is inserted into the ER membrane as a transmembrane polypeptide having its N-terminus in the cytoplasm. The transmembrane segment of the molecule serves both as a signal for chain translocation and as a membrane anchor. To study which structural features of this segment are required for its dual function, we have essentially replaced the transmembrane peptide with the C-terminal membrane-spanning segment of two proteins having a separate N-terminal translocation signal and with an artificial uncharged peptide. In each case the mutant TR molecules are efficiently translocated in vitro. In contrast, substitution of the transmembrane peptide of TR with a hydrophilic peptide results in no detectable translocation activity of the mutant TR. This suggests that the hydrophobic character of the transmembrane peptide of TR, rather than its actual amino acid sequence, is important for chain translocation and membrane binding.