Interaction between two domains of the P. yoelii MSP-1 protein detected using the yeast two-hybrid system

Several model systems of plasmodia have demonstrated the potential of the merozoite surface protein, MSP-1, to induce protective immunity. However, little is known about the function of this protein or its interaction with other surface molecules that may also serve as immunological targets. To identify potentially significant inter- and intra-molecular interactions involving MSP-1, we have utilized the yeast two-hybrid system. A cDNA activation domain library was constructed from the erythrocytic stages of the murine malarial parasite Plasmodium yoelii yoelii 17XL. A 795 bp region of Py17XL MSP-1 (bait), homologous to the Plasmodium falciparum MSP1(33) fragment, was inserted into a Gal4p DNA binding domain vector and used to screen the activation domain library (target). Several randomly selected clones that demonstrated bait-target interaction were found to express overlapping regions of Py17XL MSP-1. Deletion constructs further localized the peptide fragments retaining interaction indicating that a region within the MSP-1(38) fragment interacts with the MSP-1 bait domain. Subsequent studies confirmed this interaction, as both peptides were co-precipitated from cell lysate by a peptide tag-specific antibody. It was observed that the interaction of these two fragments significantly increased the half-life of the MSP-1(38) within yeast cells. The specific interaction described here demonstrates the potential of this approach to elucidate additional inter- or intra-molecular interactions of Py17XL MSP1 and other malarial proteins.

A cysteine residue in helixII of the bHLH domain is essential for homodimerization of the yeast transcription factor Pho4p

The yeast transcription factor Pho4p is required for expression of the phosphate-repressible acid phosphatase encoded by the PHO5 gene. Functional studies have shown that the molecule is composed of an N-terminal acidic activation domain, a central region which is necessary for interaction with a negative regulatory factor (the cyclin Pho80) and a C-terminal basic helix-loop-helix domain, which mediates DNA binding and homodimerization. In this study the homodimerization domain maps specifically to helixII of this region and a cysteine residue within this region is essential for this function. Experiments support the role of an intermolecular disulfide bond in stabilization of homodimerization, which is critical for DNA binding.

Elevated expression of stress response genes resulting from deletion of the PHO85 gene

The cyclin-dependent protein kinase Pho85 is a known negative regulatory factor for two stress response genes, PHO5 and GSY2, which encode the inducible form of acid phosphatase and glycogen synthase, respectively, in the yeast Saccharomyces cerevisiae. Cells carrying a disruption of the PHO85 gene inappropriately express both PHO5 and GSY2, resulting in the increase in phosphate scavenging and hyperaccumulation of glycogen in nutrient-rich conditions. Constitutive activation of PKA in a pho85 mutant suppresses the hyperaccumulation of glycogen. This work presents data to show that, at least in part, the suppression of glycogen biosynthesis upon activation of PKA in a pho85 mutant results from the suppression of GSY2 expression. In addition to GSY2, disruption of the PHO85 gene inappropriately triggers the derepression of two other stress response genes, HSP12 and UBI4. At least in the case of GSY2, regulation of transcription by Pho85 is not through the stress-responsive cis-promoter elements (STRE). Furthermore, Pho85 may associate with the known cyclin Pho80 in the transcriptional regulation of these genes.

Interaction of Saccharomyces cerevisiae Pho2 with Pho4 increases the accessibility of the activation domain of Pho4

In Saccharomyces cerevisiae, expression of acid phosphatase, encoded by the PHO5 gene, requires two positive regulatory factors, Pho4 and Pho2 (also called Bas2 or Grf10). Using GAL4-PHO4 fusions, we demonstrate that a functional interaction between these two proteins is necessary for transcriptional activation to occur. This functional interaction between Pho4 and Pho2 is independent of the presence of the negative regulatory factor, Pho80, which also interacts with Pho4. Interestingly, truncations of Pho4 missing amino acids 252-265, which encompass the basic region of the basic helix-loop-helix (bHLH) DNA binding motif, exhibit high transcriptional activation that is independent of the Pho2 molecule. Single amino acid mutations of highly conserved residues within this area all display this Pho2-independent phenotype. A region near the C-terminus of Pho2 appears to be critical for this interaction with Pho4. A model to account for the requirement for Pho2 in Pho4-dependent transcriptional activation is proposed.

Deletion of the gene encoding the cyclin-dependent protein kinase Pho85 alters glycogen metabolism in Saccharomyces cerevisiae

Pho85, a protein kinase with significant homology to the cyclin-dependent kinase, Cdc28, has been shown to function in repression of transcription of acid phosphatase (APase, encoded by PHO5) in high phosphate (Pi) medium, as well as in regulation of the cell cycle at G1/S. We described several unique phenotypes associated with the deletion of the PHO85 gene including growth defects on a variety of carbon sources and hyperaccumulation of glycogen in rich medium high in Pi. Hyperaccumulation of glycogen in the pho85 strains is independent of other APase regulatory molecules and is not signaled through Snfl kinase. However, constitutive activation of cAPK suppresses the hyperaccumulation of glycogen in a pho85 mutant. Mutation of the type-1 protein phosphatase encoded by GLC7 only partially suppresses the glycogen phenotype of the pho85 mutant. Additionally, strains containing a deletion of the PHO85 gene show an increase in expression of GSY2. This work provides evidence that Pho85 has functions in addition to transcriptional regulation of APase and cell-cycle progression including the regulation of glycogen levels in the cell and may provide a link between the nutritional state of the cell and these growth related responses.

Structure-function relationships of the yeast cyclin-dependent kinase Pho85

The PHO85 gene of Saccharomyces cerevisiae encodes a cyclin-dependent kinase involved in both transcriptional regulation and cell cycle progression. Although a great deal is known concerning the structure, function, and regulation of the highly homologous Cdc28 protein kinase, little is known concerning these relationships in regard to Pho85. In this study, we constructed a series of Pho85-Cdc28 chimeras to map the region(s) of the Pho85 molecule that is critical for function of Pho85 in repression of acid phosphatase (PHO5) expression. Using a combination of site-directed and ethyl methanesulfonate-induced mutagenesis, we have identified numerous residues critical for either activation of the Pho85 kinase, interaction of Pho85 with the cyclin-like molecule Pho80, or substrate recognition. Finally, analysis of mutations analogous to those previously identified in either Cdc28 or cdc2 of Schizosaccharomyces pombe suggested that the inhibition of Pho85-Pho80 activity in mechanistically different from that seen in the other cyclin-dependent kinases.

cDNA sequence and differential mRNA regulation of two forms of glial cell line-derived neurotrophic factor in Schwann cells and rat skeletal muscle

Total RNA from rat Schwann cells grown in culture and adult rat skeletal muscle was reverse transcribed, amplified for glial cell line-derived neurotrophic factor (GDNF) messenger RNA (mRNA) using the polymerase chain reaction (PCR), and the PCR products sequenced. Two forms of GDNF were detected in the PCR step, one of a predicted size (GDNF633) and a second smaller form missing a 78-base pair sequence (GDNF555). Sequence analysis demonstrated that GDNF633 is similar to the published sequence of GDNF differing only at three nucleotides. Southern and Northern blot analyses reveal that the two forms are probably derived from a single RNA species that is alternatively spliced. Interestingly, GDNF633 mRNA was found to be selectively upregulated in denervated rat skeletal muscle at 1-2 weeks following axotomy, providing evidence that the innervation status of the muscle may determine the expression profile of the two alternatively spliced forms. Given these findings, we suggest that GDNF may function as a target-derived trophic factor for neuronal populations innervating skeletal muscle, including sensory neurons and spinal cord motoneurons.

Cloning of a partial length cDNA encoding the C-terminal portion of the 75-77-kDa antigen of Trypanosoma cruzi

It has been suggested that several Trypanosoma cruzi antigens have possible protective epitopes which may be suitable vaccine candidates. We found previously that animals resistant to T. cruzi infection produced antibodies against the 75-77-kDa parasite antigen. To test the ability of the recombinant form of this antigen to protect animals from T. cruzi infection, the cDNA which encodes a portion of the 75-77-kDa antigen was cloned using a cDNA library constructed in an orientation-specific bacteriophage expression vector (lambda gt 11) from poly (A)+ RNA of Brazil strain epimastigotes. One clone, named SFS-40, was selected by screening the library using affinity purified antibodies specific for the 75-77-kDa parasite antigen as probe. The cDNA corresponding to the 1.7-kilobase insert of SFS-40 was subcloned into plasmid vectors and characterized. The cDNA sequence encodes a polypeptide of about 40 kDa. The putative product of the cDNA was homologous to members of the 70-kDa stress protein family. When epimastigotes were shifted from 29 degrees C to 37 degrees C, there was no change in the level of SFS-40 mRNA. In contrast, the 70-kDa heat shock protein mRNA of the parasite was increased about four fold by this treatment.

The relationship between the trimethylation of lysine 77 and cytochrome c metabolism in Saccharomyces cerevisiae

1. Site directed mutations were constructed in the yeast iso-1-cytochrome c gene adjacent to the lysine 77 (methylation site) codon. 2. These mutant genes were then cloned and transformed into the S. cerevisiae strain B-6642 which contains a deficiency in the iso-1-cytochrome c gene. 3. The resulting transformants were screened for cytochrome c production using gel electrophoresis. 4. Amino acid analysis of the mutated cytochromes c demonstrated varying levels of trimethyllysine formation, depending on the nature of the site directed mutation. 5. The resulting transformants were then used as tools in order to investigate the relationship between trimethyllysine formation and various aspects of cytochrome c metabolism including protein stability and heme conjugation.

Molecular analysis of the PHO81 gene of Saccharomyces cerevisiae

The PHO81 gene product is a positive regulatory factor required for the synthesis of the phosphate repressible acid phosphatase (encoded by the PHO5 gene) in Saccharomyces cerevisiae. Genetic analysis has suggested that PHO81 may be the signal acceptor molecule; however, the biochemical function of the PHO81 gene product is not known. We have cloned the PHO81 gene and sequenced the promoter. A PHO81-LacZ fusion was shown to be a valid reporter since its expression is regulated by the level of inorganic phosphate and is controlled by the same regulatory factors that regulate PHO5 expression. To elucidate the mechanism by which PHO81 functions, we have isolated and cloned dominant mutations in the PHO81 gene which confer constitutive synthesis of acid phosphatase. We have demonstrated that overexpression of the negative regulatory factor, PHO80, but not the negative regulatory factor PHO85, partially blocks the constitutive acid phosphatase synthesis in a strain containing a dominant constitutive allele of PHO81. This suggests that PHO81 may function by interacting with PHO80 or that these molecules compete for the same target.

Further investigations regarding the role of trimethyllysine for cytochrome c uptake into mitochondria

1. A mutant of the iso-1-cytochrome c gene from Saccharomyces cerevisiae has been constructed which contains an Arg codon, replacing the normal trimethylated Lys at position 77. 2. This mutated gene was cloned into a pGem 1 vector and used for the in vitro translation of yeast iso-1-cytochrome c. 3. Utilizing an in vitro mitochondria binding assay, it was found that the mutant cytochrome c could transverse the yeast mitochondrial membrane, however the amount of protein incorporated was 3-fold less that of the trimethylated wild type. 4. Omission of the protein methyltransferase from assays containing the wild type cytochrome c caused only a slight reduction (15%) in the amount of protein incorporated. 5. These results suggest while the lysine residue 77 of apocytochrome c is important for mitochondria uptake, the methylation of this residue seems to play a relatively minor role.

Molecular and expression analysis of the negative regulators involved in the transcriptional regulation of acid phosphatase production in Saccharomyces cerevisiae

The PHO80 and PHO85 gene products encode proteins necessary for the repression of transcription from the major acid phosphatase gene (PHO5) of Saccharomyces cerevisiae. The deduced amino acid sequences of these genes have revealed that PHO85 is likely to encode a protein kinase, whereas no potential function has been revealed for PHO80. We undertook several approaches to aid in the elucidation of the PHO80 function, including deletion analysis, chemical mutagenesis, and expression analysis. DNA deletion analysis revealed that residues from both the carboxy- and amino-terminal regions of the protein, amounting to a total of 21% of the PHO80 protein, were not required for function with respect to repressor activity. Also, 10 independent single-amino-acid changes within PHO80 which resulted in the failure to repress PHO5 transcription were isolated. Nine of the 10 missense mutations resided in two subregions of the PHO80 molecule. In addition, expression analysis of the PHO80 and PHO85 genes suggested that the PHO85 gene product was not necessary for PHO80 expression and that the PHO85 gene was expressed at much higher levels in the cell than was the PHO80 gene. Furthermore, high levels of PHO80 were shown to suppress the effect of a PHO85 deletion at a level close to full repression. Implications for the function of the negative regulators in this system are discussed.

Structure and expression of the PHO80 gene of Saccharomyces cerevisiae

In yeast, the repression of acid phosphatase under high phosphate growth conditions requires the trans-acting factor PHO80. We have determined the DNA sequence of the PHO80 gene and found that it encodes a protein of 293 amino acids. The expression of the PHO80 gene, as measured by Northern analysis and level of a PHO80-LacZ fusion protein is independent of the level of phosphate in the growth medium. Disruption of the PHO80 gene is a non-lethal event and causes a derepressed phenotype, with acid phosphatase levels which are 3-4 fold higher than the level found in derepressed wild type cells. Furthermore, over-expression of the PHO80 gene causes a reduction in the level of acid phosphatase produced under derepressed growth conditions. Finally, we have cloned, localized and sequenced a temperature-sensitive allele of PHO80 and found the phenotype to be due to T to C transition causing a substitution of a Ser for a Leu at amino acid 163 in the protein product.

Molecular analysis of the DNA sequences involved in the transcriptional regulation of the phosphate-repressible acid phosphatase gene (PHO5) of Saccharomyces cerevisiae

The expression of the PHO5 gene of Saccharomyces cerevisiae is transcriptionally regulated in response to the level of inorganic phosphate present in the growth medium. We have identified, by DNA deletion analysis, the sequences (upstream activator sequences) that mediate this response. The sequence 5' CTGCACAAATG 3' is present in two copies located within a 60-base-pair region. The presence of a single copy of the sequence is sufficient for the phosphate-mediated transcriptional response. In addition, a DNA fragment that contains two copies of this sequence will act to repress transcription of a CYC1-lacZ fusion when placed either upstream or downstream of the CYC1 activator sequence.

A DNA fragment containing the upstream activator sequence determines nucleosome positioning of the transcriptionally repressed PHO5 gene of Saccharomyces cerevisiae

The functional relationship of nucleosome positioning and gene expression is not known. Using high-copy plasmids, containing the yeast phosphate-repressible acid phosphatase gene (PHO5) and the TRP1/ARS1 vector system, I have determined the nucleosomal structure of the 5' region of the PHO5 gene and demonstrated that the nucleosomal positioning of this region is independent of orientation or position in the various plasmid constructions utilized. However, deletion of a 278-base pair BamHI-ClaI fragment from the 5'-flanking sequences of the PHO5 gene causes the nucleosome positioning to become dependent on orientation or position in the plasmids tested. Use of PHO5-CYC1-lACZ fusions have demonstrated that this DNA fragment contains the sequences responsible for the transcriptional regulation of the PHO5 gene in response to the level of phosphate in the growth media. The nucleosome positioning in the 5' region of PHO5 may be determined by an interaction with the sequences or machinery responsible for transcriptional regulation of the gene.

Structure of the transcriptionally repressed phosphate-repressible acid phosphatase gene (PHO5) of Saccharomyces cerevisiae

We developed a high-copy-number plasmid system containing the entire structural and regulatory sequences of the phosphate-repressible acid phosphatase (PHO5) gene and the TRP1/ARS1 replicator sequences of the yeast Saccharomyces cerevisiae to investigate the mechanism of repression-derepression of transcription. The resulting plasmid was used to transform either wild-type cells or a number of strains which contain mutations in various trans-acting regulatory loci for the production of acid phosphatase. Results of analysis of mRNA levels isolated from the transformed strains grown under repressed or derepressed conditions suggested that normal transcriptional regulation of the gene persisted, although gene copy number was significantly increased. Analysis of changes in linking number (i.e., the number of negative supercoils) of the plasmid isolated under repressed and derepressed growth conditions revealed that the transcriptionally inactive plasmid contained approximately three more negative supercoils than the transcriptionally active plasmid. This difference in topological state was similarly seen in a plasmid containing a sequence-related acid phosphatase gene (PHO11) under the same regulatory control system, but it was not seen in plasmids isolated from some strains containing mutations which caused either fully constitutive or nonderepressible production of acid phosphatase. Finally, analysis of the nucleosome positioning along the inactive gene sequence revealed that an abnormally broad internucleosomal spacer is present in a region presumed to function in the regulation of transcription by the level of Pi in the growth media.

Nuclease digestion of circular TRP1ARS1 chromatin reveals positioned nucleosomes separated by nuclease-sensitive regions

TRP1ARS1 is a circular yeast DNA of 1453 base-pairs that contains the N-5'phosphoribosyl anthranilate isomerase (TRP1) gene and a sequence important for autonomous replication (ARS1). It exists extrachromosomally in 100 to 200 copies/cell and is presumably packed in nucleosomes. TRP1ARS1 has been partially purified as chromatin from lysed spheroplasts of yeast using gel filtration. A structural analysis of mapping micrococcal nuclease and DNAase I cutting sites with an accuracy of +/- 20 base-pairs is presented. Comparison of nuclease cleavage sites in chromatin and in purified DNA reveals that regions which are protected against nuclease attack are not distributed randomly. These regions are big enough to accommodate nucleosome cores. Three nucleosomes are positioned in the so-called ARS sequences, and are stable at low and high levels of digestion. The TRP1 gene region is covered by four nucleosomes, but they are neither randomly arranged nor precisely positioned. They are not stable and rearrange or disintegrate during digestion. The nucleosomal regions are separated by two segments of DNA (A, B), each about 180 base-pairs long, which are very sensitive to DNAase I and micrococcal nuclease and therefore presumably not packed in nucleosomes. Region B is found 5' to the TRP1 gene and might be related to transcription, whereas region A is centered around the termination codon of the TRP1 gene and the putative origin of replication.

Modulation of chromatin structure associated with derepression of the acid phosphatase gene of Saccharomyces cerevisiae

We have analyzed the chromatin structure of a phosphate-repressible acid phosphatase gene (PHO5) within yeast nuclei. Under derepressed conditions (low Pi media), the gene is much more sensitive to either DNAse I or micrococcal nuclease digestion than is the repressed gene. We have mapped DNase I hypersensitive sites unique to the active gene near the 5'-end of the acid phosphatase mRNA and within a region presumed to function in the regulation of the gene by Pi. Although the gene is packaged into regularly spaced nucleosomes, no detectable phase relationship exists between nucleosomes and DNA sequence under derepressed conditions, whereas in the repressed state the nucleosomes occur in one predominant phase. These results demonstrate reversible changes in the chromatin structure of a eukaryotic gene system that directly correlate with the functional state of the gene.

Glycosylation causes an apparent block in translation of immunoglobulin heavy chain

Analysis of nascent heavy chains isolated from MPC11 (gamma 2b heavy chains) and MOPC 21 (gamma 1 heavy chains) mouse myeloma cells demonstrates an accumulation of nascent heavy chains which are slightly smaller in mass (approximately 35,000 daltons) than nascent heavy chains which have just been glycosylated (approximately 38,000 daltons). The accumulation of 35,000-dalton nascent heavy chain appears to be a consequence of the glycosylation process since tunicamycin, an inhibitor of glycosylation, abolishes the apparent translational block manifested by the accumulation of 35,000-dalton nascent chains. Tunicamycin also causes a 15 to 25% increase n the relative rate of synthesis of heavy chain compared to the corresponding rate of synthesis of the nonglycosylated light chain synthesized by the same cell. These results suggest that the translation block, caused by the glycosylation process, of heavy chain synthesis contributes to the imbalance of heavy chain and light chain biosynthesis observed in malignant and normal lymphoid cells.

Structure of sea urchin sperm chromatin core particle

Sea urchin sperm chromatin contains forms of H1, H2A, and H2B which differ from those present in adult tissues. We have delineated some effects of the variant H2A and H2B on chromatin by study of the structure of the core particle from Strongylocentrotus purpuratus sperm. The particle contains 145 base pairs of DNA and equal amounts of the four smaller histones. It sediments at 11 S and has a circular dichroism spectrum similar to that of particles containing more typical histones. The sperm core particle undergoes a shape change at low ionic strength, as observed for chicken erythrocyte particles. In contrast to these similarities, the melting profile of the sperm particle is quite different from that of erythrocyte; both the reversible transition and the irreversible denaturation of the core particle occur at higher temperatures. The sperm core particle is digested by DNase I more slowly than the core particle from chicken erythrocyte. Cutting site maps for the sperm core particle reveal the same basic organization of DNA by these histones; however, certain features of the map differ significantly from that for chicken erythrocyte, demonstrating a modulation of the canonical core particle structure by the unusual histones present in sea urchin sperm.

Formation of intermolecular disulfide bonds on nascent immunoglobulin polypeptides

The initial step of intermolecular covalent assembly of immunoglobulins molecules involves formation of heavy chain-light chain or heavy chain-heavy chain disulfide bonds. Using QAE-Sephadex chromatography to isolate microsomal nascent polypeptides, we have shown that this initial step of intermolecular covalent assembly occurs, to a substantial extent, on nascent heavy chains, as well as on completed heavy chains as previously demonstrated by others. In MPC 11 mouse myeloma cells, completed light chains are assembled covalently to nascent heavy chains, whereas in MOPC 21 mouse myeloma cells, completed heavy chains are assembled covalently to nascent heavy chains. These results are consisted with the heavy-light half-molecule being the major initial intermediate in the assembly of MPC 11 IgG2b and heavy-heavy dimer being the major initial intermediate formed in assembly of MOPC 21 IgG1. The nascent MPC 11 heavy chain must be at least 38,000 daltons in size before assembly with the light chain occurs, even though the heavy chain cysteine involved in this disulfide bond is 131 residues (approximately 15,000 daltons) from the NH2 terminus. In addition, pulse-chase labeling studies of MPC 11 cells have shown that the assembly of completed light chains with the nascent heavy chain must occur within a few minutes of the synthesis of the light chain even though a large excess of unassembled MPC 11 light chains remain inside the cell for an average time of 2 h before being secreted.

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.

Temporal relationship of translation and glycosylation of immunoglobulin heavy and light chains

The initial glycosylation of MPC 11 gamma 2b heavy chains occurs quantitatively in vivo when the nascent heavy chains reach a size of approximately 38 000 daltons. Nonglycosylated, completed MPC 11 heavy chains cannot be glycosylated in these cells. Other classes of mouse heavy chains (i.e., mu, alpha, and gamma 1) also appear to be glycosylated as nascent chains; nonglycosylated, completed heavy chains cannot be glycosylated by the cell in any of these cases. In contrast, variant MPC 11 cells synthesizing a heavy chain with a carboxy-terminal deletion appear to glycosylate some heavy chains prior to chain completion and some heavy chains after chain completion and release from the polysomes. Similar to the variant MPC 11 cells, MOPC 46B cells (which synthesize a kappa light chain containing an oligosaccharide attached to an asparagine located 28 residues from the amino terminus) glycosylate the majority of light chains after prior to chain completion but also some light chains after chain completion and release from the polysomes. In addition, it appears that, although completed MOPC 46B light chains can be glycosylated if they are present in a monomeric form, they cannot be glycosylated if they are present in a covalent dimeric form.

Addition of glucosamine and mannose to nascent immunoglobulin heavy chains

We have investigated the process of protein glycosylation in an attempt to answer the question of whether glucosamine and mannose are added to nascent chains prior to chain completion or only to completed chains after release from the ribosome. The MPC 11 mouse plasmacytoma cell line used in these studies synthesizes a glycosylated gamma2b heavy chain which accounts for 12% of the total protein synthesis. Nascent chains were separated from completed chains by ion-exchange chromatography of solubilized ribosomes on QAE-Sephadex. Our results indicate that both glucosamine and mannose are incorporated into nascent heavy chains prior to chain completion and release from the ribosome. Gel analysis of specifically immunoprecipitated nascent chains indicates that the carbohydrate moiety can be added to the nascent heavy chains very soon after the presumptive asparaginyl glycosylation site (CH2 domain) is synthesized on the ribosome.