Proc. Natl. Acad. Sci. USAVol. 73, No. 5, pp. 1559-1563, May 1976Biochemistry

A protein binding the methylated 5'-terminal sequence, m7GpppN, ofeukaryotic messenger RNA

(Artemia salina embryos/cap binding protein)

W. FILIPOWICZ*, Y. FURUICHI, J. M. SIERRA, S. MUTHUKRISHNAN, A. J. SHATKIN, AND S. OCHOARoche Institute of Molecular Biology, Nutley, New Jersey 07110

Contributed by S. Ochoa, February 27, 1976

ABSTRACT Ribosomal salt washes of Artemia salina em-bryos contain a protein(s) that binds [3HIm7GpppGpC andI3H]m7GpppGmpC, as measured by retention on nitrocellulosemembrane filters. These oligonucleotides correspond in struc-ture to the methylated 5'-terminal sequences (caps) present inmany eukaryotic mRNAs. The cap bindig protein does not bindthe unmethylated counterparts of caps, e.g., [32P]GpppGpCp,or a derivative of m7GpppGmpC containing ring-opened m7G.None of the purified initiation factors IF-MP, IF-M2A, IF-M2B,IF-M3, or IF-Ml binds the m7G-containing oligonucleotides.m7GMP, m7GDP, m7GTP, and other m7G-containing cap ana-logs, but not their unmethylated counterparts, are strong com-petitors of cap binding. Messenger RNAs having an m G-cap(e.g., globin and reovirus messenger RNAs) are also strongcompetitors of cap binding, but RNAs that have no m7G-cap(e.g., ribosomal, encephalomyocarditis virus, satellite tobacconecrosis virus, and unmethylated reovirus RNAs) do not com-pete. The cap binding protein(s) may influence the initiation ofprotein synthesis by promoting ribosomal binding of cap-con-taining messengers through recognition of the m7GpppN at their5'-terminus.

Many viral and eukaryotic mRNAs have a m7G(5')pppN "cap"at the 5'-end (3-6). This cap is important for translation and isrequired for binding the messenger, e.g., reovirus mRNA, to40S or 80S ribosomes (7-10). Consistent with these observations,the translation of messengers having an m7G-cap, but not ofthose lacking the m7G-cap, is inhibited by m7GMP throughcompetition with messenger for ribosome binding (11). In thiscommunication we report on the presence of a cap bindingprotein(s) in extracts of Artemia salina embryos and the com-petition by cap analogs and mRNAs for cap binding.

MATERIALS AND METHODSCap Binding Protein (CBP). A. salina CBP was partially

purified from ribosomal wash of developing embryos. About100 mg of wash protein, prepared as described (12) and di-alyzed against buffer A-80 [20 mM 4-(2-hydroxyethyl)-1-pi-perazineethanesulfonic acid (Hepes), pH 7.6, 80mm KCI, 0.1mM EDTA, 1 mM dithiothreitol, 5% glycerol], was applied toa DEAE-cellulose column (Whatman DE-52, 1.5 X 12 cm)equilibrated with the same buffer. The column was washedwith buffer A-80 and eluted stepwise with buffers A-180 andA-350, which contain 180 mM and 350 mM KC1, respectively.The former eluate contains initiation factor (IF)-MP; the lattercontains CBP activity. The A-350 eluate was dialyzed againstbuffer A-100 and divided into small portions for storage in

Abbreviations: AdoMet, S-adenosylmethionine; Hepes, 4-(2-hy-droxyethyl)-1-piperazineethanesulfonic acid; EMC, encephalomyo-carditis virus; STNV, satellite tobacco necrosis virus; cap, methylated5'-terminal sequence of mRNA; CBP, cap binding protein. The An-derson nomenclature is used for initiation factors (IF) from reticulo-cytes (see refs. 1 and 2).* Present address: Institute of Biochemistry and Biophysics, PolishAcademy of Sciences, 02-532 Warsaw, Poland.

1559

liquid nitrogen. Ribosomal wash from developing embryoscontains about three times as much CBP activity (about 45units/100 g of dry cysts) as ribosomal wash from dormantembryos (about 15 units/100 g).

Initiation Factors and RNAs. The rabbit reticulocyte ini-tiation factors and A. salina IF-Ml were those used previously(1, 2). The reticulocyte factors were kindly provided by Dr. W.C. Merrick and Dr. W. F. Anderson, NIH, Bethesda, Md.Reovirus (13), encephalomyocarditis virus (EMC) (14), A. salina(10), and globin (15) mRNA were as in previous work. RNA ofsatellite tobacco necrosis virus (STNV) was kindly provided byDr. A. Marcus, Institute for Cancer Research, Philadelphia, Pa.Calf lens mRNA was the kind gift of Dr. A. Berns and Dr. H.Bloemendal, University of Nijmegen, The Netherlands. 18SRNA from A. salina 40S ribosomal subunits was prepared bythe phenol extraction procedure.Caps and Cap Analogs. [3H]Methyl-labeled m7GpppGmpC

and its alkali-treated derivative containing ring-opened m7G,m7G'&pppGmpC, were prepared from reovirus mRNA syn-thesized in vitro in the presence of [3H]methyl-labeled S-ade-nosylmethionine (AdoMet) (7.5 Ci/mmol) as described (4,t,t). Methyl-poor reovirus mRNA was synthesized in the presenceof S-adenosylhomocysteine to prevent methylation (16).Monomethylated cap, i.e, [3H]methyl-labeled m7GpppGpC,and unmethylated 32P-labeledG *pppGpC weresynthesizedbycondensation of ppGpC (17, kindly provided by Dr. J. Tomasz,Institute of Biophysics, Szeged, Hungary) and either unlabeledGTP and [3H]AdoMet or [a-32P]GTP without AdoMet, usingreovirus-associated enzymes, as describedt. [3H]Methyl-m7GpppG and [32P]GC pppG were derived fromm7GpppCpCand G*pppGpC, respectively, by digestion with Penicilliumnuclease and purification by paper chromatography (4, 8).GpppN (N = G, C, U, or A) and GppG were synthesizedchemically by an anhydride exchange reaction between GMPand ppN or pG according to the procedure of Michelson (18).GpppG and GppppG from A. salina were gifts from Dr. A. H.Warner, University of Windsor, Windsor, Ont., Canada.m7GDP and m7GTP were synthesized by M. Ahmad and E.Heimer, Hoffmann-LaRoche, according to published proce-dures (19). m7GMP, m7GpppCm, and m7GpppAm were pur-chased from P-L Biochemicals.Cap Binding Assay. Unless otherwise noted, assay samples

(50 ,l) contained 30mM Hepes buffer (pH 7.6), 100 mM KC1,1.5 mM Mg(AcO)2, 0.5mM dithiothreitol, labeled cap (e.g.,[3H]m7GpppGmpC, [3H]m7GpppGpC), or cap analog (e.g.,

t G. W. Both, Y. Furuichi, S. Muthukrishnan, and A. j. Shatkin (1976)"Effect of 5'-terminal structure and base composition on polyri-bonucleotide binding to ribosomes," submitted for publication.

* Y. Furuichi, S. Muthukrishnan, J. Tomasz, and A. J. Shatkin (1976)"Mechanism of formation of reovirus mRNA 5'-terminal blockedand methylated sequence, m7GpppGmpC," submitted for publica-tion.

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10 20 30 40PROTEIN (Jg)

50

FIG. 1. Binding of [3HJm7GpppGpC as a function of (A) timeand (B) CBP concentration under the conditions of the standardassay. The amount of CBP in (A) was 52 gug.

["2PJG* pppGpC), and CBP as indicated in the legends. Whenstudying the effect of competitors we mixed the compound withlabeled cap before adding CBP. After incubation for 20 minat 00 and addition of 3 ml of ice-cold wash buffer [20 mMTris-HCI, (pH 7.5), 100 mM KC1, 1.5 mM Mg(AcO)2], thesamples were filtered on Millipore membranes (Hawpo 2500,0.45 ,Am pore size); the filters were dried and their radioactivitywas determined in Omnifluor. One unit of CBP activity is takenas the amount binding 1 pmol of [3H]m7GpppGmpC.

Translation Assays. The A. salina system consisted of salt-washed ribosomes from undeveloped embryos, a fraction fromundeveloped s105 (250 jg of protein), prepared as previouslydescribed (15), to supply chain elongation components, a

DEAE-cellulose fraction (104 gg of protein) from s105 of de-veloping embryos, comprising the protein retained in bufferA-100 and eluted with buffer A-350, as a source of initiationfactors, and other components as in previous work (1). The as-

says were as previously described (1) except that the samples(25 ,ul) contained 2 mM rather than 3 mM Mg(AcO)2, 0.15 mMspermine (20), and 0.5 A20 unit of washed 80S ribosomes, andwere incubated at 250. (A26o unit is that amount of material thatgives an absorbance of 1 when dissolved in 1 ml of solvent whenthe light path is 1 cm.) Translation by wheat germ extracts wasas described (10) except for a final volume of 25 Al with 2.5 mMMg(AcO)2 and 0.1 mM spermine.

RESULTS

Cap bindingIn an attempt to determine if protein(s) selectively bindm7G-capped mRNA for subsequent attachment to 40S ribo-somal subunits during protein synthesis in eukaryotes, meth-ylated and unmethylated reovirus mRNAs were tested withvarious fractions from A. salina extracts for differential re-

tention on membrane filters. Both types of reovirus RNA boundextensively, preventing detection of any m7G-dependentmRNA binding. However, when m7GpppGpC and GpppGpC,oligonucleotides similar to the 5'-terminal cap structure ofmethylated and unmethylated reovirus mRNA, were used toassay for cap binding, a protein(s) was detected in crude extracts(S23) of A. salina that selectively bound the m7G-containingstructure. The differential cap binding activity was maximalin the ribosomal salt wash fraction, which was then purified byDEAE-cellulose chromatography. The degree of purificationwas slight; the specific activity of the DEAE-cellulose fractionwas only two to three times higher than that of the ribosomalwash. The binding activity is rather labile. The activity was

largely destroyed by heating for 5 min at 65° and was reducedby 80% after gel filtration on Sepharose 6B or Sephadex G-200.The complex formed by the CBP preparation is also fairlyunstable. When the kinetics of binding is followed at 00 and 300

Table 1. Binding of [ 3HJm'GpppGpC and analogsby protein(s) present in A. salina embryos

Exp. Sample Labeled compound Bindingno. no. (pmol) (pmol)

1 1 [3HJm7GpppGmpC (0.65) 0.1102 [3HJm7GpppGmpC* (0.65) 0.005

2 1 [3Hlm7GpppGpC (1.0) 0.2422 [32P GpppGpC (1.0) 0.003

3 1 [3HJm'GpppG (1.0) 0.2822 [32PjGpppG (1.0) 0.201

4 1 [32PJGpppG (1.0) 0.2372 [32P]GpppGpC (1.0) 0.02 03 [3HJGTP (1.0) 0.242

5 1 [3HJm'GpppGpC (0.86) 0.3732 [32P]GpppGpC (0.86) 0.037

Standard binding assay was used in each case, except for Exp. 1,which was done at 30°. In Exps. 1-4 CBP was a DEAE-cellulosefraction eluted between 0.18 and 0.35 M KCI; in Exp. 5 the fractionwas one eluted between 0.1 and 0.35 M KCl. The amounts of CBP-containing fraction used (in mig of protein) were 46 in Exps. 1-3,40 in Exp. 4, and 65 in Exp. 5. The specific radioactivity of thedimethylated and monomethylated compounds was 6000 and 3000cpm/pmol. respectively. The 32P-labeled compounds contained1000 cpm/pmol. [3H]GTP = 11,000 cpm/pmol.* Imidazole ring of 5'-terminal m7G opened by alkali treatment (4).

(Fig. 1A), a maximum is reached in about 20 min at 00, andthere is no decay when incubation is continued for a further 20min. However, at 300 a lower maximum is reached in 5 min andthere is progressive decay of the complex with increasing timeof incubation. Low stability of the CBP-cap complex is alsoindicated by the observation that the activity retained on Mil-lipore filters decreased by 40 and 60% when, after filtration,the filters were washed once or twice, respectively, with anadditional 3 ml of wash buffer. Filtration, after the sampleswere allowed to stand for 1 hr at 00, after addition of the washbuffer, decreased the retention of cap radioactivity by almost90%.

Although complex formation occurred in the absence ofadded Mg2+, it decreased by about 50% when the assay wasconducted without added Mg2+ in the presence of 1.0 mMEDTA. The KC1 concentration was optimal between 40 and150 mM. At 1.0 M KCl complex formation was reduced byabout 60%. Under the conditions of the standard assay at 00 capbinding was proportional to the protein concentration up toabout 30 .ug/50 ,ul (Fig. 1B).The structural requirements for cap binding are illustrated

in Table 1. m7G-bearing caps, such as m7GpppGmpC (sample1, Exp. 1) or m7GpppGpC (sample 1, Exps. 2 and 5), werebound, whereas cap analogs with an open imidazole ring in them7G (sample 2, Exp. 1) or lacking the 7-methyl group on the5'-terminal guanosine residue (sample 2, Exps. 2 and 5; sample2, Exp. 4) were not. Thus, the presence of the 7-methyl groupon the 5'-terminal guanosine is essential for cap binding but thatof the 2'-O-methyl group on the second guanosine residue isnot. Previously, it was shown that messengers lacking the 7-methyl group on the 5'-terminal guanosine were poorly trans-lated (7) and that removal of the m7G by ,B-elimination abol-ished translation of the mRNA (8-10). The fact that GTP(sample 3, Exp. 4) and GpppG (sample 1, Exp. 4) were alsobound suggests the possibility that they bind to a protein otherthan CBP in the preparation. This binding is prevented by thepresence of an extra nucleotide residue (pC) in GpppGpC(compare samples 1 and 2, Exp. 4). The observation that GTPdoes not inhibit cap binding and that GpppG is a weak inhibitor

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Table 2. Comparison of cap binding by CBP and initiation factors

Cap binding (pmol)

[3HJm7GpppGpC (a) [32P]GpppGpC (a)or or

Exp. no. Protein (,lg) [3HIm7GpppGmpC (b) [3Hm7GApppGmpC (b)

1 CBP (32) (a) 0.277 (a) 0.02IF-M2A (1.3) (a) 0.002 (a) 0.02IF-M2B (9.3) (a) 0.001 (a) 0.07IF-M3 (4.3) (a) 0.004 (a) 0.07IF-Mi (1.3) (b) 0.003

2 CBP (37) (b) 0.297IF-MP (1.1) (b)0.06 (a) 0.04IF-M4 (4.8) (b) 0.002IF-M5 (7.1) (b) 0.005

3 IF-MP (1.1 ) (b) 0.004 (b) 0.00

Conditions were those of standard binding assay. The concentration of labeled caps was 0.02 ,uM. The specific radioactivities (cpm/pmol)of caps were as follows: Exp. 1, [3H]m7GpppGpC, 3000; [32P]GpppGpC, 908; Exps. 2 and 3, [3Hlm7GpppGmpC, 6000; [32P]GpppGpC, 118;[3Hm7G-pppGmpC, 6000. CBP and IF-Ml were from A. salina; rabbit reticulocytes were the source of the other purified proteins.

(see below) supports the belief that a protein(s) other than CBPis responsible for the binding of these compounds.None of the rabbit reticulocyte initiation factors, assayed at

concentrations at which they act optimally in mRNA translationin the A. salina system (1), displayed cap binding activity (Table2). This was true of factors IF-MP, IF-M2A, IF-M2B, and IF-M3, as well as of two other factors, IF-M4 and IF-M5, purifiedmore recently by Anderson's group. However, mixtures ofseveral factors were not tried. IF-M4 seems to correspond toIFEMC (21), a factor which according to Staehelin et al. (22) isrequired for translation of other messengers besides EMC RNA.The identity of IF-M5 is not clear. IF-Mi (EIF-1), a factor thatpromotes the AUG-dependent binding of Met-tRNAj to 40Sribosomal subunits but does not function in mRNA translation(1), assayed at higher than optimal concentrations for thebinding reaction (23), was also devoid of cap binding activity(Table 2).Cap analogs inhibited cap binding, and the degree of inhi-

bition paralleled the structural similarities between cap andanalog. m7GDP and m7GTP were strong inhibitors of thebinding of [3H]m7GpppGpC by CBP (Fig. 2A). Inhibition in-creased with increasing concentrations up to 0.1 mM, whenbinding was inhibited by about 90%. Thus, the inhibition ap-

2g 120

(D 100zaz 80mW 60

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zu

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CONCENTRATION (pM)

pears to be competitive in nature. m7GpppAm and m7GpppGmwere also strong inhibitors; 50% inhibition was obtained at0.01-0.02 ,uM, about equal to the concentration of cap. GTPdid not inhibit up to 0.7 mM. m7GMP was a much weaker in-hibitor than its di- and tri-phosphorylated relatives. Curiously,m7GMP stimulated cap binding (up to about 30%) at concen-trations below 0.1 mM. Strong (70-90%) inhibition by m7GMPrequired rather high concentrations (1-4 mM). Unlike GTP,the unmethylated compounds, GppG, GpppG, and GppppG,inhibited cap binding, the extent of inhibiton increasing withthe number of phosphates (Fig. 2B), but these compounds weremuch less inhibitory than m7GDP and m7GTP. Substitution ofG by C, U, or A, as in GpppC, GpppU, or GpppA, decreasedthe ability to inhibit [3H]m7GpppGpC binding (Fig. 2B).

Fig. 3 shows the effect of mRNAs on cap binding by CBP.mRNAs bearing a m7G-cap, like reovirus (4), globin (8), andA. salina (10) mRNA, strongly inhibited cap binding butmRNAs that have no cap, like STNA( (24, 25) and EMC RNA(unpublished results, Y. Furuichi, A. LaFiandra, and A. Shatkin)did not inhibit. A. salina 18S ribosomal RNA was weakly in-hibitory at high concentrations and so was Escherschia coli ri-bosomal RNA (not shown). Methyl-poor reovirus mRNA in-hibited cap binding to a much less extent than its methyl-rich

10 100

FIG. 2. Competition with [3H]m7GpppGpC (cap) binding as a function of the concentration of competitor. Abscissa, concentration plottedDn a logarithmic scale. Ordinate, relative binding values (binding in the absence of competitor taken as 100%). (A) [3H]cap, 1 pmol; CBP, 52ug. 100% binding = 0.19 pmol. There was no competition by GTP even at very high concentrations, e.g., 100% binding at 0.72 mM. (B) [3H]cap,).86 pmol; CBP, 40Mgg; 10(0% binding, 0.2 pmol. Cap binding was 50% inhibited at the following approximate concentrations of competitor: m7GMP,).6 mM; m7GDP, 1 MM; m7GTP, 0.5 MM; m7GpppGm, <0.02 MM; m7GpppAm, 0.02 ,M; GppG, 0.1 mM; GpppG, 40 ,uM; GppppG, 10MgM; GpppC,ypppU, and GpppA, each around 0.2 mM.

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z 60 NO mn

7540; \ (mothyl poor) ~I-40

\c \ A. salina mRNA

20 - Ins mRNA

roo mRNA (methylated)gIobinmRNA3

0 4 8 1 2 '28 3RNA (ug/50pl)

FIG. 3. Competition of [3H]m7GpppGmpC (cap) binding bymRNAs: inhibition as a function of the RNA concentration. Bindingwith no added RNA (0.125 pmol) was taken as 100%. The cap con-

centration was 0.65 pmol/50 Al and that ofCBP 46 jtg/50 gl through-out. reo, reovirus.

counterpart. Lens mRNA also inhibited cap binding, indicatingthat it bears a m7G-cap at its 5'-terminus. The molar ratio ofglobin mRNA/m7GpppGmpC was about 3 for 50% and about15 for 90% inhibition of cap binding. These values are probablyoverestimates because the presence of other RNA-bindingproteins in the partially purified CBP would reduce the ef-fective concentration of mRNA caps that could compete withfree caps (m7GpppGmpC) for CBP. The results demonstratethat CBP can recognize caps in mRNA.

Inhibition of mRNA translation by methylatednucleotidesBaglioni and coworkers have shown that m7GMP inhibitstranslation of m7G-containing mRNAs by a wheat germ system(11). Marcus and collaborators have further shown that m7GMPinhibits the formation of 806Sribosomal-Met-tRNAiMet complexwith m7G-capped RNAs such as tobacco mosaic virus RNA andalfalfa mosaic virus RNA, but not with STNV RNA, which hasno caps (26). We have studied the effect of m7G-containingnucleotides and their nonmethylated counterparts on thetranslation of globin and reovirus mRNA by the A. salina andwheat germ systems to see whether it correlates with their effecton cap binding by CBP. As shown in Table 3, this clearly wasthe case. Although there is a correlation between the ability ofthe cap analogs to inhibit protein synthesis and cap binding,translation generally is much more resistant to the inhibitorsthan cap binding. Moreover, the differences between GTP andm7GTP, on the one hand, and between m7GMP and m7GTP,on the other hand, were less pronounced when measured bytranslation inhibition. It may be noted that GMP and GTPproduced some inhibition of translation at concentrations atwhich they had no effect on cap binding, possibly due to a

chelating effect on Mg2+. As was true in the case of cap binding(see Fig. 2), m7GpppGm and m7GpppAm were strong inhibitorsof mRNA translation but their 7-methyl lacking analogs,GpppG and GpppA, were less inhibitory at similar concentra-tions when tested in the wheat germ extract with reovirusmRNA (Table 3).

DISCUSSIONThe presence in A. salina ribosomal salt washes of a protein(s)that binds the m7G(5')pppN cap present at the 5'-terminus of

Table 3. Inhibition of mRNA translationby methylated nucleotides

Inhibition of translation (%)

Wheat germA. salina system system

Inhibitor Globin Reovirus Globin Reovirus(mM) mRNA mRNA mRNA mRNA

GMP (0.8) 0 6 22 27m7GMP (0.8) 65 88 95 99GTP (0.14) 20 24 5 10m7GTP (0.18) 57 84 86 92GpppG (0.16) - 40m7GpppGm (0.18) 78 93 95 98GpppA (0.2) - 30m7GpppAm (0.2) 79 96 95 99

Translation assay was as described in Materials and Methods.The incorporation of [14C]leucine in samples without inhibitorwas (in pmol): A. salina system, globin mRNA 16, reovirus mRNA10; wheat germ system, globin mRNA 70, reovirus mRNA 103. Thefollowing concentrations (mM) of competitor inhibited globinmRNA translation by 50% (i50). A. salina system: m7GMP, 0.4;m7GTP, 0.15; m7GpppGm, 0.06; m7GpppAm, 0.08. Wheat germsystem: m7GMP, 0.15; m7GTP, 0.04; m7GpppGm or m7 GpppAm,0.03. The i5o values for reovirus mRNA translation were as follows.A. salina system: m7GMP, 0.08; m7GTP, 0.05; m7GpppGm, 0.03;m7GpppAm, 0.04. Wheat germ system: m7GMP, 0.05; m7GTP,0.03; m7GpppGm or m7GpppAm, 0.02.

many viral and eukaryotic mRNAs suggests that this protein(s)may recognize and promote the ribosomal binding of m7G-bearing mRNAs. This is consistent with the pattern of inhibitionby cap analogs of both CBP cap binding (Fig. 2) and mRNAtranslation (Table 3) and with inhibition of cap binding bym7G-containing, but not m7G-lacking mRNAs (Fig. 3). Thestructural specificity of CBP binding for cap and cap analogsand of competition with cap binding (Table 1 and Fig. 2) showsthat a m7G-polyphosphate structure is essential for binding. Wedo not know whether this protein(s) is a ribosomal protein oran initiation factor, but it does not seem to be identical to anyof several initiation factors assayed singly for cap binding ac-tivity (Table 2). CBP has also been detected in native 40S ri-bosomal subunits from rabbit reticulocytes (kindly providedby Dr. G. Blobel, Rockefeller University). For reasons that arenot clear, the binding activity of the native subunits was low,but it was more than 50% inhibited by globin mRNA and notinhibited by STNV RNA. Furthermore, protein synthesis wasincreased several-fold by addition of CBP (DEAE-cellulosefraction) to a globin mRNA directed A. salina system that wassaturated with respect to the known initiation factors listed inTable 2 (unpublished results). These findings suggest an in-volvement of CBP in the initiation of m7G-capped mRNAtranslation.Cap binding activity is also present in the supernatant s105

fraction from A. salina extracts in considerably higher amountsthan ribosome-associated CBP (unpublished results). However,the slOS cap binding activity differs from CBP in several ways.The s105 binding activity is not inhibited by m7G-cappedmRNAs. Furthermore, at 370 it cleaves m7GMP fromm7GpppGpC, a property absent from CBP. The s105 activityappears to correspond to the m7G-specific pyrophosphatasepreviously detected in HeLa cell extracts (27). Consistent withthis possibility, the purified HeLa cell pyrophosphatase alsohydrolyzes m7GpppGpC at 370 but not at 00 to yield m7GMP.It does not cleave caps in mRNA at 370 (27) or bind mRNA at

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00 under cap binding assay conditions (D. L. Nuss, personalcommunication). The presence of m7G-specific pyrophos-phatase activity in A. salina supernatant slOS fraction mayexplain why m7GTP, m7GpppGm, and m7GpppAm inhibitedtranslation to an extent that was less than expected on the basisof their ability to inhibit cap binding.

It will be of interest to survey extracts from other eukaryotesfor CBP and to determine the function(s) of highly purifiedCBP in protein synthesis.

We thank Miss Christa Melcharick for excellent technical asisance.J.M.S. is a postdoctoral trainee of the Institute of Molecular Biology,Madrid, Spain.

1. Filipowicz, W., Sierra, J. M., Nombela, C., Ochoa, S., Merrick,W. C. & Anderson, W. F. (1976) "Polypeptide chain initiationin eukaryotes: Initiation factor requirements for translation ofnatural messengers," Proc. Natl. Acad. Sct. USA 73, 44-48.

2. Nombela, C., Nombela, N. A., Ochoa, S., Safer, B., Anderson, W.F. & Merrick, W. C. (1976) "Polypeptide chain initiation in eu-karyotes: Mechanism of formation of initiation complex," Proc.Nat!. Acad. Sci. USA 73,298-301.

3. Furuichi, Y. & Miura, K.-I. (1975) "A blocked structure at the5'-terminus of mRNA from cytoplasmic polyhedrosis virus,"Nature 253,374-375.

4. Furuichi, Y., Morgan, M., Muthukrishnan, S. & Shatkin, A. J.(1975) "Reovirus messenger RNA contains a methylated, blocked5'-terminal structure m7G(5')ppp(5')GmpCp," Proc. Natl. Acad.Sci. USA 72,362-366.

5. Adams, J. M. & Cory, S. (1975) "Modified nucleosides and bizarre5'-termini in mouse myeloma mRNA," Nature 255,28-33.

6. Dasgupta, R., Shih, D. S., Saris, C. & Kaesberg, P. (1975) "Nu-cleotide sequence of a viral RNA fragment that binds to eukar-yotic ribosomes," Nature 256, 624-628.

7. Both, G. W., Banerjee, A. K. & Shatkin, A. J. (1975) "Methylationdependent translation of viral messenger RNAs in vitro," Proc.Natl. Acad. Sci. USA 72,1189-1193.

8. Muthukrishnan, S., Both, G. W., Furuichi, Y. & Shatkin, A. J.(1975). "5'-Terminal 7-methylguanosine in eukaryotic mRNAis required for translation," Nature 255,33-37.

9. Both, G. W., Furuichi, Y., Muthukrishnan, S. & Shatkin, A. J.(1975) "Ribosome binding to reovirus mRNA in protein synthesisrequires 5'-terminal 7-methylguanosine," Cell 6, 185-195.

10. Muthukrishnan, S., Filipowicz, W., Sierra, J. M., Both, G. W.,Shatkin, A. J. & Ochoa, S. (1975) "mRNA methylation and pro-tein synthesis in embryos of brine shrimp Artemia salina," J. Biol.Chem. 250, 9336-9341.

11. Hickey, E. D., Weber, L. A. & Baglioni, C. (1976) "Inhibition ofintiation of protein synthesis by 7-methylguanosine-5'-mono-phosphate," Proc. Natl. Aced. Sci. USA 73, 19-23.

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