Activation of helix-loop-helix proteins Id1, Id2 and Id3 during neural differentiation

Id is a nuclear factor containing a helix-loop-helix (HLH) motif. It has been reported that Id functions as an inhibitor of cell differentiation and its gene expression is down-regulated during cell differentiation. We have characterized three Id-related cDNAs, Id1, Id2 and Id3, isolated from nerve growth factor (NGF)-stimulated PC12 cells. Structural analysis revealed that all three Id's contain putative phosphorylation sites for cyclic AMP-dependent kinase, casein kinase II and protein kinase C near and inside the HLH motifs. In contradiction to the previous reports, Northern blot analysis revealed that NGF induces rapid and transient increase of all three Id gene transcriptions. Furthermore, in situ hybridization of rat embryo showed that all three Id genes are highly expressed in neural precursors rather than differentiated neural cells. These results indicate that Id family members may function as immediate-early gene products, and that the expression of the Id family may play an important role in the early stage of neural differentiation.

Induction of erythroid-specific gene expression in lymphoid cells

Erythropoietin (Epo) is a cytokine which specifically regulates differentiation and proliferation of erythroid progenitor cells. We report here that Epo receptor expressed in interleukin 3-dependent lymphoid Ba/F3 cells transmits both differentiation and growth signals. Epo stimulation of these cells leads to activation of transcription and/or translation of the erythroid-specific transcription factors GATA-1 and SCL, followed by the accumulation of both alpha- and beta-globin chains. These results suggest that expression and activation of the Epo receptor regulates erythroid-specific gene expression and might play a role in determining a cell lineage in vivo and that GATA-1 and SCL may exert their effects after Epo binds to its receptor. It was further found that chimeric receptors composed of extracellular domains of Epo receptor and cytoplasmic domains of interleukin 2 or interleukin 3 receptors could also induce erythroid-specific gene expression in Ba/F3 cells. Taking these data together with previous observations, we conclude that interaction of the extracellular domains of the Epo receptor with other membrane components is essential for transmission of both the erythroid differentiation and the growth signals.

Ligand-dependent activation of chimeric receptors with the cytoplasmic domain of the interleukin-3 receptor beta subunit (beta IL3)

beta IL3 (formerly known as AIC2A), a beta subunit of the murine interleukin-3 receptor (IL-3R), is not only required for formation of the high affinity receptor but is also important for signal transduction. To examine the function of beta IL3 in signal transduction, we constructed several chimeric receptors consisting of the intracellular portion of beta IL3 and the extracellular portion of other members of the cytokine receptor superfamily, i.e. the human interleukin-2 receptor beta chain (hIL-2R beta), the human interleukin-4 receptor (hIL-4R), and the murine erythropoietin receptor (mEpoR). These chimeric receptors and normal cytokine receptors were expressed in an IL-3-dependent murine pro-B cell line, Ba/F3, and an IL-2-dependent murine T cell line, CTLL2. Regardless of the origin of the extracellular domain, these chimeric receptors were functional in Ba/F3 cells; they stimulated proliferation and induced tyrosine phosphorylation in response to the cytokine corresponding to the extracellular domain. However, the response of transfectants expressing chimeric receptors was similar to, but not identical with, the response of Ba/F3 cells to mIL-3. We present evidence that the IL-4R and EpoR probably have an additional component which is involved in signal transduction.

Erythropoietin receptor binds to Friend virus gp55 through other membrane components

Direct interactions of Friend spleen focus-forming virus glycoprotein gp55 with either erythropoietin receptor (EpoR) or interleukin (IL)2 receptor beta chain (IL2R) (but not with IL3 receptor) have been reported to induce factor-independent prolonged proliferation of erythroid or lymphoid cells. In order to clarify the molecular mechanism by which EpoR-gp55 complex transmits an aberrant growth signal in the absence of erythropoietin, various chimeric receptors constituted with IL2R, EpoR or IL3 receptor were constructed. It was found that coexpression of gp55 and the chimeric receptors containing the cytoplasmic domains of EpoR and the extracellular domains of IL3 (or IL2) receptor in IL3-dependent Ba/F3 cells results in factor-independent growth. Since gp55 in cell membrane has only a two amino acid tail in the cytoplasmic domains and thus cannot interact with EpoR in cytoplasm, our data suggest that gp55 does not bind EpoR directly but interacts with EpoR through third membrane component(s).

Tyrosine kinase activation through the extracellular domains of cytokine receptors

Interaction of cytokines with their membrane receptors induces the proliferation and differentiation of a specific lineage of haematopoietic progenitors. The molecular mechanism of cytokine receptor-mediated signal transduction is unclear because these receptors do not have tyrosine kinase activity. Interleukin-3 and erythropoietin, however, induce transient tyrosine phosphorylation of a common set of proteins as a growth signal, and interleukin-2 induces phosphorylation of an overlapping but distinct set of proteins. Here we show that chimaeric receptors consisting of the extracellular domains of the erythropoietin receptor and the cytoplasmic domains of the interleukin-2 (or interleukin-3) receptor induce an erythropoietin-dependent tyrosine phosphorylation in interleukin-3-dependent Ba/F3 cells; however, chimaeric receptors composed of the extracellular domains of the interleukin-2 receptor and the cytoplasmic domains of the erythropoietin (or interleukin-3) receptor apparently transmit an interleukin-2-dependent signal. Our results indicate that these cytokines transmit distinct signals for activation of specific tyrosine kinases through the extracellular rather than cytoplasmic domains of the receptors.

Functionally essential cytoplasmic domain of the erythropoietin receptor

The cytoplasmic domains of the erythropoietin receptor essential for signal transduction were identified by assessing a series of truncated and deletional mutant receptors. A 91-amino acid region proximal to the transmembrane domain was required for growth signaling. In this region, residues between 353Pro and 362His and between 278Gln and 308Leu appeared to constitute the essential cytoplasmic domains. These two domains contain the conserved amino acids common in the cytokine receptor superfamily, which indicates that these domains in the cytoplasmic regions of the erythropoietin receptor may be important for interaction with common signal transducers or protein tyrosine kinases.

Tryptophan residue of Trp-Ser-X-Trp-Ser motif in extracellular domains of erythropoietin receptor is essential for signal transduction

The Trp-Ser-X-Trp-Ser motif commonly exists just outside the transmembrane domains of all cytokine receptors so far isolated. The role of this conserved motif in erythropoietin receptor was examined by assessing a series of mutant receptors on erythropoietin-induced signal transduction. Replacement of one of the two conserved Trp residues in the motif to Gly was found to completely abolish the binding of erythropoietin to the receptor and also to lose the ability to transduce the factor-dependent growth signal. While the mutants with one Ser residue converted to Gly or Ala retained full biological activities, the replacement of both conserved Ser residues diminished the functions of the receptor. Furthermore, the receptors lacking a part or all of the Trp-Ser-X-Trp-Ser motif did not respond to erythropoietin. The Trp-Ser-X-Trp-Ser motif, especially Trp residue, located in extracellular domains of the erythropoietin receptor thus appears to play a critical role in receptor-mediated signal transduction.

Growth signal erythropoietin activates the same tyrosine kinases as interleukin 3, but activates only one tyrosine kinase as differentiation signal

By Western blotting with anti-phosphotyrosine-specific antibody, we demonstrated that both erythropoietin (Epo) and interleukin 3 (IL3) induce rapid and transient tyrosine phosphorylation of a common set of proteins of 45, 55, 69, 87, 90, 95 and 160 KDa as a growth signal in Epo- and IL3-dependent FD-M6 cells. In contrast, only two proteins of 87 and 90 KDa were transiently phosphorylated in Epo-induced erythroid differentiation of SKT6 cells. Furthermore, no tyrosine phosphorylation was observed in dimethyl sulfoxide-induced differentiation of SKT6 cells. Taken together with other observations, these results indicate that Epo, IL3 and GM-CSF activate the same tyrosine protein kinases as growth signal and that Epo-induced differentiation signal uses only a part of the tyrosine kinase pathway.

Isolation of a cDNA encoding a potential soluble receptor for human erythropoietin

Analysis of human erythropoietin receptor-encoding cDNAs revealed the possible existence of a soluble receptor lacking the transmembrane and cytoplasmic domains.

GATA-1 transactivates erythropoietin receptor gene, and erythropoietin receptor-mediated signals enhance GATA-1 gene expression

Erythropoietin is a cytokine which specifically regulates the proliferation and differentiation of erythroid progenitor cells. The expression of erythropoietin receptor on the cell membrane of the progenitor cells is a critical event during the erythroid differentiation process. In order to clarify the tissue-specific and differentiation stage-specific expression of the erythropoietin receptor gene, its transcriptional regulation was examined by transient expression assay, gel mobility shift assay and DNase I footprinting. The results clearly showed that GATA-1 transactivates the gene expression through a single GATA motif located around -200 bp upstream from the ATG codon in a dose dependent manner. Furthermore, Northern blot analysis revealed that erythropoietin receptor-mediated signals strongly enhanced GATA-1 gene expression in accordance with the appearance of hemoglobin-positive cells. Taken together with other observations, these results suggested the following scheme of erythroid differentiation: 1)GATA-1 is expressed in the early stage of blood cell development; 2) GATA-1 transactivates the erythropoietin receptor gene; 3) erythropoietin binds its receptor and the receptor-mediated signals enhance GATA-1 gene expression in erythroid progenitor cells; and 4) GATA-1 finally transactivates hemoglobin synthesis-related genes and globin genes in relatively matured erythroid cells.

Constitutive expression of exogenous c-myb gene causes maturation block in monocyte-macrophage differentiation

A nuclear protooncogene c-myb has been hypothesized to play an important role in hematopoiesis, but little is known about the physiological function of the c-myb gene products. To study the role of c-myb gene expression in monocyte-macrophage differentiation and proliferation, we introduced exogenous c-myb gene into murine myelomonocytic leukemia WEHI-3B(D+) cells which can be induced to differentiate into mature monocytes with granulocyte-colony stimulation factor (G-CSF) and actinomycin D. Expression of the transfected gene was found to result in elevated levels of c-myb transcripts, which were not subject to normal down-regulation by differentiation induction. This constitutive expression of c-myb gene allowed the c-myb transfectants to differentiate into promonocytes with G-CSF and actinomycin D, but blocked further maturation from promonocytes to mature monocytes. It is concluded that normal down-regulation of c-myb gene expression during monocyte-macrophage differentiation is required for the maturation of promonocytes to mature monocytes.

Characterization of murine erythropoietin receptor genes

We have isolated and characterized the murine genomic and complementary DNAs encoding erythropoietin (Epo) receptor from Epo-responsive and unresponsive mouse erythroleukemia cells. Two classes of Epo receptor cDNAs were isolated from Epo-responsive cells. One is a 55,000 Mr membrane-bound Epo receptor, and the other is a 29,000 Mr soluble Epo receptor lacking the transmembrane and cytoplasmic domains. As a result of alternative splicing, two insert sequences containing termination codons are produced, and the encoded polypeptide diverges four amino acids upstream from the transmembrane domain, adding 20 new amino acids before terminating. Amino acid sequence of the Epo receptor cDNA isolated from Epo-responsive cells was identical with that of Epo-unresponsive cells, indicating that Epo-responsiveness does not depend upon the primary structure of the Epo receptor (binding) protein. Analysis of 6.6 x 10(3) base-pairs (kb) genomic DNA segments covering complete Epo receptor gene and promoter regions revealed that potential regulatory elements (NF-E1, GF-1 or Eryf 1) for erythroid-specific and differentiation stage-specific gene expression are located in the promoter and 3' noncoding regions.

Transmembrane signaling during erythropoietin- and dimethylsulfoxide-induced erythroid cell differentiation

Erythropoietin is a glycoprotein factor which specifically regulates the proliferation and differentiation of erythroid progenitor cells. We have investigated here the biochemical mechanisms of erythroid differentiation on mouse erythroleukemia SKT6 cells which can be induced to differentiate either with erythropoietin or dimethyl sulfoxide (Me2SO). cAMP-elevating agents, such as forskolin and 3-isobutyl-1-methyl-xanthine, caused spontaneous erythroid differentiation, and these agents showed the stimulatory effects on erythropoietin- or Me2SO-induced differentiation. An adenylate cyclase inhibitor, 2',5'-dideoxyadenosine, blocked erythropoietin-induced differentiation. The intracellular cAMP level was rapidly increased by addition of erythropoietin but not by Me2SO. These observations suggest that erythroid differentiation induced by erythropoietin is mediated, at least in part, through the cAMP-dependent pathway. When the effect of erythropoietin and Me2SO on the intracellular Ca2+ level was examined using fura 2, no acute change was observed. Measurements of the levels of inositol 1,4,5-trisphosphate and diacylglycerol following stimulation with erythropoietin or Me2SO showed that phosphatidylinositol turnover did not change significantly after erythropoietin stimulation but decreased gradually after Me2SO induction. Taken together, these results indicate that a complex signaling network including the cAMP-dependent pathway is involved in the erythroid differentiation process.

Down-regulation of c-myb gene expression is a prerequisite for erythropoietin-induced erythroid differentiation

The role of nuclear protooncogenes during erythroid cell differentiation was examined by transfecting exogenous c-fos and c-myb genes into mouse erythroleukemia cells, which can be induced to differentiate either with erythropoietin (Epo) or dimethyl sulfoxide. Expression of exogenous c-myb or c-fos oncogene completely inhibited Epo-induced erythroid differentiation but only partially inhibited dimethyl sulfoxide-induced differentiation. Normally Epo-induced differentiation leads to a drastic decline of c-myb mRNA levels and an increase of c-myc transcripts in the early stage of differentiation. Cells expressing exogenous c-fos gene, however, maintained high levels of c-myb mRNA after Epo treatment. This high level of c-myb transcripts was found to be due to block of transcription shutoff (or transcriptional activation) rather than to mRNA stabilization. It is concluded that the down-regulation of endogenous c-myb gene expression is a prerequisite for commitment of Epo-induced erythroid differentiation and that expression of c-myb gene may be indirectly regulated by c-fos gene product. We also concluded that early down-regulation of c-myc gene expression is not essential for erythroid differentiation and that gene regulation of chemically induced erythroid differentiation may differ from that of Epo-induced differentiation.

Characterization of erythropoietin receptor on erythropoietin-unresponsive mouse erythroleukemia cells

A membrane receptor for erythropoietin was identified in various erythropoietin-unresponsive mouse erythroleukemia cells. Scatchard analyses of the binding of human 125I-labeled erythropoietin to T3C1-2-0, K-1, GM86 and 707 cells showed the presence of a single class of binding sites with apparent Kd values of 0.27-0.78 nM, which are slightly higher than those of erythropoietin-responsive cells. The number of binding sites varied from 110 to 930 per cell. Crosslinking of 125I-erythropoietin to its binding sites with disuccinimidyl suberate revealed the existence of a single binding protein with molecular mass of 63 kDa. No binding sites with higher molecular mass, as observed in erythropoietin-responsive cells, were detected, nor was any specific binding observed to the non-erythroid hematopoietic cell or to the human erythroleukemia cells examined.

Specific binding of erythropoietin to its receptor on responsive mouse erythroleukemia cells

Erythropoietin (Epo) is a glycoprotein factor that specifically regulates the proliferation and differentiation of erythroid progenitor cells. Here we describe the isolation of Epo-responsive mouse erythroleukemia cell line SKT6, the characterization of the specific binding of biologically active 125I-labeled human Epo (125I-Epo) to its membrane receptor, and, finally, report information concerning the molecular structure of the receptor. About 75% of erythroid colony-forming precursor cell-like colonies derived from SKT6 cells were hemoglobin-positive after 3- to 4-day exposure to Epo in methylcellulose culture. Radioiodinated Epo bound specifically to SKT6 cells, and Scatchard analysis of the data showed a high affinity for 125I-Epo (Kd = 0.15 nM) but displayed only a small number of specific receptors (approximately equal to 470 per cell). Membrane components that specifically interact with 125I-Epo were identified by covalent crosslinking with disuccinimidyl suberate, and three receptor species with apparent Mr 63,000, 94,000, and 119,000 were found in membrane from SKT6 cells, suggesting the complex structure of the receptor molecules. Specific bindings were also detected in all of the Epo-unresponsive Friend erythroleukemia cells examined, and cross-linking study revealed the presence of only the 63,000 species as a binding site.

Sequential expression of proto-oncogenes during a mouse erythroleukemia cell differentiation

To investigate the possible involvement of proto-oncogenes in the regulation of cell differentiation and proliferation, their transcriptional levels were measured by Northern blot analysis during the process of dimethyl sulfoxide-induced differentiation of mouse erythroleukemia cells. The differentiation into erythrocytes was accompanied by sequential transient expression of the proto-oncogenes in the order of c-fos, c-myb, c-myc and c-k-ras, during the first 48 hr of differentiation induction. Following the transient expression of these proto-oncogenes, adult type sss-globin and embryonic type yl-globin gene transcripts appeared and accumulated in terminally differentiated cells. The expression of ten other proto-oncogenes examined here were not detected, nor dramatically changed during differentiation process.

Isolation and characterization of a genomic DDD mouse interleukin-3 gene

A chromosomal DNA segment containing the entire gene for interleukin-3 (IL-3) was isolated from DDD mouse erythroleukemia cells, and its gene organization and nucleotide sequence were determined. Enhancer-like sequences in the second intron and G + C-rich sequence in the 5'-flanking region may play a role in the regulation of tissue-specific and inducible gene expression. Southern blot analyses revealed that constitutively IL-3-producing WEHI-3 cells have the rearranged IL-3 gene, and that genomic DNAs prepared from the adult and fetal mouse liver have the same organization of the IL-3 gene. No IL-3 gene transcript was detected in the mouse erythroleukemia cells by Northern blot analysis.

Two non-allelic human interferon alpha genes with identical coding regions

A hitherto undescribed human interferon-alpha (IFN-alpha) gene, IFN-alpha 13, has been isolated and characterized. Its entire coding sequence is completely identical with that of IFN-alpha 1; the 5' and 3' non-coding regions differ by 4.5% and 3.8%, respectively. As the two genes are not allelic, we conclude that the similarity is due to a very recent gene conversion event. Both genes are expressed in virus-induced leukocytes.

Structure and expression of human IFN-alpha genes

Copy DNA (cDNA) was prepared from induced leucocyte poly(A) RNA and cloned in Escherichia coli. IFN-alpha cDNA clones were isolated by subculture cloning with the use of a translation hybridization assay. Definitive identification of the clones was based on the production of an interferon-like protein by the transformed bacteria. Different IFN-alpha cDNAs, with characteristic target cell specificities, were identified. The cloned cDNAs typically encode a mature polypeptide of 166 (or, for IFN-alpha 2, 165) amino acids and a signal sequence of 23 amino acids. A human chromosomal library was screened with IFN cDNA and 17 distinct IFN-alpha-related sequences were isolated and identified, of which 7 proved to be nonallelic authentic genes and 4 pseudogenes; 6 sequences remain to be elucidated. Taking into account the work of Goeddel and his colleagues, 13 non-allelic authentic genes and 6 pseudogenes can be distinguished. In addition, 9 genes believed to be allelic to the 13 authentic genes have been sequenced. The IFN-alpha genes may be classified into two major subfamilies, which diverged at least 33 Ma ago, but perhaps much earlier, if sequence rectification occurred. At least one IFN-alpha gene appears to have resulted by a recombinational event between members of the subfamily I and II. IFN-beta is distantly related to IFN-alpha's and may have diverged from a common ancestor at least 500 Ma ago. Both IFN-alpha and IFN-beta genes differ from most other genes of higher organisms by being devoid of introns. The mouse was found to possess an IFN-alpha gene family of a size similar to that of man; the murine genes also do not have introns. IFN-alpha genes devoid of their signal sequence were joined to prokaryotic promoters to produce the mature interferons in E. coli in high yield. IFN-alpha 2, purified to homogeneity, has been crystallized by T. Unge and B. Strandberg (Uppsala). Hybrid genes consisting of IFN-alpha 1 and IFN-alpha 2 segments were constructed and expressed in E. coli; the target cell specificities of such hybrids were dependent on the arrangement of the segments and were different from those of either parent. The chromosomal gene for HuIFN-alpha 1 was introduced into mouse L cells to study the mechanism of its expression. Correct transcription was only detected after induction (with Newcastle disease virus); expression was transient, with the same kinetics as those of the endogenous mouse IFN mRNA. Natural murine IFNs and human IFN-beta and IFN-gamma are glycosylated. Because E. coli cells transformed with the genes of eukaryotic glycoproteins are not expected to yield correctly glycosylated polypeptides, we prepared lines of hamster cells permanently transformed with hybrid plasmids, which contained an IFN gene linked to the SV40 early promoter, as well as dihydrofolate reductase as a selective marker. After intracellular amplification of the introduced genes, cell lines were obtained which constitutively produced IFN at about 40 000 units ml-1 and could be propagated for at least several months.

The binding of transfer ribonucleic acids to 5 S and 5.8 S eukaryotic ribosomal ribonucleic acid-protein complexes

Rat liver 5 S and 5.8 S rRNAs were oxidized with periodate and the 3' termini were coupled to Sepharose 4B through an adipic acid dihydrazide spacer. Ribosomal proteins were passed through the nucleic acid affinity columns to form ribonucleoprotein complexes containing the nucleic acid and the proteins that bind to it (5 S . L6, L7, L19; and 5.8 S . L6, L19, S9, S13). Pure isoaccepting species of yeast initiator-tRNA (tRNAfMet) and elongator-tRNAs (tRNAmMet, and tRNAPhe) were chromatographed on the ribosomal ribonucleoprotein affinity columns. The three rRNAs were bound to the 5 S and 5.8 S ribosomal ribonucleoprotein complexes. The elongation and initiation ternary complexes, EF-1 alpha . GTP . Phe-tRNAPhe, and eIF-2 . GTP . Met-tRNAfMet, also were bound to both ribosomal ribonucleoprotein affinity columns, whereas the binary complex EF-1 alpha . GTP and puromycin were not.

Characterization of the binding of rat liver ribosomal proteins L6, L8, L19, S9, and S13 to 5.8 S ribosomal ribonucleic acid

The interaction of rat liver ribosomal proteins L6, L8, L19, S9, and S13 with 5.8 rRNA was characterized by nitrocellulose membrane filtration. Binding approached saturation with the five proteins; the apparent association constants (K'a), measured at 4 degrees C and 22 degrees C, ranged from 0.2 to 18 X 10(5) M-1. The molar ratio of ribosomal protein and rRNA in the complexes at saturation approximated 1, indicating there is one binding site for each of the five proteins on the nucleic acid. A number of ribosomal proteins, including some previously suspected from affinity chromatography of associating weakly, did not form a complex with 5.8 S rRNA.

Characterization of the binding of rat liver ribosomal proteins L6, L7, and L19 to 5 S ribosomal ribonucleic acid

The binding of rat liver ribosomal proteins L6, L7, and L19 to 5 S rRNA was characterized by nitrocellulose membrane filtration. Binding could be saturated with the three proteins; the apparent association constants (Ka'), measured at 4 degrees C and 22 degrees C, ranged from 1.3 to 6.8 x 10(5) M-1. The molar ratio of ribosomal protein and rRNA in the complex at saturation approximated 1, indicating there is one binding site for each of the three proteins on the nucleic acid. A large number of rat liver ribosomal proteins, including some previously suspected of associating weakly, did not form a complex with 5 S rRNA.

Identification by affinity chromatography of the rat liver ribosomal proteins that bind to Escherichia coli 5 S ribosomal ribonucleic acid

The eukaryotic and prokaryotic ribosomal proteins that bind to Escherichia coli 5 S rRNA were identified by affinity chromatography. The E. coli ribosomal proteins that associated with the nucleic acid were L5, L18, and L25 confirming earlier findings using the same and different procedures. The rat liver ribosomal proteins that associated with E. coli 5 S rRNA were L6, L7, L19, L35a, and S9; several of those proteins also bind to rat liver 5 S rRna (L6, L19) and to 5.8 S rRna (L6, L19, and S9).

Isolation of eukaryotic ribosomal proteins. Purification and characterization of the 60 S ribosomal subunit proteins La, Lb, Lf, P1, P2, L13', L14, L18', L20, and L38

The proteins of the large subunit of rat liver ribosomes were separated into seven groups by stepwise elution from carboxymethylcellulose with LiCl at pH 6.5. Ten proteins (La, Lb, Lf, P1, P2, L13', L14, L18', L20, and L38) were isolated from three groups (A60, B60, and D60) by ion exchange chromatography on carboxymethylcellulose and DEAE-cellulose, and by filtration through Sephadex. The amount of protein obtained varied from 0.3 to 3.8 mg. Two of the proteins (La and L18') had no detectable contamination; the impurities in the others were not greater than 8%. The molecular weight of the proteins was estimated by polyacrylamide gel electrophoresis in sodium dodecyl sulfate; the amino acid composition was determined. Several additional acidic proteins were identified: P1a and P1b are phosphorylated derivatives of P1; P2a, P2b, and P2c are phosphorylated derivatives of P2. P1 and P2 are distinct proteins but both have large amounts of alanine (20.4 and 17.5 mol %).

Isolation of eukaryotic ribosomal proteins. Purification and characterization of 60 S ribosomal subunit proteins L3, L6, L7', L8, L10, L15, L17, L18, L19, L23', L25, L27', L28, L29, L31, L32, L34, L35, L36, L36', and L37'

The proteins of the large subunit of rat liver ribosomes were separated into seven groups by stepwise elution from carboxymethylcellulose with LiCl at pH 6.5. Twenty-one proteins (L3, L6, L7', L8, L10, L15, L17, L18, L19, L23', L25, L27', L28, L29, L31, L32, L34, L35, L36, L36', and L37') were isolated from three groups (C60, E60, and F60) by ion exchange chromatography on carboxymethycellulose and by filtration through Sephadex. The amount of protein obtained varied from 0.3 to 25 mg. Nine of the proteins (L6, L8, L18, L27', L28, L29, L34, L36, and L36') had no detectable contamination: the impurities in the others were no greater than 9%. The molecular weight of the proteins was estimated by polyacrylamide gel electrophoresis in sodium dodecyl sulfate; the amino acid composition was determined.