Transfected mutant p53 gene increases X-ray-induced cell killing and mutation in human fibroblasts immortalized with 4-nitroquinoline 1-oxide but does not induce neoplastic transformation of the cells

We introduced the mutant p53 gene (codon 273Arg-His) into human fibroblasts (SUSM-I cells) previously immortalized with 4-nitroquinoline I-oxide (4NQO) and obtained 2 clonal cell lines (SUSM-i/p53-1 and SUSM-1/p53-6) expressing the mutant p53. Since the genetic background of SUSM-1/p53 is the same as that of SUSM-1 except for the presence of the mutant p53, we expected to obtain more information on the mechanisms of p53 functions without the influence of other genetic differences by comparing cellular characteristics of both cell lines. SUSM-1/p53 cells became about twice as sensitive to the cytotoxic effects of X-rays as their parent SUSM-1 cells. Mutation frequency was determined by the appearance of hypoxanthine guanine phosphoribosyl transferase deficient (6-thioguanine resistant) cells. As a result, the mutation frequency of SUSM-1/p53 cells was about 5 times that of SUSM-1 cells transfected with or without the vector plasmid alone. Furthermore, when the SUSM-1/p53 cells were exposed to X-rays, the mutation frequency increased to about twice that of the non-irradiated SUSM-1/p53 cells. However, SUSM-1/p53 cells showed neither anchorage-independent growth in soft agar nor tumorigenicity in nude mice. These results indicate that the mutant p53 gene itself, which generally works in a dominant-negative way on cellular carcinogenesis, is not sufficient for neoplastic transformation of immortalized human cells, and that additional genetic change(s) may be necessary for transformation.

[Phenotypic analysis of myeloma cells]

Recent 2-color phenotypic analysis using anti-CD38 antibody reveals that plasma cells alone locate at CD38strong positive (CD38++) fraction and expression of adhesion molecules such as VLA-5 and MPC-1 can define VLA-5-MPC-1- immature, VLA-5-MPC-1+ intermediate and VLA-5+ MPC-1+ mature plasma (myeloma) cells. Furthermore, phenotypic analysis of plasma cells with anti-CD19 and -CD56 antibodies can distinguish normal (polyclonal) plasma cells from malignant (monoclonal) plasma cells; normal plasma cells from various tissues are all CD19+ CD56-, while malignant plasma cells are mostly CD19- CD56+. Therefore, this 2-color phenotypic analysis is very useful for differential diagnosis of bone marrow plasmacytosis, that is, myeloma, benign monoclonal gammopathy or polyclonal gammopathy, and furthermore contributes to understanding of differentiated stages of myeloma cells (immature, intermediate or mature myeloma cells).

Human fibroblasts (KMST-6/RAS cell line) transformed with 60Co gamma-rays and c-Ha-ras oncogene produce a large amount of granulocyte colony-stimulating factor (G-CSF); production is enhanced by cAMP, theophylline, and butyrate

Human fibroblasts (KMST-6/RAS cell line), which was malignantly transformed in vitro with 60Co gamma-rays and the c-Ha-ras oncogene, produced a large amount of granulocyte colony-stimulating factor (G-CSF). The production was greater during the logarithmic growth phase than during the stationary phase. cAMP and theophylline, alone or in combination, and butyrate significantly enhanced G-CSF production, but dexamethasone or 5-azacytidine did not. Enhanced production of G-CSF by these agents was regulated at the posttranscriptional level. Neither the expression of the ras oncogene nor the tumorigenicity of the cells correlated with the production of G-CSF.

Differentiation of early plasma cells on bone marrow stromal cells requires interleukin-6 for escaping from apoptosis

The bone marrow (BM) is well known to be the major site of Ig production in secondary immune responses; thus, the microenvironment of BM is considered to be essential for final differentiation of plasma cells. We identified in the peripheral blood (PB) early plasma cells (CD38++CD19+VLA-5-) committed to entering the BM. The sorted early plasma cells rapidly entered apoptosis in vitro, but these cells could survive and further differentiate into mature plasma cells (CD38 CD19+) just as BM plasma cells in the presence of a BM-derived stromal cell line (KM-102). Culture supernatants of KM-102 cell lines could also support survival of these cells, and antibody to interleukin-6 (IL-6) completely blocked the effect of these supernatants. Furthermore, recombinant IL-6, but not IL-1 or IL-3, could support their survival and their differentiation into mature plasma cells (CD38 CD19+VLA-5+) with expression of VLA-5 mRNA. Therefore, here is direct evidence that early plasma cells found in the PB differentiated into mature plasma cells with stromal cell-derived IL-6 in vitro; thus, BM stromal cells control the final checkpoint of plasma cell differentiation with secretion of IL-6 in the BM.

Recognition of mitochondria-targeting signals by a cytosolic import stimulation factor, MSF

MSF, a mitochondrial import stimulation factor purified from rat liver cytosol, is an ATP-dependent precursor protein conformational modulator. As a step toward understanding the specificity of substrate recognition by MSF, various synthetic peptides were examined for their ability to induce MSF ATPase activity. The peptides corresponding to various mitochondria-targeting signal sequences elicited significant ATPase activity. MSF bound the synthetic mitochondrial signal peptides, and ATP hydrolysis caused dissociation of the peptides from MSF. Basic amino acid residues in the signal peptides seemed to be essential for recognition. Thus, MSF is a member of the polypeptide chain-binding protein family with unique recognition specificity and is distinct from the hsp70 family of proteins.

MSF, a novel cytoplasmic chaperone which functions in precursor targeting to mitochondria

Mitochondrial import stimulation factor (MSF) unfolds wheat germ lysate synthesized aggregated mitochondrial precursor proteins and stimulates their mitochondrial import in an ATP dependent manner. Here we analysed the function of MSF mainly by utilizing chemically pure adrenodoxin precursor (pAd). MSF bound to the unfolded pAd and prevented it from losing import competence and also restored the import competence of the aggregated pAd dependent on ATP hydrolysis. The import incompetent aggregated mitochondrial precursors induced the ATPase activity of MSF and the activity was strongly inhibited by isolated mitochondrial outer membrane (OM) but not by trypsin treated outer membrane (tOM). The precursor induced ATPase activity of N-ethylmaleimide (NEM)-treated MSF was not inhibited by OM. In this context, the MSF-precursor complex specifically bound to OM and binding was abolished both by the treatment of OM with trypsin and by the treatment of MSF with NEM. These results show that MSF is a novel cytoplasmic chaperone protein with a mitochondrial precursor-targeting function.

Structural requirement for recognition of the precursor proteins by the mitochondrial processing peptidase

We examined the structural characteristics of the extension peptides responsible for the recognition by the mitochondrial processing peptidase by using preadrenodoxin, which has a long extension peptide of 58 amino acid residues, as the substrate. The deletion of various parts of the extension peptide of pre-adrenodoxin indicated that more than 40 amino acid residues and the presence of basic amino acid residues in the distal portion (20-40 amino acid residues upstream of the cleavage site) were necessary for the recognition of the precursor by the peptidase. The processing of preadrenodoxin was strongly inhibited by the synthetic peptide corresponding to the middle portion of the extension peptide, whereas the peptide corresponding to the amino-terminal portion exhibited weak inhibition of the processing. The replacement of arginine residues in the middle portion of the extension peptide with neutral amino acids resulted in a great decrease in the processing. We conclude that basic amino acids at a position distal to the cleavage site are necessary for the recognition of the precursor proteins by the processing peptidase and that basic amino acids required for the mitochondrial targeting and those for the recognition by the peptidase are separately located in the extension peptide of pre-adrenodoxin.

cDNA cloning and characterization of mitochondrial import stimulation factor (MSF) purified from rat liver cytosol

We identified a liver cytosolic protein factor that stimulated the import of wheat germ lysate-synthesized precursor proteins into mitochondria. It was termed mitochondrial import stimulation factor or MSF [Hachiya, N. et al. (1993) EMBO J. 12, 1579-1586]. It consisted of 32-kDa (MSFL) and 30-kDa (MSFS) polypeptides as assessed by SDS-PAGE. MSF recognized the presequence portion of mitochondrial precursor proteins and catalyzed the depolymerization and unfolding of in vitro synthesized mitochondrial precursor proteins in an ATP-dependent manner. We report here the cDNA cloning and characterization of MSF. Microsequencing of MSFL and MSFS showed that they belonged to a highly conserved, widely distributed eukaryotic protein family, collectively designated as 14-3-3 proteins. We cloned the cDNA of MSFL and that of one component of MSFS (MSFS1) from a rat liver cDNA library. The cloned cDNAs were separately expressed in Escherichia coli and the expressed proteins were purified to homogeneity. The purified recombinant MSFL and MSFS1 stimulated mitochondrial import of adrenodoxin precursor (pAd) synthesized in vitro with wheat germ lysate translation system. Recombinant MSFL or MSFS1 had the ability to bind with denatured pAd and they kept the precursor in an import-competent state. Rabbit polyclonal antibodies raised against the recombinant proteins inhibited the import-stimulation activity of rat liver cytosol as well as that of authentic purified MSF. Identification of MSF as 14-3-3 proteins establishes a novel function for this family of proteins and indicates their role as cytosolic chaperones to aid many important cellular events.

The transmembrane region of microsomal cytochrome P450 identified as the endoplasmic reticulum retention signal

Microsomal-type cytochrome P450s are integral membrane proteins bound to the membrane through their N-terminal transmembrane hydrophobic segment, the signal anchor sequence. To elucidate the determinants that enable the P450s to be located in the ER, we constructed cDNAs encoding chimeric proteins in which a secretory form of carboxyesterase, carboxyesterase Sec, was connected to the N-terminus of the full-length or truncated forms of a microsomal-type P450, P450(M1), and the constructed plasmids were expressed in COS cells. Since carboxyesterase Sec is an N-glycosylated secretory protein, endo H treatment could be used to determine whether these chimeric proteins were located in the ER or not. Carboxyesterase Sec with the N-terminal 20 amino acids, containing the transmembrane region, of P450(M1), was located in the ER, as determined from the endo H sensitivity of the expressed protein and immunofluorescence staining of the cells. As the expressed protein exhibited carboxyesterase activity, it was not retained in the ER through the BiP-dependent quality control system recognizing unfolded proteins. Another chimeric protein construct in which carboxyesterase Sec was connected to the C-terminal region of rat UDP-glucuronosyltransferase (UDP-GT), that contained a double-lysin ER retention motif, was also located in the ER, as determined from the endo H sensitivity and immunofluorescence staining. On the other hand, the sugar moiety of the carboxyesterase Sec connected to the transmembrane segment of UDP-GT, Sec/GTd, was partially resistant to the endo H treatment. From the results of immunofluorescent staining and cell fractionation, it was concluded that the Sec/GTd product was located in the Golgi apparatus. These observations indicated that the N-terminal hydrophobic segment of P450(M1) is sufficient for the ER membrane retention, whereas the transmembrane segment of UDP-GT is not. To determine whether microsomal P450s are recycled between the ER and Golgi compartments or not, a DNA construct encoding cathepsin D connected to the N-terminus of P450(M1) was prepared and expressed in COS cells. The fusion protein was phosphorylated, but the phosphorylation was sensitive to alkaline phosphatase. As a control, authentic cathepsin D was subjected to phosphorylation of its oligosaccharide chain that was resistant to the alkaline phosphatase treatment. Since GlcNAc-P-transferase, which forms the alkaline phosphatase-resistant phosphodiester in the sugar chains of lysosome-targeting proteins, is located in the Golgi apparatus, it was concluded that the oligosaccharide chain of the cathepsin D portion of the fusion protein was not phosphorylated, and that the chimeric protein did not go to the Golgi apparatus.(ABSTRACT TRUNCATED AT 400 WORDS)

Reconstitution of import-competent outer membrane vesicles from mammalian mitochondria

Protein insertion into mitochondrial outer membrane (OM) vesicles isolated from Neurospora crassa has recently been reported. The N. crassa OM vesicles retained the features of the intact mitochondria concerning the dependency of insertion on the receptor protein [A. Mayer et al. (1993) J. Cell Biol. 121, 1233-1243]. In this study, OM vesicles were purified from bovine adrenal cortex mitochondria, and unilamellar proteoliposomes were reconstituted from OM vesicles using heptyl beta-thioglucoside. Both OM vesicles and the reconstituted outer membrane vesicles (ROM) were able to import porin, but unable to import the precursor of adrenodoxin, which translocates across both the outer and inner membranes of intact mitochondria. Porin insertion into both OM vesicles and ROM was inhibited in the presence of purified recombinant adrenodoxin precursor and also by ATP depletion, and was dependent on the trypsin-sensitive membrane surface factor, suggesting that the purified OM vesicles as well as ROM retained the properties of the intact OM concerning porin insertion. The protein import machinery of OM seems to be functional for the outer membrane protein without the participation of the inner membrane. The successful reconstitution of the protein import activity from solubilized OM will pave the way for further biochemical characterization of the protein import machinery of OM.

Increase in production of hepatocyte growth factor by human embryonic lung fibroblasts in the process of aging in culture

It was determined whether human hepatocyte growth factor (hHGF)-producing ability would change in the human embryonic lung fibroblast cell strains (MRC-5 and IMR-90) until the cells senesced in culture. The effects of phorbol 12-myristate 13-acetate (PMA), dexamethasone, and transforming growth factor-beta 1 (TGF-beta 1) on hHGF production were also studied in these cell strains. For stimulation of DNA synthesis of adult rat hepatocytes in primary culture, hHGF secreted by MRC-5 cells at 39.9 and 69.8 population doubling levels (PDLs) showed almost the same activity as recombinant hHGF. Secretion of hHGF by MRC-5 cells increased about threefold between 37.3 and 67.8 PDLs. IMR-90 cells also showed about a threefold increase in hHGF secretion with increased passage from 37.8 to 66.0 PDL. Both cell strains showed almost the same ratio of hHGF amount in the cell extracts to that secreted into the medium around 40 and 70 PDLs. Northern blot analysis showed that the transcriptional level of the hHGF gene in MRC-5 cells increased about three-fold from 42.0 to 73.6 PDL in culture. These findings indicated that hHGF production increased in both cell strains with aging in culture. Production of hHGF in both cell strains was remarkably stimulated by treatment with 10 nM PMA. On the other hand, hHGF production in both cell strains was slightly suppressed by treatment with 1 microM dexamethasone. TGF-beta at a concentration of 5 ng/ml prominently inhibited hHGF production in both cell strains. The response of both cell strains to these regulators for hHGF production was almost the same around 40 and 70 PDLs in culture.

Manipulation of renal disposition of human recombinant superoxide dismutase by chemical modification

The renal disposition characteristics of superoxide dismutase (SOD) and its derivatives, including macromolecular conjugates with polyethylene glycol and carboxymethyl-dextran, a cationized derivative, and glycosylated derivatives with galactose and mannose, were studied in the isolated perfused rat kidney. Renal disposition processes, such as glomerular filtration, tubular reabsorption, and uptake from the capillary side, were quantitatively determined by single-pass indicator dilution experiments under filtering and nonfiltering kidney conditions. Native SOD had a high glomerular filtration rate (40% of that of inulin) and was effectively reabsorbed in the tubules, while no significant uptake was observed from capillary side. Macromolecular conjugates showed restricted glomerular filtration due to an increase in molecular size. Cationization of SOD greatly enhanced its association with the tissue, not only from the luminal side but also from the capillary side, based upon electrostatic interaction. Galactosylated and mannosylated SOD showed reduced tubular reabsorption and increased exposure of the luminal surface to the enzyme. In addition, a small but significant uptake of mannosylated SOD from the capillary side was observed. This uptake was dose-dependent and completely inhibited by mannan, suggesting that mannose receptor-mediated endocytosis existed in the capillary side of the kidney. Thus, we can manipulate the renal disposition profiles of SOD by changing its physicochemical or biological properties through chemical modification.

Improvement of therapeutic effect of human recombinant superoxide dismutase on ischemic acute renal failure in the rat via cationization and conjugation with polyethylene glycol

Therapeutic effect of superoxide dismutase (SOD) and three derivatives: a conjugate with polyethylene glycol (SOD-PEG2), a cationized derivative (cSOD), and a mannosylated derivative (Man-SOD), on acute renal failure induced by ischemia/reperfusion was studied in rats. SOD and derivatives were administered intravenously to the rat after nephrectomy of the right kidney and before and after 60 min occlusion of the left renal artery. At 48 hr after reperfusion, the renal function was evaluated by determining the urinary excretion rate of 14C-inulin injected intravenously. No therapeutic effect on the impaired renal function was shown in the case of low dose SOD (2600 unit/kg) treatment. In contrast, administration of cSOD which was shown to be taken up by the isolated perfused kidney from its capillary side and SOD-PEG2 which maintained high plasma concentration exhibited significant therapeutic effect, as did SOD at ten-fold higher dose (26,000 unit/kg). On the other hand, renal damage was promoted by Man-SOD. Thus, the present study demonstrated that chemical modification may improve the therapeutic effect of SOD on the ischemic acute renal failure and increased SOD concentration in the renal vascular space is an important factor for the improved effect.

Immortalization of normal human cells is a multistep process and a rate limiting step of neoplastic transformation of the cells

Three normal human fibroblast strains were immortalized by treatment with either 4-nitroquinoline 1-oxide (SUSM-1 and OUMS-24F lines) or 60Co gamma rays (KMST-6 line). Immortalization of normal human cells with these agents required repeated treatments. Before immortalization, most of the cells treated with these mutagenic agents showed more or less chromosome aberrations, but immortalized cells appeared very rarely. This suggests that several mutational events are involved in the immortalization process itself. The immortalized cells showed polygonal or epithelial-like morphology, prominent numerical and structural abnormalities of chromosomes, requirement of serum growth factors for their growth, and no tumorigenicity. The immortalized KMST-6 and OUMS-24F were further transformed into neoplastic ones by the ras oncogene, but not by transfection with the mutant p53 gene. Normal human fibroblasts which are mortal were not immortalized nor neoplastically transformed with the ras oncogene or the mutant p53. These findings strongly indicate that the immortalization of human cells is prerequisite for neoplastic transformation of the cells.

Importance of the proline-rich region following signal-anchor sequence in the formation of correct conformation of microsomal cytochrome P-450s

A proline-rich region is present following the signal-anchor sequence in the amino-terminal portion of all known microsomal cytochrome P-450s. To assess the functional significance of the proline residues in this region, we systematically altered these residues of cytochrome P450(M1) (P450 2C11); one, two, and three proline residues out of the five in the region were exchanged for alanine residues. The wild-type and the mutated proteins were expressed in the fission yeast Schizosaccharomyces pombe under the control of nmt1 promoter. The wild-type and the mutated proteins were all highly expressed in the yeast cells (5-9% of the total membrane protein). The expressed wild-type P450(M1) showed a typical carbon monoxide difference spectrum of P-450 and the activity of testosterone hydroxylation, whereas all the mutated proteins constructed in the present study showed no characteristic P-450 spectrum, suggesting that the substitution of the proline residues in this region resulted in a defect of proper heme incorporation. Furthermore, the mutated proteins in which more than one proline residues had been exchanged were more sensitive to trypsin digestion than the wild type. From these results, we propose that the proline residues in the proline-rich region are crucial for the formation of the correct conformation of microsomal P-450 molecules.

[Optical density of human macular pigment in spectral sensitivity measurements]

Optical density of the human macular pigment was measured by studying the spectral sensitivities to 1 degree, 200 ms test flashes on a 430 nm monochromatic background in the fovea and 10 degrees outside the fovea. Narrow-band (6 to 10 nm half band width) interference filters were used for the test lights. The difference between the spectral sensitivity in the fovea and outside the fovea coincided with the optical density of the macular pigment estimated previously by psychophysical means. We suggest that this method is useful for investigating the optical density of human macular pigment.

Trimeric mutant bacteriorhodopsin, D85N, shows a monophasic CD spectrum

The structure of mutant bacteriorhodopsin (bR), D85N, was examined by CD and X-ray diffraction at pH 7. The absorption maximum of D85N at pH 7 is located at 605 nm, which is similar to the acid-blue form of wild-type bR. D85N shows a monophasic CD band, the maximum of which is at 575 nm, although the crystalline arrangement and the trimeric structure is maintained. The acid-blue form of wild-type bR shows a biphasic CD despite the similarity in absorption spectra.

Core glycosylation of cytochrome P-450(arom). Evidence for localization of N terminus of microsomal cytochrome P-450 in the lumen

It was found that cytochrome P-450(arom) purified from human placenta microsomes is glycosylated, and the sugar chain was cleaved with endoglycosidase H (Endo H). The core glycosylation of P-450(arom) was examined with two heterologous expression systems, cultured insect cells and in vitro translation system. The P-450(arom) protein expressed in the insect cells was glycosylated, and the sugar chain was sensitive to Endo H. It was also glycosylated when translated with the wheat germ cell-free system in the presence of rough microsomal membrane, and the sugar chain could be removed by Endo H treatment. Since the P-450(arom) molecule has two potential glycosylation sites (Asn-12 and Asn-180), we replaced each of the 2 asparagine residues with alanine by site-directed mutagenesis and examined the glycosylation of the two mutant proteins in the cell-free system. The core glycosylation did not occur when the Asn-12 residue was mutated, whereas the mutant protein with modified Asn-180 residue was glycosylated. These results demonstrated that the potential glycosylation site (Asn-12) in the N-terminal portion of P-450(arom) is the site of glycosylation. We conclude that the N terminus of P-450(arom) is translocated across the endoplasmic reticulum membrane to be glycosylated at the luminal side.

Neoplastic transformation and characterization of human fibroblasts by treatment with 60Co gamma rays and the human c-Ha-ras oncogene

Human fibroblasts (KMST-6) immortalized by treatment with 60Co gamma rays were further neoplastically transformed by transfection of the c-Ha-ras oncogene from human lung cancer. The ras-transfected cells formed undifferentiated fibrosarcoma in nude mice. One of the tumors was recultured and a neoplastic human fibroblast line, KMST-6/RAS, was established. To analyze multistep carcinogenesis of human cells, the cellular characteristics of these genetically matched immortalized (KMST-6) and neoplastic (KMST-6/RAS) cell lines were studied in detail. KMST-6/RAS cells showed an increased saturation density, colony formation on confluent monolayers of normal human fibroblasts, proliferation in neomycin-containing medium, anchorage-independent growth, and enhanced expression of the transfected c-Ha-ras oncogene, whereas the immortalized cells did not demonstrate these characteristics. Unexpectedly, growth of KMST-6/RAS cells was serum-dependent, although they were neoplastic. Interestingly, the neoplastic cells did not show the criss-crossing or piling up growth pattern characteristic of transformed rodent fibroblasts.

Recoverin alters its surface properties depending on both calcium-binding and N-terminal myristoylation

The solution structure and calcium-dependent structural changes of recoverin, a 23 kDa calcium binding protein of vertebrate photoreceptors, have been studied by small-angle X-ray scattering and CD, as well as the effect of N-terminal myristoylation. The CD spectrum is not affected by N-terminal myristoylation, but strongly affected by Ca2+, indicating that N-terminal myristoylation alone does not cause a conformational change. The major conformational change in recoverin induced by Ca2+ is characterized as a decrease in the alpha-helical content of the protein and an increase in global size upon removal of Ca2+. In the presence of Ca2+, unmyristoylated recoverin is monomeric and globular in solution, while N-terminal myristoylation brings about aggregation. In the absence of Ca2+, unmyristoylated recoverin tends to aggregate, while myristoylated recoverin becomes monomeric and globular. These observations indicate that recoverin changes its surface properties depending on both calcium binding and N-terminal myristoylation. Melittin interacts non-specifically only with the myristoylated recoverin in the absence of Ca2+. This may be indicative of the properties of the interaction between recoverin and its normal physiological target enzyme.

Detection of a novel 9-kDa endoplasmic reticulum membrane protein in mammalian cells by chemical cross-linking with translocating nascent peptides

It is widely accepted that a proteinous channel participates in the co-translational translocation of proteins across the endoplasmic reticulum (ER) membrane. Truncated mRNAs encoding N-terminal 70, 75, 88, and 110 amino acids of interleukin 2 were translated with wheat germ cell-free system in the presence of rough microsomal membrane (RM), and integral membrane proteins were probed with the translocating nascent peptides by using a cross-linking reagent (DSS). Two membrane proteins, 9 kDa (Cp9, cross-linking partner 9 k) and 39 kDa (Cp39, cross-linking partner 39 k), were cross-linked with the 75 amino acids nascent peptide. When NEM-treated RM was used for the translocation reaction, neither Cp9 nor Cp39 proteins were cross-linked. When the translation products were treated with puromycin before the cross-linking, both proteins were not cross-linked. The cross-linked products of Cp9 and Cp39 were not extracted by alkaline extraction of the membrane, not sensitive to endoglycosidase H, and did not bind to Con A-Sepharose. These results indicate that both of the cross-linking partners were nonglycosylated integral membrane proteins. Cp39 was cross-linked with the 70, 88, and 110 amino acid nascent peptides as well as the 75 amino acid peptide, whereas Cp9 reacted only with the nascent peptides consisting of 70 and 75 amino acid residues. Even after the digitonin treatment of the RM with the translocating intermediates, the cross-linked products with Cp9 and Cp39 were detected. Cp9 and Cp39 seem to be tightly associated with the ribosome-nascent peptide complex.(ABSTRACT TRUNCATED AT 250 WORDS)