CD36 does not play a direct role in HDL or LDL metabolism

CD36 and scavenger receptor class B, type I (SR-BI) are both class B scavenger receptors that recognize a broad variety of ligands, including oxidized low density lipoprotein (oxLDL), HDL, anionic phospholipids, and apoptotic cells. In this study we investigated the role of mouse CD36 (mCD36) as a physiological lipoprotein receptor. We compared the association of various lipoprotein particles with mCD36 and mSR-BI expressed in COS cells by adenovirus-mediated gene transfer. mCD36 bound human oxLDL and mouse HDL with high affinity. Human LDL bound poorly to mCD36, indicating that mCD36 is unlikely to play a significant role in LDL metabolism. The ability of mCD36 to mediate the selective uptake of cholesteryl esters (CE) from receptor-bound HDL was assessed. In comparison with mSR-BI, mCD36 inefficiently mediated the selective uptake of CE. Hepatic overexpression of mCD36 in C57BL/6 mice by adenovirus-mediated gene transfer did not result in significant alterations in plasma LDL and HDL levels. We conclude that mCD36, while able to bind HDL with high affinity, does not contribute significantly to HDL or LDL metabolism.

Apolipoprotein A-II modulates the binding and selective lipid uptake of reconstituted high density lipoprotein by scavenger receptor BI

High density lipoprotein (HDL) represents a mixture of particles containing either apoA-I and apoA-II (LpA-I/A-II) or apoA-I without apoA-II (LpA-I). Differences in the function and metabolism of LpA-I and LpA-I/A-II have been reported, and studies in transgenic mice have suggested that apoA-II is pro-atherogenic in contrast to anti-atherogenic apoA-I. The molecular basis for these observations is unclear. The scavenger receptor BI (SR-BI) is an HDL receptor that plays a key role in HDL metabolism. In this study we investigated the abilities of apoA-I and apoA-II to mediate SR-BI-specific binding and selective uptake of cholesterol ester using reconstituted HDLs (rHDLs) that were homogeneous in size and apolipoprotein content. Particles were labeled in the protein (with (125)I) and in the lipid (with [(3)H]cholesterol ether) components and SR-BI-specific events were analyzed in SR-BI-transfected Chinese hamster ovary cells. At 1 microg/ml apolipoprotein, SR-BI-mediated cell association of palmitoyloleoylphosphatidylcholine-containing AI-rHDL was significantly greater (3-fold) than that of AI/AII-rHDL, with a lower K(d) and a higher B(max) for AI-rHDL as compared with AI/AII-rHDL. Unexpectedly, selective cholesterol ester uptake from AI/AII-rHDL was not compromised compared with AI-rHDL, despite decreased binding. The efficiency of selective cholesterol ester uptake in terms of SR-BI-associated rHDL was 4-5-fold greater for AI/AII-rHDL than AI-rHDL. These results are consistent with a two-step mechanism in which SR-BI binds ligand and then mediates selective cholesterol ester uptake with an efficiency dependent on the composition of the ligand. ApoA-II decreases binding but increases selective uptake. These findings show that apoA-II can exert a significant influence on selective cholesterol ester uptake by SR-BI and may consequently influence the metabolism and function of HDL, as well as the pathway of reverse cholesterol transport.

HDL modification by secretory phospholipase A(2) promotes scavenger receptor class B type I interaction and accelerates HDL catabolism

During inflammatory states plasma levels of high density lipoprotein (HDL) cholesterol and apolipoprotein A-I (apoA-I) are reduced. Secretory group IIa phospholipase A(2) (sPLA(2)) is a cytokine-induced acute-phase enzyme associated with HDL. Transgenic mice overexpressing sPLA(2) have reduced HDL levels. Studies were performed to define the mechanism for the HDL reduction in these mice. HDL isolated from sPLA(2) transgenic mice have a significantly lower phospholipid content and greater triglyceride content. In autologous clearance studies, (125)I-labeled HDL from sPLA(2) transgenic mice was catabolized significantly faster than HDL from control mice (4.24 +/- 1.16 vs. 2.84 +/- 0.1 pools per day, P < 0.008). In both sPLA(2) transgenic and control mice, the cholesteryl ester component of HDL was more rapidly catabolized than the protein component, indicating a selective uptake mechanism. In vitro studies using CHO cells transfected with scavenger receptor class B type I (SR-BI) showed that sPLA(2)-modified HDL was nearly twice as efficient as a substrate for cholesteryl ester transfer. These data were confirmed in in vivo selective uptake experiments using adenoviral vector overexpression of SR-BI. In these studies, increased hepatic selective uptake was associated with increased (125)I-labeled apolipoprotein uptake in the kidney. We conclude that during inflammation sPLA(2) hydrolysis of HDL phospholipids alters the lipid composition of the particle, allowing for more efficient SR-BI-mediated selective cholesteryl ester uptake. This enhanced SR-BI activity generates HDL remnants that are preferentially catabolized in the kidney.

Expression of serum amyloid A protein in the absence of the acute phase response does not reduce HDL cholesterol or apoA-I levels in human apoA-I transgenic mice

Plasma concentrations of high density lipoprotein (HDL) cholesterol and its major apolipoprotein (apo)A-I are significantly decreased in inflammatory states. Plasma levels of the serum amyloid A (SAA) protein increase markedly during the acute phase response and are elevated in many chronic inflammatory states. Because SAA is associated with HDL and has been shown to be capable of displacing apoA-I from HDL in vitro, it is believed that expression of SAA is the primary cause of the reduced HDL cholesterol and apoA-I in inflammatory states. In order to directly test this hypothesis, we constructed recombinant adenoviruses expressing the murine SAA and human SAA1 genes (the major acute phase SAA proteins in both species). These recombinant adenoviruses were injected intravenously into wild-type and human apoA-I transgenic mice and the effects of SAA expression on HDL cholesterol and apoA-I were compared with mice injected with a control adenovirus. Plasma levels of SAA were comparable to those seen in the acute phase response in mice and humans. However, despite high plasma levels of murine or human SAA, no significant changes in HDL cholesterol or apoA-I levels were observed. SAA was found associated with HDL but did not specifically alter the cholesterol or human apoA-I distribution among lipoproteins. In summary, high plasma levels of SAA in the absence of a generalized acute phase response did not result in reduction of HDL cholesterol or apoA-I in mice, suggesting that there are components of the acute phase response other than SAA expression that may directly influence HDL metabolism.

SR-BII, an isoform of the scavenger receptor BI containing an alternate cytoplasmic tail, mediates lipid transfer between high density lipoprotein and cells

The scavenger receptor class B, type I (SR-BI), binds high density lipoprotein (HDL) and mediates selective uptake of cholesteryl ester from HDL and HDL-dependent cholesterol efflux from cells. We recently identified a new mRNA variant that differs from the previously characterized form in that the encoded C-terminal cytoplasmic domain is almost completely different. In the present study, we demonstrate that the mRNAs for mouse SR-BI and SR-BII (previously termed SR-BI.2) are the alternatively spliced products of a single gene. The translation products predicted from human, bovine, mouse, hamster, and rat cDNAs exhibit a high degree of sequence similarity within the SR-BII C-terminal domain (62-67% identity when compared with the human sequence), suggesting that this variant is biologically important. SR-BII protein represents approximately 12% of the total immunodetectable SR-BI/II protein in mouse liver. Subcellular fractionation of transfected Chinese hamster ovary cells showed that SR-BII, like SR-BI, is enriched in caveolae, indicating that the altered cytoplasmic tail does not affect targeting of the receptor. SR-BII mediated both selective cellular uptake of cholesteryl ether from HDL as well as HDL-dependent cholesterol efflux from cells, although with approximately 4-fold lower efficiency than SR-BI. In vivo studies using adenoviral vectors showed that SR-BII was relatively less efficient than SR-BI in reducing plasma HDL cholesterol. These studies show that SR-BII, an HDL receptor isoform containing a distinctly different cytoplasmic tail, mediates selective lipid transfer between HDL and cells, but with a lower efficiency than the previously characterized variant.

Adenoviral vector-mediated overexpression of serum amyloid A in apoA-I-deficient mice

Serum amyloid A (SAA) is an acute phase reactant that can become the predominant apolipoprotein of high density lipoprotein (HDL) during severe inflammatory states. However, the function of SAA is unknown. To study the ability of SAA to form HDL in the absence of apolipoprotein A-I, we expressed the mouse SAA pI 6.15 (CE/J) isoform in apolipoprotein A-I knock-out (apoA-I (-/-)) mice using a recombinant adenovirus. As a control, apoA-I (-/-) mice were injected with an adenovirus expressing human apoA-I. High level expression of plasma SAA was obtained in the absence of any endogenous acute phase SAA production. SAA expression increased plasma HDL cholesterol levels about 2-fold, but to a lesser extent than the expression of apoA-I (about 10-fold). The HDL particles isolated by density ultracentrifugation from SAA-expressing mice were heterogeneous in size and composition and rich in free cholesterol as well as apoE and apoA-IV. Of the SAA expressed in the plasma, only a small fraction (4%) was associated with HDL particles in contrast to expressed apoA-I, of which 62% was associated with HDL. We conclude that SAA is unable to substitute for apoA-I in HDL particle formation.

Alternative forms of the scavenger receptor BI (SR-BI)

The class B, type I scavenger receptor has been implicated as a receptor for high density lipoprotein (HDL). We have isolated a murine cDNA clone encoding an alternative form of SR-BI that differs in the putative cytoplasmic domain of the receptor. This variant form, likely the result of alternative mRNA splicing, is designated SR-BI.2. SR-BI.2 mRNA was detected in mouse tissues known to express SR-BI and tissue-specific differences in the relative abundance of SR-BI.2 were apparent. In mouse adrenal glands, SR-BI.2 represented approximately one-third of total SR-BI mRNA, whereas in mouse testes, SR-BI.2 represented the major mRNA species (79% of total). SR-BI.2 was also detected in the human cell lines examined, namely HeLa, HepG2, and THP-1 cells. CHO cells transfected with the mouse SR-BI.2 cDNA expressed significant levels of SR-BI.2 protein and acquired the ability to take up fluorescent lipid (DiI) from DiI-HDL. Alternative splicing of SR-BI represents a potentially important process for the regulation of SR-BI expression and function.

Structure of the mouse Saa4 gene and its linkage to the serum amyloid A gene family

The serum amyloid A (SAA) proteins are a polymorphic family of apolipoproteins associated with high-density lipoproteins (HDL). Three distinct subfamilies have been identified: (i) a cytokine-induced acute phase subfamily that is hepatically produced and can become the major apolipoprotein on HDL (SAA1, SAA2); (ii) a peripherally produced acute phase SAA3 that is only a minor HDL apolipoprotein; and (iii) a constitutive subfamily (SAA4) that is a minor normal HDL apolipoprotein comprising more than 90% of the SAA during homeostasis. Here we define the structure of the Saa4 gene. Similar to other Saa family members, it has four exons and three introns. It is 4588 bp long from the transcription start site to the end of the 3'-untranslated region and is approximately 20% larger than other Saa genes. We have located Saa4 11 kb upstream from Saa3 and 5 kb downstream from Saa1, with the pseudogene approximately 1 kb from the 5' end of Saa4. Saa4 has the same orientation as most other Saa family members, with only Saa2 having an opposing orientation. These data promote our understanding of the evolution of the Saa family. They enhance our ability to develop the mouse as a transgenic and gene deletion model to advance the understanding of the function of these apolipoproteins.

Complex regulation of transforming growth factor beta 1, beta 2, and beta 3 mRNA expression in mouse fibroblasts and keratinocytes by transforming growth factors beta 1 and beta 2

Regulation of transforming growth factor beta 1 (TGF beta 1), TGF beta 2, and TGF beta 3 mRNAs in murine fibroblasts and keratinocytes by TGF beta 1 and TGF beta 2 was studied. In quiescent AKR-2B fibroblasts, in which TGF beta induces delayed stimulation of DNA synthesis, TGF beta 1 autoregulation of TGF beta 1 expression was observed as early as 1 h, with maximal induction (25-fold) after 6 to 12 h. Increased expression of TGF beta 1 mRNA was accompanied by increased TGF beta protein production into conditioned medium of AKR-2B cells. Neither TGF beta 2 nor TGF beta 3 mRNA, however, was significantly induced, but both were apparently down regulated at later times by TGF beta 1. Protein synthesis was not required for autoinduction of TGF beta 1 mRNA in AKR-2B cells. Nuclear run-on analyses and dactinomycin experiments indicated that autoregulation of TGF beta 1 expression is complex, involving both increased transcription and message stabilization. In contrast to TGF beta 1, TGF beta 2 treatment of quiescent AKR-2B cells increased expression of TGF beta 1, TGF beta 2, and TGF beta 3 mRNAs, but with different kinetics. Autoinduction of TGF beta 2 mRNA occurred rapidly with maximal induction at 1 to 3 h, enhanced TGF beta 3 mRNA levels were observed after 3 h, and increased expression of TGF beta 1 occurred later, with maximal mRNA levels obtained after 12 to 24 h. Nuclear run-on analyses indicated that TGF beta 2 regulation of TGF beta 2 and TGF beta 3 mRNA levels is transcriptional, while TGF beta 2 induction of TGF beta 1 expression most likely involves both transcriptional and posttranscriptional controls. In BALB/MK mouse keratinocytes, minimal autoinduction of TGF beta 1 occurred at only the 12- and 24-h time points and protein synthesis was required for this autoinduction. The results of this study provide an example in which TGF beta 1 and TGF beta 2 elicit different responses and demonstrate that expression of TGF beta 1, and TGF beta 3 are regulated differently. The physiological relevance of TGF beta 1 autoinduction in the context of wound healing is discussed.

Cell-surface expression and purification of human CD4 produced in baculovirus-infected insect cells

CD4 is an integral membrane glycoprotein that acts as the cellular receptor for human immunodeficiency virus (HIV). A cDNA encoding full-length CD4 was inserted into the genome of Autographa californica nuclear polyhedrosis virus under transcriptional regulation of the viral polyhedrin gene promoter. The recombinant virus was used to infect insect cells, which resulted in the abundant expression of CD4 as evaluated by flow cytometry and immunoblot analysis. Recombinant CD4 expressed on the surface of infected insect cells was immunologically indistinguishable from human CD4 when using 11 different anti-CD4 monoclonal antibodies. The extraction of infected cells by phase-transition separation with Triton X-114 followed by immunoaffinity chromatography yielded a single protein detected by NaDodSO4/PAGE using silver staining. N-terminal sequence analysis of the purified recombinant protein showed that CD4 produced in Sf9 cells is efficiently cleaved from the precursor protein. Immunoblot analysis under nondenaturing conditions showed that the purified protein reacted with the anti-CD4 monoclonal antibody Leu-3a. The potential use of the recombinant membrane-associated CD4 in anti-HIV therapy is discussed.

Expression and characterization of recombinant TGF-beta 2 proteins produced in mammalian cells

Recombinant DNA plasmids coding for transforming growth factor beta 2 (TGF-beta 2) precursor and a hybrid TGF-beta 1(NH2)/beta 2(COOH) molecule consisting of the amino-terminal precursor portion of transforming growth factor-beta 1 (TGF-beta 1) linked in phase to the carboxyl terminus of mature TGF-beta 2 were constructed and transfected into COS cells. Both plasmids directed the synthesis of active TGF-beta 2 which was secreted into the supernatants of transfected cells. The TGF-beta 2 was secreted in a latent form, as an acidification step was required to demonstrate optimal biological activity. Using site-specific anti-peptide antibodies, we show that precursor and mature forms of TGF-beta 2 are produced. A stable Chinese hamster ovary (CHO) cell line expressing the hybrid TGF-beta 1(NH2)/beta 2(COOH) protein was isolated. This cell line secreted both precursor and mature forms of TGF-beta 1(NH2)/beta 2(COOH); acidification was required to demonstrate biological activity. Protein sequence analysis of recombinant TGF-beta 2 produced by this CHO clone demonstrated that correct proteolytic cleavage had occurred, suggesting that the processing signals contained within the TGF-beta 1 amino portion can function in producing mature TGF-beta 2. Receptor binding studies showed that TGF-beta 2 specifically bound predominantly to type III receptors on the surface of human palatal mesenchymal cells. The availability of active TGF-beta 2 should aid in determining its potential therapeutic use as a growth modulator.

Structural and sequence analysis of TGF-beta 2 cDNA clones predicts two different precursor proteins produced by alternative mRNA splicing

Analysis of cDNA clones coding for human and simian transforming growth factor-beta 2 (TGF-beta 2) revealed the existence of two types of TGF-beta 2 precursor proteins of 414 amino acids (TGF-beta 2,414) and 442 amino acids (TGF-beta 2,442) in length. TGF-beta 2,442 contains a 29-amino-acid insertion in the amino terminus of the precursor region that replaces an Asn residue located at position 116 in TGF-beta 2,414. Of these 29 amino acids, three are cysteines, suggesting a more extensive disulfide-bond mediated secondary structure for TGF-beta 2,442 than for TGF-beta 2,414. Northern blot analysis using probes specific for the insert in TGF-beta 2,442 indicated that this protein is encoded by a minor 5.1-kb mRNA species present in human and simian cells. Since the DNA sequences flanking the insert are identical between clones coding for the two precursor protein, we suggest mRNAs coding for these proteins arise via differential splicing. Evidence is also presented that indicates that additional TGF-beta 2 mRNA heterogeneity is due to alternate polyadenylation. We propose that the 414-amino-acid precursor be referred to as TGF-beta 2a and the 442-amino-acid precursor be referred to as TGF-beta 2b.

Transforming growth factor-beta 2: cDNA cloning and sequence analysis

We have obtained a cDNA clone coding for human transforming growth factor (TGF)-beta 2. The clone was isolated from a tamoxifen-treated human prostatic adenocarcinoma cell line (PC-3) using oligonucleotide probes based on the partial amino acid sequence of purified TGF-beta 2. The cDNA sequence predicts that TGF-beta 2 is synthesized as a 442-amino-acid polypeptide precursor from which the mature 112-amino-acid TGF-beta 2 subunit is derived by proteolytic cleavage. The proteins coded for by the human TGF-beta 1 and TGF-beta 2 cDNAs show an overall homology of 41%. The mature and amino-terminal precursor regions show 71% and 31% homology, respectively. Northern blot analysis identified TGF-beta 2 transcripts of 4.1, 5.1, and 6.5 kb using mRNA from several different sources. Analysis of polyadenylated RNA from tamoxifen-treated PC-3 cells showed that these cells contain higher numbers of transcripts for TGF-beta 1 than for TGF-beta 2, although they produce more TGF-beta 2 protein than TGF-beta 1. This suggests that there is a post-transcriptional level of regulation for the production of these proteins.

Type 1 transforming growth factor beta: amplified expression and secretion of mature and precursor polypeptides in Chinese hamster ovary cells

Recombinant type 1 transforming growth factor beta (TGF-beta) was expressed to high levels in CHO cells by using dihydrofolate reductase (dhfr) gene amplification. The expression plasmid was derived from the pSV2 vectors and contained, in tandem, the simian TGF-beta and mouse dhfr cDNAs. Transcription of both cDNAs was controlled by the simian virus 40 early promoter. Stepwise selection of transfected CHO cells in increasing concentrations of methotrexate yielded cell lines that expressed amplified TGF-beta nucleic acid sequences. The expression plasmid DNA was amplified greater than 35-fold in one of the methotrexate-selected transfectants. The major proteins secreted by these cells consisted of latent TGF-beta and TGF-beta precursor polypeptides, as judged by immunoblots by using site-specific anti-peptide antibodies derived from various regions of the TGF-beta precursor. Levels of recombinant TGF-beta protein secreted by these cells approached 30 micrograms/24 h per 10(7) cells and required prior acidification for optimal activity; nonacidified supernatants were approximately 1% as active as acidified material. Antibodies directed toward sequences present in the mature growth factor readily identified a proteolytically processed recombinant TGF-beta which, on sodium dodecyl sulfate-polyacrylamide gels, comigrated with highly purified natural TGF-beta. In addition to mature recombinant TGF-beta, site-specific antibodies demonstrated the existence of larger TGF-beta precursor polypeptides. The availability of biologically active recombinant type 1 TGF-beta and precursor forms should provide a means to examine the structure, function, and potential in vivo therapeutic use of this growth factor.

Bovine and human cDNA sequences encoding a putative benzodiazepine receptor ligand

cDNAs containing the entire coding sequence of endozepine, a putative ligand of the benzodiazepine receptor, were isolated from bovine and human cDNA libraries. These libraries were constructed using a novel oligonucleotide adapter molecule that allowed us to combine the use of G/C tailing with the preservation of the unique Eco RI site in the vector, lambda gt10. The amino acid sequences derived from these cDNA clones are identical to those previously determined for the purified proteins and are homologous to a related rat protein termed diazepam-binding inhibitor. The endozepine proteins are highly conserved, as illustrated by the finding that the nucleotide sequences of the coding regions are 93% conserved between the bovine and human forms. Analysis of these sequences indicates that endozepine is not, as expected, derived from a precursor molecule containing a transient signal peptide. Moreover, Northern analyses using the cloned cDNAs as hybridization probes indicate that the 650-nucleotide endozepine mRNA is expressed in a number of peripheral tissues in addition to brain. These observations may be consistent with a recent report describing the presence in peripheral tissues of benzodiazepine receptors on the outer mitochondrial membrane (Anholt et al., 1986). In addition to the endozepine cDNAs, we also isolated a bovine cDNA clone which encodes a larger protein, a portion of which is homologous to endozepine. This related protein may be synthesized in a precursor form containing putative signal peptide and membrane-spanning domains.

Immunological detection of the messenger RNA cap-binding protein

The 24-kilodalton messenger RNA cap-binding protein (CBP) was purified from the rabbit reticulocyte postribosomal supernatant fraction using an affinity resin consisting of the p-aminophenyl gamma-ester of m7GTP coupled to Sepharose. The affinity-purified CBP was used to raise a goat antiserum. Anti-CBP antibodies were purified by adsorption to CBP coupled to either Controlled-Pore Glass or diazobenzyloxymethyl paper. The affinity-purified antibodies reacted specifically with only the 24-kilodalton polypeptide in whole reticulocyte lysate and in initiation factors prepared from the same source. During a conventional (nonaffinity) purification of CBP from a high salt extract of the ribosomal pellet, immunological reactivity paralleled the ability to reverse cap analogue inhibition of translation, indicating that the 24-kilodalton polypeptide present in the postribosomal supernatant fraction is immunologically cross-reactive with the CBP purified from ribosomes. Fractionation of whole reticulocyte lysate by sucrose gradient sedimentation followed by immunoblotting revealed that CBP was present in the supernatant fraction and the region of the gradient corresponding to ribosomal subunits but not in mono- or polysomes. The CBP to ribosome ratio was found to be approximately 0.02, assuming that the m7GTP-Sepharose retains all of the protein. This is considerably lower than that of other initiation factors and suggests that CBP may be the limiting polypeptide factor involved in the initiation of protein synthesis. The antibodies also inhibited the translation of a capped messenger RNA (globin). Inhibition of the translation of an uncapped RNA (satellite tobacco necrosis virus) was also observed, but to a lesser degree than with globin mRNA.

Cloning and sequence analysis of a cDNA for rat transforming growth factor-alpha

Transforming growth factors (TGFs) are mitogenic polypeptides produced most conspicuously by transformed cells and conferring on normal cells several phenotypic alterations associated with transformation. TGFs comprise two distinct sets of molecules: TGF-alpha s are structurally similar to epidermal growth factor (EGF), binding to and inducing the tyrosine phosphorylation of the EGF receptor in a manner indistinguishable from that of EGF. In addition, the 50-amino acid rat TGF-alpha has 33 and 44% homologies with mouse and human EGFs, respectively, and shares with EGFs a conserved pattern of three disulphide bridges. Thus, it has been proposed that TGF-alpha s belong to a family of EGF-like polypeptides. TGF-beta s, on the other hand, display no measurable binding to EGF receptors, but potentiate the growth-stimulating activities of TGF-alpha. Here we report the isolation of a complementary DNA clone encoding rat TGF-alpha. This cDNA hybridizes to a 4.5-kilobase (kb) messenger RNA that is 30 times larger than necessary to code for a 50-amino acid polypeptide and is present not only in retrovirus-transformed rat cells but also at lower levels in normal rat tissues. The nucleotide sequence of the cDNA predicts that TGF-alpha is synthesized as a larger product and that the larger form may exist as a transmembrane protein. However, unlike many polypeptide hormones (including EGF), cleavage of the 50-amino acid TGF-alpha from the larger form does not occur at paired basic residues, but rather between alanine and valine residues, suggesting the role of a novel protease.

Purification of the messenger RNA cap-binding protein using a new affinity medium

The p-aminophenyl gamma-ester of 7-methylguanosine 5'-triphosphate (m7GTP) was synthesized and coupled to Sepharose 4B. A 0.5 M salt extract of rabbit reticulocyte ribosomes was passed over a column containing the affinity medium. After extensive washing, a solution of m7GTP was passed through the column, and a single polypeptide species of 24 kilodaltons (kDa) was eluted. This had an electrophoretic mobility identical with that of the mRNA cap-binding protein. This assignment was confirmed by the fact that the eluted material was enriched nearly 200-fold in the ability to specifically bind 32P-labeled capped oligonucleotides. A control affinity medium consisting of GTP similarly coupled to Sepharose failed to retain the 24-kDa species. The postribosomal supernatant fraction yielded slightly more of the 24-kDa species than the ribosomal wash fraction when passed over this affinity medium.