Inhibition of eukaryotic protein chain initiation by vanadate (protein synthesis)

Transcription

1 Proc. Nati Acad. Sci. USA Vol. 8, pp , June 1983 Biochemistry Inhibition of eukaryotic protein chain initiation by vanadate (protein synthesis) RAjINDER SINGH RANU Department of Microbiology and the Graduate Program in Cellular and Molecular Biology, Colorado State University, Fort Collins, Colorado 8523 Communicated by Raj C. Bose, December 3, 1981 ABSTRACT Vanadate inhibits protein chain initiation in rabbit reticulocyte lysates. The evidence that supports this conclusion is as follows: (i) the biphasic kinetics of inhibition in which protein synthesis is maintained at the control rate for 1-2 min is followed by an abrupt decline in the rate of synthesis; (ii) inhibition is associated with a marked disaggregation of polyribosomes and a concomitant increase in 8S ribosomes; and (iii) vanadate concentrations that inhibit protein chain initiation do not inhibit polypeptide chain elongation or the aminoacylation of trna. In partial reactions of protein chain initiation, vanadate concentrations that inhibit protein synthesis have no detectable effect on the formation of eukaryotic initiation factor eif-2-promoted ternary comple with Met-tRNAf and GTP and on the assembly of 4S ribosomal subunit-met-trnaf complees. On the addition of mrna, the 4S ribosomal subunit-met-trnaf complees also are transformed into 8S ribosome-mrna-met-trnaf complees, termed 8S initiation complees. In vanadate-treated samples, however, these 8S initiation complees are defective and unable to proceed beyond this step. The requirement of vanadium in trace amounts as an essential nutrient has been recognized for some time (1, 2). The vanadium compounds in moderately high levels can be highly toic (1, 2). Although the biological role of vanadium at the molecular level is not known, recent interest in vanadate has arisen as a result of the findings of Cantley et al (3) that vanadate is a potent inhibitor of membrane Na+,K+-ATPase. Vanadate also inhibits a variety of other enzymes-e.g., myosin ATPase, dynein ATPase, Ca2+-ATPase (sarcoplasmic reticulum), Mg2+- ATPase, and adenylate kinase (4, 5). Almost all of these enzymes are phosphohydrolases, and frequently a phosphoenzyme intermediate is involved in the mechanism of action of these enzymes. Current evidence suggests that vanadate competes with phosphate for the enzyme-binding site (4). The protein biosynthesis is dependent on a series of reactions that require ATP and GTP hydrolysis-e.g., aminoacylation, GTP- and ATP-dependent initiation of polypeptide, and the GTP-dependent elongation and termination of polypeptide (6-8). The eukaryotic protein synthesis also is regulated by ATPdependent protein kinases that are activated in the presence of double-stranded RNA or by heme deficiency (9-11). These considerations and the apparent selective inhibition of the ATPases by vanadate prompted the eamination of the effect of vanadate on protein synthesis in eukaryotes. The results presented in this report show that vanadate preferentially inhibits protein chain initiation. MATERIALS AND METHODS The materials utilized in these studies were obtained from the following sources: ammonium metavanadate (analytical reagent The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C solely to indicate this fact grade) from Baker; cetyltrimethylammonium bromide and poly(uridylic acid) from Sigma; purified 9S globin mrna from Searle (Wycombe, England); and [3S]methionine (1,2 Ci/ mmol; 1 Ci = 3.7 X 11 Bq), [14C]leucine (32 mci/mmol), and [14C]phenylalanine (27 mci/mmol) from New England Nuclear. Sparsomycin (NSC 59729) was provided by Natural Products Branch, Division of Cancer Treatment, National Cancer Institute. The noncapped satellite tobacco necrosis virus (STNV) RNA was provided by J. Clark (University of Illinois, Urbana, IL). The sources of other reagents have been described (12). The following procedures also have been described: preparation of rabbit reticulocyte lysates and proteinsynthesis reaction mitures, assay of protein synthesis, the preparation of purified eif-2 (12), and the preparation of [3S]- Met-tRNAf (1, cpm/pmol) (12, 13). Inhibition of Protein Synthesis by Vanadate. Rabbit reticulocyte lysate-based protein-synthesis reaction mitures (25.ul) containing 1 pum hemin were incubated at 3 C with various concentrations of vanadate. At intervals, aliquots (5,ud) were removed and protein synthesis was assayed (12). The vanadate solutions used in this study were prepared fresh each day in deionized distilled water. Assay of Poly(uridylic Acid)-Dependent Polyphenylalanine Synthesis in Lysates. Rabbit-reticulocyte-lysate reaction mitures (25 u1) containing 1 pum hemin were incubated at 3 C with (25,ug) or without poly(uridylic acid) in the presence of 8 mm Mg2+. Under these conditions, the endogenous natural mrna-dependent protein synthesis is completely suppressed, and maimal poly(uridylic acid)-dependent polyphenylalanine synthesis is observed. At intervals, aliquots were removed and protein synthesis was assayed (12). Assay of Aminoacylation of trna. The aminoacylation assay was carried out under conditions of protein synthesis (12). Edeine (5,uM) was added to the reaction miture to block initiation of protein synthesis (14). At intervals, aliquots (5,ul) were removed and transferred to 1 ml of 1% cold trichloroacetic acid containing.5 mm methionine or leucine (see Fig. 4 legend). The precipitate was collected on Millipore filter and was washed etensively with cold 5% trichloroacetic acid (15). The filters were dried and radioactivity was assayed. The assay of eukaryotic initiation factor eif-2-dependent ternary comple (eif-2'gtp'met-trna) formation and the assay of the formation of 4S ribosomal subunit-met-trnaf complees in lysates have been described (12, 13, 16). Analysis of the Distribution of Polyribosomes. The polyribosome distribution in protein-synthesis reaction mitures was analyzed in sucrose density gradients (1-45%) in buffer A (2 mm Tris HCl, ph 7.6/8 mm KCl/2 mm magnesium acetate). Aliquots of reaction miture (25,ul) were removed and transferred to 1 1,u of ice-cold buffer A. The sample was layered over a sucrose density gradient and then was centrifuged at 38, Abbreviation: eif, eukaryotic initiation factor.

2 Biochemistry: Ranu rpm in a Spinco SW-5. 1 rotor for 2 hr at 4TC. The absorbance profile was monitored in a ISCO density gradient monitor at 254 nm. The fractions from the gradients were analyzed for the radioactivity associated with the nascent polypeptide chains according to Darnbrough et al (13). Assay of Formation of 8S Initiation Complees. The formation of 8S ribosome-mrna-met-trnaf complees, termed 8S initiation complees, was determined by shift assay in lysate protein-synthesis reaction miture (33 y1) containing 2,M hemin. Incubation with or without 2-3 AuM vanadate was at 3TC for 5 min. Sparsomycin (4,M) was then added, and incubation was continued for another 3.5 min, at which time [35S]Met-tRNAf (1, cpm) and 2 Ag of globin mrna were added. After 2 min of incubation, the sample was diluted with 13 td of ice-cold buffer B (1 mm Hepes, ph 7.6/8 mm KCV 2 mm magnesium acetate). The sample was layered over a 5.2- ml 1-35% sucrose density gradient in buffer B. The samples were centrifuged at 45, rpm in a Spinco SW-5. 1 rotor for 2 hr at 2 C. The absorbance profile of the gradients was monitored at 254 nm. The fractions were then analyzed for radioactivity as described by Darnbrough et al (13). Assay of the Formation of Initiation Dipeptide (Methionylvaline) of Globin. The micrococcal nuclease-treated lysate (16) protein-synthesis reaction mitures (4 Al) containing 24,uCi of [3S]methionine and.7 Ag of purified globin mrna were incubated at 3 C in the presence of sparsomycin (.2,uM) or vanadate (3,AM). A control without added mrna also was included [the micrococcal nuclease-treated lysate system itself shows a high rate of formation of 8S initiation complees because of the presence of mrna fragments; these fragments compete with the added globin mrna in the formation of 8S initiation complees (unpublished data; ref. 17)]. The samples were incubated for 1 min. The reaction was stopped by the addition of 18,l of ice-cold buffer A containing sparsomycin (.2 AM) or vanadate (3,M). The samples were layered on top of respective sucrose density gradients (1-45% in buffer A) containing sparsomycin or vanadate and centrifuged at 49, rpm in a Spinco SW-5. 1 rotor for 2 hr at 2 C. Fractions from mono- and polyribosome regions of the gradient were pooled. The ribosome-bound radioactivity (to trna) was etracted with phenol at ph 5.2 in the presence of carrier trna (1 mg/ml). Proc. Natd Acad. Sci. USA 8 (1983) 3149 The radioactive peptides were stripped of trna by eposure to 1% trimethylamine. The samples were applied to Whatman 3 MM filter paper strips along with internal standards [methionine (1 gg) and methionylvaline (2 pg)] and were subjected to ascending chromatography at room temperature in butanol/ acetic acid/h2, 45:5:12.5 (vol/vol) (15). The positions of the markers (methionine and methionylvaline) were located with ninhydrin. The paper was cut into 1.5-cm pieces and assayed for radioactivity. RESULTS AND DISCUSSION The effect of vanadate on eukaryotic protein synthesis was eamined in rabbit reticulocyte lysates because in this system the in vitro rates of protein chain initiation and elongation approach the in vivo rates (12). Moreover, the requirement of heme for the maintenance of protein synthesis, first observed in intact cells, is preserved in lysates (12). Vanadate strongly inhibited protein synthesis (Fig. 1A). The inhibition of synthesis with 1-4,uM vanadate showed a concentration dependence. Beyond these concentrations, the inhibition reached a plateau. The kinetics of inhibition in the presence of 1,uM vanadate showed that protein synthesis during the first 4-6 min was maintained at the control rate, followed by a progressive decline in the rate of synthesis. In the presence of 2,uM vanadate, synthesis at the control rate was maintained only for the first 1-2 min, and then there was an abrupt decline in the rate of synthesis. However, after this sharp decline, a synthesis at 5-1% of the control rate was preserved. These biphasic kinetics of inhibition of protein synthesis suggest that vanadate inhibits protein chain initiation. The eperimental results in Fig. 2 support this view. The vanadateinduced inhibition of protein synthesis was associated with the disaggregation of polyribosomes and a concomitant increase in 8S ribosomes. There was a marked decline in the radioactivity associated with the nascent polypeptides in the polyribosome region. The polyribosome profile and the radioactivity associated with the polyribosome fraction during the first minute of incubation in the control and vanadate-treated sample were similar (results not shown). After 3 min of incubation, there was a pronounced disaggregation of polyribosomes into 8S ribosomes and a marked decline in the radioactivity associated with 1 A B * / ~~~~~2 i=5 /=4 _S 1 l 3~~~~~~~~~~~~~U Vanadate, 1M Time, min FIG. 1. Inhibition ofprotein synthesis by vanadate. Protein-synthesis reaction mitures (25 sal) containing 1 MM hemin were incubated at 3C for various times with or without the indicated vanadate concentrations, and 5-,l aliquots were removed for assay. (A) Protein synthesis after 4 min of incubation with vanadate at various concentrations. (B) Protein synthesis at indicated incubation times without vanadate (-) or with 1 pm (-) or 2 pm (A) vanadate.

3 315 Biochemistry: Ranu Proc. Natl. Acad. Sci. USA 8 (1983) U - 1~~~~ - e.25 A Fraction FIG. 2. Effect of vanadate on polyribosomes. Protein-synthesis reaction mitures (11,l) containing1,m hemin and [35S]methioniine (14,uCi) were incubated with 2,uM vanadate (D, E, or F) or without vanadate (A, B. and C) at 3TC. After 3 (A and D), 6 (B and E), and 12 (C and F) min of incubation, 25-,ul aliquots were removed for the analysis of polyribosomes. (Inset) For protein synthesis assay, 5-,ul aliquots were taken out at 3-, 6-, 12-, 2-, and 4-min intervals. *, Without vanadate; A, with 2,uM vanadate. cs the polyribosomes in vanadate-treated sample. This finding is consistent with the observed kinetics of inhibition shown in Fig. 1B and Fig. 2F Inset. The low level of radioactivity that remained associated in the polyribosomal region in vanadate-treated samples (Fig. 2 E and F) after the onset of inhibition of protein synthesis is a reflection of the reduced rate (5-1% of the conco 9-4 P,-~E QO C.) Vanadate, AM Time,min FIG. 3. Effect of vanadate on poly(uridylic acid)-dependent polyphenylalanine synthesis. Protein synthesis reaction mitures (25,ul) were incubated at 3 with or without vanadate for various time intervals, and aliquots were removed-for assay. (A) Polyphenylalanine synthesis after 6 min of incubation with vanadate at various concentrations. (B) Polyphenylalanine synthesis at various incubation times without vanadate (e) or with 2 pm vanadate (A). 9 3 trol) of synthesis that is maintained under these conditions (Fig. 1B and Fig. 2F Inset). The conclusion drawn from the data in Fig. 1 and Fig. 2 is supported further by the eperiments (Fig. 3) in which poly- (uridylic acid)-dependent polyphenylalanine synthesis was assayed in lysates in the presence of 8 mm Mg2+. Under these conditions, initiation of protein synthesis dependent on natural mrna is bypassed. Hence, a measure of the effect of vanadate on polypeptide chain elongation is afforded. The similar kinetics of polyphenylalanine synthesis in control and samples treated with 2,uM vanadate (Fig. 3B) suggest that vanadate has no measurable effect on polypeptide chain elongation. At vanadate concentrations higher than 5,tM, however, some inhibition of polypeptide chain elongation began to emerge (Fig. 3A) Ṫhe results in Fig. 3B also suggest that vanadate does not inhibit the aminoacylation of trna of phenylalanine. Similarly, the results in Figs. 1 and 2, in which ["4C]leucine and [3S]methionine, respectively, were used for protein-synthesis assay, are indicative of the fact that, here too, the effect on the aminoacylation of trna of leucine and methionine may not be a factor in the vanadate-dependent inhibition of protein synthesis. This conclusion is born out by the eperimental results in Fig. 4 showing the similar rate of formation of the Leu-tRNA and the Met-tRNA in control and vanadate-treated samples. The negative results on aminoacylation and polypeptide chain elongation, the biphasic kinetics of inhibition, and the disaggregation of polyribosomes with a concomitant increase in 8S ribosomes provide compelling evidence that vanadate under

4 CY3 '--4 C) z U: 4.. Biochemistry: Ranu Time, min FIG. 4. Effect of vanadate on the formation of Leu-tRNA and MettRNA. Lysate protein synthesis reaction miture (3 ud) containing [35S]methionine (2,Ci) or ['4C]leucine were incubated at 3 C with (A) or without () 2,uM vanadate. At intervals, aliquots were assayed for aminoacylation. these conditions inhibits protein chain initiation. It should be pointed out that the effect of ecess magnesium and ATP on vanadate-induced inhibition of protein synthesis was also eamined (in addition to 2 mm Mg2' and 1 mm ATP already present in the lysate protein-synthesis reaction miture). The 2 Proc. Natl. Acad. Sci. USA 8 (1983) 3151 AuM vanadate-induced inhibition was not prevented by the addition of ecess Mg2+ (25-8 AtM) or MgATP (5-4,AM; results not shown). Vanadate also inhibited protein synthesis when addition was made after 1 min of preincubation of protein-synthesis reaction mitures at 3TC (results not shown). The assembly of an 8S initiation comple in eukaryotes involves a series of discrete steps. (i) The formation of a ternary comple (eif-2 GTP Met-tRNAf) with Met-tRNAf and GTP is promoted by the initiation factor eif-2 and several ancillary IFs (6, 9, 18-2). (ii) The ternary comple binds to 4S ribosomal subunit and forms complees of 4S ribosomal subunit-mettrnaf (6, 18). (iii) Insertion of mrna into the 4S ribosomal subunit-met-trnaf preinitiation complees is promoted by several IFs and the cap binding protein (6). This step also requires ATP (6). (iv) Finally, the joining of the 6S ribosomal subunits to the 4S initiation comple is catalyzed by eif-5 (6). The effect of vanadate on the formation of the eif-2-promoted ternary complees was determined: vanadate concentrations that inhibited protein synthesis had no detectable effect on ternary comple formation (results not shown). The formation of steadystate levels of the 4S ribosomal subunit-met-trnaf complees also was measured in situ in lysates in the presence and absence of vanadate. This assay is also a measure of the formation of ternary complees under natural conditions, the conditions under which vanadate strongly inhibits protein synthesis (Fig. 1). Here again, vanadate had no measurable effect on the formation of 4S ribosomal subunit-met-trnaf complees Control A 4S 88 B 4S II ~~~~8 9 8j.25 1 ii Q Va Id Vanadate I' 4 Z~~~C I'~~~~~~~- Fraction FIG. 5. Formation of 8S initiation complees. (A and B) Controls incubated without (A) and with (B) 9S globin mrna. (C and D) Vanadatetreated samples incubated without (C) and with (D) 9S globin mrna.

5 3152 Biochemistry: Ranu (results not shown; see data in Fig. 5). This finding also is supported by the fact that addition of eogenous eif-2 did not relieve the vanadate-induced inhibition of protein synthesis (results not shown) and, therefore, suggests that the inhibition of lysate protein synthesis by vanadate does not involve the activation of the heme-regulated protein kinase and the phosphorylation of eif-2 (9, 12). These negative results focus the binding of mrna and the subsequent joining of 6S ribosomal subunits as the potential targets for further investigation into the molecular basis of the vanadate-induced inhibition of protein synthesis. The formation of 8S initiation complees from 4S ribosomal subunit- Met-tRNAf complees in the presence of mrna represents not only a measure of the formation of 4S ribosomal subunit-mettrnaf-mrna complees but also their competence in the joining of 6S ribosomal subunits. The results are presented in Fig. 5. In this eperiment, lysates were first incubated with or without vanadate for 5 min. They then were incubated for 3.5 min with sparsomycin before the addition of [ws]met-trnaf. Under these conditions there was almost eclusive labeling of 4S subunits with ['S]Met-tRNAf (Fig. 5 A and C) in agreement with the previous finding (13). The radioactivity associated with 4S ribosomal subunit in vanadate-treated sample was 2- to 2.5- fold higher, reflecting the fact that under this condition there were more subunits available. Upon the addition of 9S globin mrna (Fig. 5 B and D) the formation of 8S initiation complees was observed. In the vanadate-treated sample there was a marked increase in the amount of radioactivity associated with 8S ribosomes (Fig. 4D). There was also a notable decrease in the number of ribosomal subunits and a concomitant increase in 8S ribosomes, as judged by the absorbance profile. This marked increase in the formation of 8S initiation complees in vanadate-treated samples supports the view that because of the vanadate-induced inhibition of initiation, more vacant 8S ribosomes and ribosomal subunits are available for participation in these reactions of initiation. These same results were observed when globin mrna was replaced by tobacco mosaic virus RNA (data not shown). In the absence of sparsomycin, the radioactivity of [35S]Met-tRNAf in 8S initiation complees in the control sample moved into the polyribosome region (data not shown; see Fig. 2), suggesting that these 8S initiation complees proceed with the mrnadirected assembly of polypeptide. The assay of the formation of initiation dipeptide (methionylvaline) in vanadate-treated samples showed that even though there was a noticeable increase in radioactive Met-tRNAf associated with 8S initiation complees compared with controls, only a small fraction of this radioactivity was transferred into initiation dipeptide (Table 1). In the vanadate-treated samples, therefore, these 8S initiation complees appear to be defective in one of the subsequent steps of translation. The kinetics of protein synthesis (Fig. 1B) show that elongation of globin chains (on ribosomes already engaged in the assembly of polypeptide) can normally proceed in the presence of vanadate. This observation and the finding that inactive 8S initiation complees are formed in vanadate-treated samples suggest that vanadate interacts with ribosomes only at Proc. Natl. Acad. Sci. USA 8 (1983) Table 1. Effect of vanadate on the formation of initiation dipeptide (methionylvaline) of globin [3"S]Met, [3"S]Met-Val, Ep. Additions cpm cpm 1 Control 7,517 2,84 Globin mrna + sparsomycin 7,529 2,97 Globin mrna + vanadate 9,25 3,48 2 Control 6,484 2,28 Globin mrna + sparsomycin 7,577 2,723 Globin mrna + vanadate 9,223 3,181 the time of initiation and not subsequently. The ability to select an isolated event in reactions involved in the initiation of polypeptide chain is of considerable value in the elucidation of the mechanism of assembly of an initiation comple. Vanadate may permit the identification of those factors involved in the formation of functional 8S initiation complees involving mrna and ribosomes. This investigation was supported by National Science Foundation Grant PCM , Biomedical Research Support Grant 2 SO 7RR , and the U.S. Department of Agriculture Animal Health and Disease Research Program. 1. Schwartz, K. (1974) in Trace Elements Metabolism in Animals, eds. Hoekstra, W. G., Suttie, J. W., Ganther, H. E. & Mertz, W. (University Park Press, Baltimore), Vol. 2, pp Underwood, E. J. 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