Posttranslational association of immunoglobulin heavy chain binding protein with nascent heavy chains in nonsecreting and secreting hybridomas

A rat monoclonal antibody specific for immunoglobulin (Ig) heavy chain binding protein (BiP) has allowed the examination of the association of BiP with assembling Ig precursors in mouse B lymphocyte-derived cell lines. The anti-BiP monoclonal antibody immunoprecipitates BiP along with noncovalently associated Ig heavy chains. BiP is a component of the endoplasmic reticulum and binds free intracellular heavy chains in nonsecreting pre-B (mu+, L-) cell lines or incompletely assembled Ig precursors in (H+, L+) secreting hybridomas and myelomas. In the absence of light chain synthesis, heavy chains remain associated with BiP and are not secreted. The association of BiP with assembling Ig molecules in secreting hybridomas is transient and is restricted to the incompletely assembled molecules which are found in the endoplasmic reticulum. BiP loses affinity and disassociates with Ig molecules when polymerization with light chain is complete. We propose that the association of BiP with Ig heavy chain precursors is a novel posttranslational processing event occurring in the endoplasmic reticulum. The Ig heavy chains associated with BiP are not efficiently transported from the endoplasmic reticulum to the Golgi apparatus. Therefore, BiP may prevent the premature escape and eventual secretion of incompletely assembled Ig molecules.     

Ultrastructural features of the plasma cells in "non-secretory" myeloma.

A case of "non-secretory" multiple myeloma is described. The diagnosis was based on the clinical picture, typical radiological findings, and infiltration of the bone marrow by myeloma cells which showed specific immunofluorescence staining mainly with antisera for IgM and kappa light chains. An attempt is made to explain the absence of pathological proteins in the serum, based on the ultrastructural findings of the myeloma cells, which showed "buddings" of the cell membranes containing endoplasmic reticulum and cytoplasmic material. It is suggested that the cells of the "non-secretory" type of multiple myeloma possess a normal excretory mechanism, but the pathological proteins are prevented to be secreted in the serum being surrounded by portions of the cell membrane.     

Immunoglobulin synthesis by free polysomes of mouse myeloma cells

Free polyribosomes isolated from mouse myeloma cells in tissue culture synthesize immunoglobulin chains. The presence of these peptide chains in the cytoplasm of intact myeloma cells has been investigated. Some immunoglobulin chains were observed, but it could not be ruled out that these were originally inside cisternae of the endoplasmic reticulum, which were broken during hogenization. We have also investigated the transport of the hypothetical cytoplastic immunoglobulins into the cisternae of the endoplasmic reticulum after incubation with radioactive amino acids and subsequent chase in the absence of protein synthesis. A model to account for synthesis of immunoglobulins on free polysomes is presented. This model assigns specificity for translation on membrane-bound polysomes to the N-terminal region of secretory proteins.     

Degradation of unassembled soluble Ig subunits by cytosolic proteasomes: evidence that retrotranslocation and degradation are coupled events.

Many aberrant or unassembled proteins synthesized in the endoplasmic reticulum (ER) are degraded by cytosolic proteasomes. To investigate how soluble glycoproteins destined for degradation are retrotranslocated across the ER membrane, we analyzed the fate of two IgM subunits, mu and J, retained in the ER by myeloma cells that do not synthesize light chains. Degradation of mu and J is prevented by proteasome inhibitors, suggesting that both chains are retrotranslocated to be disposed of by proteasomes. Indeed, when proteasomes are inhibited, some deglycosylated J chains that no longer contain intrachain disulfide bonds accumulate in the cytosol. However, abundant glycosylated J chains are still present in the ER at time points in which degradation would have been almost complete in the absence of proteasome inhibitors, suggesting that retrotranslocation and degradation are coupled events. This was confirmed by protease protection and cell fractionation assays, which revealed that virtually all mu chains are retained in the ER lumen in a glycosylated state when proteasomes are inhibited. Association with calnexin correlated with the failure of mu chains to dislocate to the cytosol. Taken together, these results suggest that active proteasomes are required for the extraction of Ig subunits from the ER, though the requirements for retrotranslocation may differ among individual substrates.     

Extinction of B-cell surface differentiation markers in hybrids between murine B-lymphoma and myeloma cells

Fusion between murine B-lymphoma cells bearing membrane IgM with either IgG or light chain secreting myeloma, resulted in cell hybrids synthesizing and secreting large quantities of IgM. In contrast, the hybrids did not secrete IgD even though it is also present on the surface of the B-lymphoma cells. B-Cell surface markers such as the IgM, IgD, Ia and the Fc receptor, which were present on the B-lymphoma cells, but not the myeloma cells were not expressed on the surface of the hybrids. Hybrids which secrete IgM and retain the B-cell membrane differentiation antigens were not detected, even when selection was done under conditions which favor the growth of the lymphoma parental cells.     

Glycoprotein quality control in the endoplasmic reticulum. Mannose trimming by endoplasmic reticulum mannosidase I times the proteasomal degradation of unassembled immunoglobulin subunits

Quality control in the endoplasmic reticulum must discriminate nascent proteins in their folding process from terminally unfolded molecules, selectively degrading the latter. Unassembled Ig-mu and J chains, two glycoproteins with five N-linked glycans and one N-linked glycan, respectively, are degraded by cytosolic proteasomes after a lag from synthesis, during which glycan trimming occurs. Inhibitors of mannosidase I (kifunensine), but not of mannosidase II (swainsonine), prevent the degradation of mu chains. Kifunensine also inhibits J chain dislocation and degradation, without inhibiting secretion of IgM polymers. In contrast, glucosidase inhibitors do not significantly affect the kinetics of mu and J degradation. These results suggest that removal of the terminal mannose from the central branch acts as a timer in dictating the degradation of transport-incompetent, glycosylated Ig subunits in a calnexin-independent way. Kifunensine does not inhibit the degradation of an unglycosylated substrate (lambda Ig light chains) or of chimeric mu chains extended with the transmembrane region of the alpha T cell receptor chain, implying the existence of additional pathways for extracting proteins from the endoplasmic reticulum lumen for proteasomal degradation.     

Two types of mu chain complexes are expressed during differentiation from pre-B to mature B cells.

Immunoglobulin mu chains synthesized in murine pre-B cells are known to be associated with surrogate light chains designated as omega (omega), iota (iota) and B34. In addition to these molecules, we identified the complexes of polypeptides (50, 40, 27 and 15.5 kd) associated with surface or intracellular mu chains of pre-B cell lines. Most of these polypeptides were continuously synthesized and associated with mu chains in virgin B cells lines, although some of them scarcely bound to the mu kappa dimer or mu 2 kappa 2 tetramer concomitantly present in the same clone or population. However, in mature B cells they were no longer detectable except B34. Cross-linking of micron chains on the surface of pre-B cells resulted in an increase in intracellular free Ca2+, indicating that the micron chain complex on the surface of pre-B cell lines acted as a signal transduction molecule. However, the receptor cross-linkage of pre-B cell lines did not induce the increased inositol phospholipid metabolism usually observed in virgin and mature B cell lines. These results suggest that, during the differentiation from pre-B to mature B cells, the cells express two types of mu chain complexes which exhibit different structures as a whole and possess different signal transducing capacities.     

Synthesis, processing, and secretion of recombinant human factor VIII expressed in mammalian cells

The synthesis, processing, and secretion of factor VIII expressed from heterologous genes introduced into Chinese hamster ovary cells has been studied. The results show factor VIII to be synthesized as a primary translation product of approximately 230 kDa that can be detected in the lumen of the endoplasmic reticulum. In this compartment, the majority of the factor VIII is in a complex with a resident protein of the endoplasmic reticulum, binding protein, and may never appear in the medium. Some factor VIII transits the endoplasmic reticulum to the Golgi apparatus, where it is cleaved to generate the mature heavy and light chains. In the absence of von Willebrand factor in the medium, the secreted heavy and light chains are unassociated and subsequently degraded. In the presence of von Willebrand factor in the medium, the heavy and light chains are secreted as a stable complex and activity accumulates linearly with time. The utilization and complexity of asparagine-linked carbohydrate present on the secreted recombinant-derived factor VIII and human plasma-derived factor VIII were compared and found to be very similar. In both cases, the asparagine-linked carbohydrate moieties on the heavy chain are primarily of the hybrid or complex-type. In contrast, the factor VIII from both sources contains a high-mannose type of asparagine-linked carbohydrate on the light chain.     

Role of beta2-microglobulin in the intracellular processing of HLA antigens

The biosynthesis of HLA-A, -B, and -C antigens was examined in the two lymphoblastoid cell lines DAUDI and RAJI. In RAJI cells the HLA-A, -B, and -C antigen heavy chains become core-glycosylated in the endoplasmic reticulum as evidenced by their sensitivity to endo-H digestion and tunicamycin treatment. Beta2-Microglobulin is present in excess in the endoplasmic reticulum of the RAJI cells and associates with the heavy chain at the time of synthesis of the heavy chain. Pulse-chase experiments demonstrated that the RAJI HLA-A, -B, and -C antigen heavy chains become terminally glycosylated since their changed characteristics included resistance to endo-H digestion, sensitivity to neuraminidase treatment, and incorporation fucose. DAUDI HLA-A, -B, and -C antigen heavy chains are synthesized normally and become core-glycosylated but not terminally glycosylated. Other glycosylated cell surface proteins, like the HLA-DR antigens, display normal glycosylation in DAUDI cells. Therefore it is unlikely that the absence of terminally glycosylated HLA-A, -B, and -C antigen heavy chains is the result of a general defect in the biosynthetic machinery of DAUDI cells. However, DAUDI cells lack the ability to synthesize beta2-microglobulin, the common subunit of all HLA-A, -B, and -C antigens. Therefore, it seems reasonable to conclude that beta2-microglobulin is of importance for intracellular transport of newly synthesized HLA-A, -B, and -C antigens.     

A selective defect in IgM antigen receptor synthesis and transport causes loss of cell surface IgM expression on tolerant B lymphocytes.

To explore the biochemical basis for maintaining immunological tolerance by functional inactivation of self-reactive B lymphocytes, transgenic mice carrying rearranged anti-lysozyme immunoglobulin transgenes and a lysozyme transgene were used as a source of large numbers of tolerant self-reactive B cells. Antigen receptors of the IgD isotype were expressed at normal levels on tolerant B cells, contained the heterodimeric MB1/B29 signalling component of the receptor complex and were structurally indistinguishable from IgD on nontolerant B cells. In contrast, cell surface expression of IgM receptor complexes on tolerant B cells was greatly reduced, despite normal expression of mRNA encoding the receptor components. Three-fold fewer immunoreactive mu heavy chains were detectable after a short period of biosynthetic labelling and the immunoreactive mu chains produced were paired with kappa light chains and assembled normally into intact receptor complexes containing the MB1/B29 heterodimer. Nascent IgM receptor complexes nevertheless failed to be processed into an endoglycosidase H-resistant form in the tolerant B cells and thus appeared to be selectively blocked in their transport from the endoplasmic reticulum to the medial Golgi. These findings demonstrate that intracellular trafficking of antigen receptor complexes is regulated by exposure to receptor stimuli at the cell surface causing a long-lasting decrease in surface receptor expression on tolerant B cells.     

Surface mu heavy chain expressed on pre-B lymphomas transduces Ca2+ signals but fails to cause growth arrest of pre-B lymphomas.

Little is known about the role of signals transduced by cell surface IgM (sIgM) expressed during early B cell development. A subclone (1.6) of the late pre-B cell lymphoma 70Z/3.12 was used to study signal transduction by surface mu heavy (H) chain before and after transition to the early immature B cell stage, and the functional consequences thereof. Although kappa L chain expression can be induced on 1.6 cells by LPS or cytokines, immunoprecipitations indicated that the non-induced 1.6 cells expressed mu H chain with an alternative protein(s) which may be a surrogate light chain(s). Consistent with this, anti-mu but not anti-kappa or anti-lambda antibodies caused transient Ca2+ mobilization in noninduced 1.6 cells. The Ca2+ signal was derived from both intracellular stores and Ca2+ influx in either noninduced cells or in cells that had been preinduced to express kappa L chain. Thus, the ability of mu H chain to mobilize Ca2+ as a second messenger does not depend upon the expression of mature L chains. The immature B lymphomas, WEHI-231 and CH1, express mature forms of IgM and undergo growth arrest when stimulated by anti-mu antibody. In contrast, signals generated by mu H chain on either noninduced or preinduced 1.6 cells or in the sIgM+ pre-B cell transfectant 300-19 mu lambda 36/8 did not cause growth arrest. These results suggest that mu H chain expressed on pre-B cells is capable of mobilizing Ca2+, but that this signal alone is insufficient to induce growth arrest in the pre-B cell.     

Co-translational modification of nascent immunoglobulin heavy and light chains

We have investigated the in vivo co-translational covalent modification of nascent immunoglobulin heavy and light chains. Nascent polypeptides were separated from completed polypeptides by ion-exchange chromatography of solubilized ribosomes on QAE-Sephadex. First, we have demonstrated that MPC 11 nascent heavy chains are quantitatively glycosylated very soon after the asparaginyl acceptor site passes through the membrane into the cisterna of the rough endoplasmic reticulum. Nonglycosylated completed heavy chains of various classes cannot be glycosylated after release from the ribosome, due either to rapid intramolecular folding and/or intermolecular assembly, which cause the acceptor site to become unavailable for the glycosylation enzyme. Second, we have shown that the formation of the correct intrachain disulfide loop within the first light chain domain occurs rapidly and quantitatively as soon as the appropriate cysteine residues of the nascent light chain pass through the membrane into the cisterna of the endoplasmic reticulum. The intrachain disulfide loop in the second or constant region domain of the light chain is not formed on nascent chains, because one of the cysteine residues involved in this disulfide bond does not pass through the endoplasmic reticulum membrane prior to chain completion and release from the ribosome. Third, we have demonstrated that some of the initial covalent assembly (formation of interchain disulfide bonds) occurs on nascent heavy chains prior to their release from the ribosome. The results are consistent with the pathway of covalent assembly of the cell line, in that completed light chains are assembled onto nascent heavy chains in MPC 11 cells (IgG₂b), where a heavy-light half molecule is the major initial covalent intermediate; and completed heavy chains are assembled onto nascent heavy chains in MOPC 21 cells (IgG₁), where a heavy chain dimer is the major initial disulfide linked intermediate.     

Mouse pre-B cells synthesize and secrete mu heavy chains but not light chains

The immunoglobulins produced by the earliest recognizable B cell precursors (pre-B cells) were characterized in the mouse and human. Immunofluorescent analysis revealed no evidence of surface IgM components, and only mu heavy chains could be detected intracytoplasmically in pre-B cells. Surface IgM components could not be isolated from intact fetal liver cells that lacked sIgM+ B lymphocytes but possessed pre-B cells. Pre-B cells were shown to synthesize and secrete mu heavy chains but not light chains by immunochemical analysis. These mu chains constituted less than 0.01% of TCA precipitable protein synthesized and secreted by fetal liver cells during an 8 hr labelling period. Migration of both intracellular and secreted mu chains on SDS-PAGE suggested that they were smaller than mu chains secreted by mouse and human plasmacytomas. These data indicate that mu chain synthesis precedes light chain expression during B cell ontogeny and suggest a new role for pre-B cells in the generation and expression of a diverse immunoglobulin repertoire.     

Intracellular trafficking and degradation of unassociated proalpha2 chains of collagen type I

Procollagen I is a trimer consisting of two proalpha1(I) chains and one proalpha 2(I) chain. In certain cases of mild osteogenesis imperfecta, abnormal proalpha1(I) chains are degraded very soon after synthesis. As a consequence, the cells produce excess proalpha2(I) chains, which cannot form trimers and are not secreted. The objective of this work was to determine the intracellular fate of unassociated proalpha2(I) chains. Mov13 mouse fibroblasts, which do not synthesize proalpha1(I) mRNA, but do produce proalpha2(I) mRNA, were incubated with radioactive amino acids using pulse-chase protocols, and proteins were analyzed by gel electrophoresis, autoradiography, and Western blotting. Mov13 cells produced proalpha2(I) chains that were degraded intracellularly within 30 min. Degradation was inhibited when cells were treated with brefeldin-A, which blocks transit from endoplasmic reticulum to Golgi. Fixed cells exposed to various immunofluorescence markers and imaged by confocal laser scanning microscopy showed that proalpha2(I) chains colocalized with Golgi and lysosome markers. Degradation was inhibited and chains were secreted when cells were treated with wortmannin, which blocks trafficking to lysosomes. These results demonstrate that unassociated proalpha2(I) chains leave the endoplasmic reticulum, transit the Golgi, and enter lysosomes where they are degraded.     

Control of IgM synthesis in the murine pre-B cell line, 70Z/3'

The murine 70Z/3 tumor resembles a pre-B cell in synthesizing only intracellular mu-chains and no detectable light chain. However, one kappa gene is already rearranged, and after overnight incubation with lipopolysaccharide (LPS), most of the cells are induced to synthesize light chain. The induced cells display IgM on their surface, but do not secrete IgM. Thus, 70Z/3 cells resemble cells poised at the pre-B cell/B lymphocyte border. We have examined synthesis and post-translational modification of mu-chains in uninduced and induced 70Z/3 cells. Isolation of mu-chains and peptide maps demonstrated that both populations synthesize intracellular forms that correspond to membrane-specific mum and secretion-specific mus chains. These intracellular forms have completed only the first of the two glycosylation steps characteristic of eukaryotic cells. After induction by LPS, L chain synthesis commences, mum and mus synthesis are both increased twofold to threefold (due to an increased rate of synthesis rather than decreased degradation), and both complex with L chain to form mu2L2 tetramers. Furthermore, the glycosylation of a subset of the mum chains is completed, and these are placed on the membrane. However, unglycosylated mu2L2 tetramers can be placed on the membrane, so glycosylation is not a requirement. These data suggest that L chain may not be sufficient for externalization of mum and mus chains. These data support the idea that the controls of membrane placement and secretion of mu chains are post-translational and that different mechanisms operate for mum and mus chains.