Proteins of Newcastle disease virus and of the viral nucleocapsid

JOURNAL OF VIROLOGY, Oct. 1969, p. 388-393 Copyright @ 1969 American Society for Microbiology Vol. 4, No 4 Printed in U S.A. Proteins of Newcastle Disea se Virus and of the Viral Nucleocapsid ILAN BIKEL AND PETER H. DUESBERG Department of Molecular Biology and Virus Laboratory, University of California, Berkeley, California 94720 Received for publication 28 May 1969 Newcastle disease virus was found to contain three major proteins. The structure unit of the viral nucleocapsid appears to be monomeric and to consist of a single large protein of an approximate molecular weight of 62,000. Newcastle disease virus (NDV), a subgroup II myxovirus, consists of a helically arranged ribonucleic acid (RNA) containing nucleocapsid which is enclosed by a lipoprotein envelope (18, 24). The viral envelope contains at least two distinct active proteins, a hemagglutinin and a viral neuraminidase, but the total number of the different proteins of the viral envelope is unknown. The structures of the nucleocapsids of several parainfluenza viruses including NDV are similar to that of tobacco mosaic virus (TMV) with regard to several physical and chemical properties. The nucleoproteins of parainfluenza viruses and of TMV have a diameter of about 20 nm (11, 24) and a density of about 1.3 g/ml (in sucrose or CsCl) (1, 21), and contain about 4 to 5% RNA and 95 to 96% protein (11, 13). The lengths of the two kinds of nucleoproteins were found to be proportional to the size of the RNA molecules which they include. Tobacco mosaic virus has a length of 0.3 Am and contains an RNA of a molecule weight of 2 X 106 (11) and the nucleoprotein of parainfluenza viruses contains an RNA of about 6.5 X 105 (3, 18) and has a length of about 1 ,um (1, 12). Although it is known that the structure units of TMV consist of monomeric proteins, it is not known whether the capsomers of the nucleocapsids of parainfluenza viruses also consist of single proteins or whether they consist of several proteins. The subject of the present investigation was the isolation of the different proteins of the virus. Three distinct major proteins of NDV were isolated by polyacrylamide gel electrophoresis. A single protein was found to be associated with the viral nucleocapsid. Similar independent findings were recently reported by Evans and Kingsbury (6). METHODS AND MATERIALS Virus growth. The NDV strain, NDV-Beaudette, obtained from D. W. Kingsbury, St. Jude Hospital, Memphis, Tenn., was used in all experiments. The virus was grown on lung cultures of 15- to 17-day-old chick embryos. The lungs were dispersed by stirring in tris(hydroxymethyl)aminomethane (Tris) saline containing Pronase (2.5 mg/ml) for 30 to 60 min at room temperature. Cultures were seeded at 2 X 107 to 4 X 107 cells per 10-cm plastic dish and cultured for 1 day prior to infection (19). After the medium was removed, the cells were incubated with about 3 ml of a twofold dilution of stock virus in Tris-saline. Stock virus consisted of allantoic fluid of infected chick embryos containing about 109 plaque-forming units (PFU)/ml (4). After incubation for 30 min at 37 C, the inoculum was replaced by 6 ml of amino acid-free (except for glutamine) medium 199 supplemented with 0.1% (w/v) lactalbumin hydrolysate, 0.1% (w/v) glucose, and 0.2 pAg of Fungazone per ml. Incubation was continued for 24 hr in the presence of 10 to 50,uc/ plate of 3H-amino acids (specific activity, 5 c/mmole), or 14C-amino acids (specific activity, 0.3 c/mmole) or 50 to 100 p&c/plate of 3H-uridine. At the end of this period, the medium was removed for virus purification. The cells had become rounded and partly detached from the plate. The hemagglutinin (HA) titer of such medium was usually about 160 HA units per ml. Virus purification. This process was a modification of the procedure described previously (4). Virus was purified in essentially two steps. First, the virus was concentrated from the medium of virus-producing cells by precipitation with an equal volume of saturated ammonium sulfate. The pellet was then redissolved in standard buffer [0.1 M NaCl, 0.01 M Tris (pH 7.4), 1 mt ethylenediaminetetraacetate (EDTA)] and concentrated by sedimentation on a sucrose cushion of a greater density than that of the virus. The concentrated virus was then transferred from the density interface and after appropriate dilution layered on a preformed sucrose density gradient in the same buffer. After centrifugation, viral infectivity coincided with radioactivity and optical density in a density range from 1.20 to 1.25 g/ml. From 50 to 100% of the 388 VOL. 4, 1969 PROTEINS OF NDV starting infectivity can thus be recovered as purified virus. Disintegration of the virus. One A2M0 or about 104 HA units of purified NDV in 100,uliters of low salt buffer [0.01 M NaCl, 0.01 M Tris (pH 7.4), and 1 mM EDTA] were mixed with an equal volume of 2 to 4% (w/v) Na deoxycholate (DOC) in the same buffer and incubated for 30 to 60 min at room temperature (2, 12, C. Blair, Ph.D. thesis, University of California, 1968). If higher virus to detergent ratios were used, incomplete degradation or aggregation of split products was observed, suggesting that a certain stoichiometry of detergent and virus substrate is necessary for complete disruption. Isolation of viral RNA. Isolation of the viral RNA was as described previously (4). Isolation of proteins of virus or nucleocapsid. Isolation of the proteins of virus or nucleocapsids and polyacrylamide gel electrophoresis were as described recently (5) with the following modifications. After electrophoresis, the gels were sliced in stainless-steel gel slicers (Diversified Scientific Instruments, Mountain View, Calif.). The slices were dissolved by the addition of 50,uliters of 1 M piperidine and 0.5 ml of NCS (Nuclear-Chicago Corp., Des Plaines, Ill.) and by shaking for 4 hr at 37 C or incubating at room temperature overnight. Thereafter, 5 ml of toluene-based scintillation fluid was added and each sample was counted in a Tri-Carb liquid scintillation 389 counter (Packard Instrument Co., Inc., Downers Grove, Ill.) RESULTS Isolation and characterization of the viral nucleocapsid. The nucleocapsid of NDV was released from the virion by incubation with 1 to 2% DOC as described above. The nucleocapsid was then isolated from the split products of the viral envelope by sucrose density gradient centrifugation (see legend of Fig. 1). Under the conditions of this experiment, equilibrium density was attained by the fast sedimenting nucleocapsid (see below), whereas the slowly sedimenting split products of the viral envelope remained on top of the gradient. As shown in Fig. 1A, the density of the nucleocapsid of NDV was 1.27 g/ml which is higher than that of the intact virion (1.23 g/ml) under the same conditions (4). The density of the nucleocapsid of NDV is the same as that of the nucleocapsid of Sendai virus in sucrose (C. Blair, Ph.D. thesis, University of California, 1968) and very similar to the density of 1.30 g/ml of the nucleocapsids of other parainfluenza viruses, including NDV in CsCl (1, 9). Alternatively, the nucleocapsid of Fraction Number FIG. 1. Characterization of the nucleocapsid of ND V. (A) Equilibrium sucrose-density gradient sedimentation of DOC-disrupted ND V. '4C-amino acid- and 3H-uridine-labeled ND V in low salt buffer (0.5 ml) containing 1% DOC were layered over a sucrose-D20 density gradient (4) 20 to 65% (w/v) in standard buffer containing 0.1 M NaCI, 0.01 M Tris, pH 7.4, and I mnz EDTA. After centrifugation for 3 hr at 300,000 X g in a SW-65 Spinco rotor at 4 C, 6 drop fractions were collected. Solution density was determined by weighing 100 ,uliter samples of fractions. Radioactivity was determined by counting appropriate samples after I to I dilution with1H20 in 5 volumnes of NCS and 5 ml of toluene-based scintillation fluid. Symbols: A, 14C-amino acid-labeled ND V; *, 3Huridine-labeled NDV; OL, solution density. (B) Velocity sedimentation of DOC-disrupted ND V. A sample of the DOC-treated virus preparation used in (A) was mixed with 600 ,ug of TMV and 20 ,ug of pancreatic-ribonuclease and centrifuged through a 10 to 25% (w/v) sucrose density gradient in standard buffer. After centrijugation in a Spinco S W-65 rotor for 16 min at 65,000 rev/miln at 7 C, fractions were collected. Absorbancy at 260 nm (0) was measured and radioactivity was determined from samples as described for A. (C) Sedimentation of the RNA of the nucleocapsid of ND V. The remainder offractions 8-12 of the experiment described in B were pooled and the RNA was extracted by the phenol-SDS method (4). The RNA was redissolved in 200 ,uliters of standard buffer and analyzed by sucrose gradient sedimentation as described for B. Sedimentation was for 2 hr at 65,000 rev/min at 7 C. 390 BIKEL AND DUESBERG disrupted virus was isolated from the viral split products by velocity sedimentation. The sedimentation coefficient (S,) of the NDV nucleocapsid could be estimated by the method of Martin and Ames (17) to be about 225S (Fig. 1B) using TMV as a 2005 (21) sedimentation marker. This S, is compatible with the values of about 200S determined by Kingsbury and Darlington (12) and about 250S determined by Hosaka (9) and C. Blair (Ph.D. thesis, 1968). By definition, a nucleocapsid contains viral RNA (15). The 225S component released from NDV by DOC can therefore be identified as the viral nucleocapsid by isolation of its RNA. As shown in Fig. 1C, The RNA of the 225S component had a S, of about 52S as determined by the method of Martin and Ames (17) using TMV-RNA as a 315 sedimentation marker (11). A S,. of 57S has previously been determined for the intact RNA of NDV by sedimentation in the analytical ultracentrifuge (4). Electrophoresis of the proteins of the virion and of the nucleocapsid. The proteins isolated by the phenol-SDS method (5) of 3I-amino acidlabeled NDV were coelectrophoresed with '4C-TMV protein and bovine serum albumin (BSA) on polyacrylamide gel at pH 8.1 containing 0.1 % SDS (Fig. 2). Three major viral proteins (NDV1, NDV2, NDV3) were obtained (Fig. 2). An estimate of the molecular weight of the NDV J. VIROL. proteins can be made on the basis of the relationship between electrophoretic mobility and molecular weight as described by Shapiro et al. (22). Using BSA as a 67,000 molecular weight marker and TMV protein as a 16,500 molecular weight marker, the approximate molecular weight of NDV1 was estimated to be 45,000, NDV2 to be 62,000, and NDV3 to be about 100,000. To determine which and how many of the three major viral proteins are components of the nucleocapsid, the proteins of 3I-amino acidlabeled NDV and density gradient purified 14C-amino acid-labeled nucleocapsid were isolated together by the phenol method (5) and analyzed by polyacrylamide gel electrophoresis. The result indicates that the nucleocapsid predominantly contains protein NDV2 and very small amounts of proteins NDV1 or NDV3 (Fig. 3). Proteins NDV, and NDV3 are presumably proteins of the viral envelope such as the hemagglutinin and the neuraminidase, although this has not been demonstrated. The protein of the nucleocapsid obtained from virus treated simultaneously with DOC and Pronase (100 Aug/ml, 30 min, 20 C) had a higher electrophoretic mobility than the protein of E & 5000- E 0 a0 .4 I 250000 0 10 20 30 40 50 60 Distance Moved In Millimeters 70 FIG. 2. Electropherogram of the proteins of 3Hamino acid-labeled ND V (-), '4C-labeled TMV (A), and BSA. The radioactive proteins were isolated by phenol extraction (5). The proteins were dissolved in about 100 l.diters of buffer containing 0.01 M Tris (pH 8.1), 1 mM EDTA, 2 mM dithiothreitol, 0.2% (w/v) SDS, 10% (v/v) glycerol and phenol red. Electrophoresis was for 4 hr at 10 v/cm in a 5% polyacrylamide gel as described previously (5) until the phenol red marker had migrated about 6 cm. Subsequent to electrophoresis the gel was incubated for 30 min in 10% trichloroacetic acid until the BSA band could be located. The gel was then divided into I mm slices and counted. 0 10 50 40 30 20 Distance Moved In Millimeters FIG. 3. Coelectrophoresis of the total protein of 3H amino acid-labeled NDV (0) and 14C amino acidlabeled ND V nucleocapsid (A). The viral nucleocapsid was released from the virus by incubation with 2% (w/v) DOC and purified by sucrose gradient sedimentation (Fig. 1B). Electrophoresis was as described in Fig. 2. PROTEINS OF NDV VOL. 4, 1969 391 nucleocapsid untreated with this enzyme (Fig. 4), although the S, of the nucleocapsid was little affected by such pronase treatment. This indicates that the protein of the nucleocapsid is susceptible to degradation with Pronase, whereas the RNA of the nucleocapsid is resistant to ribonuclease E [Fig. lB and C, (12)1. e-7 400 If the total protein of 3H-amino acid-labeled virus was coelectrophoresed with "4C-labeled protein of the viral nucleocapsid at pH 3.8 in 8 M urea, a different pattern was obtained. Only 0 ~~~TMV two 3H-protein components were resolved, which !200 migrated as distinct components (Fig. 5). The rest of the 3H-NDV protein failed to penetrate the gel or formed a rather high background 100 between the top of the gel and the two 3H-protein peaks. The 14C-protein of the viral nucleocapsid, however, migrated as a single component. It 0 coincided with a single peak of the total 3H-NDV 0 10 20 30 protein. The peak is presumably the protein Distance Moved In Millimeters component which was defined as NDV2 when the proteins of the virion were electrophoresed in FIG. 5. Electropherograms of the total protein of 3H-amino acid-labeled ND V (0), 14C-amino acidSDS (Fig. 2). Preliminary experiments indicate that the labeled viral nucleocapsid (A) and 50 jig of TMV proThe proteins were isolated as described for Fig. protein subunits of the three distinct helical 2.tein.After precipitation with 5 volumes of alcohol the nucleoproteins of influenza virus (3a) also con- proteins were dissolved in a solution containing 8 M sist of only one single kind of protein (Duesberg, urea, 0.01 M acetic acid, 2 mxi dithiothreitol, I mM EDTA, and enoughl methylene blue to serve as a tracking e 7500 e dye. Electrophoresis was in a 6%o polyacrylamide gel for 5 hr at 10 v/cm at pH 3.8 in 8 M urea (5). After electrophoresis, the gel was stained with amido black to locate the carrier TMV protein. The gel was then sliced and the radioactivity was determined. unpublished data; Fig. 6) after electrophoresis at pH 8.1 in 0.1 %hO SDS. E DISCUSSION That the protein of the nucleocapsid of NDV migrated as a single component as anion at pH 8 in 0.1 % SDS, where separation is thought to be only a function of molecular weight, and as cation at pH 3.8 in 8 M urea, where separation 2500 is a function of both molecular weight and charge, suggests that it is a single molecule. An accidental coincidence between different proteins in one of the two employed electrophoretic systems would probably have been resolved in the other system 10 0 20 40 50 60 because the electropherograms of the viral 30 Distance Moved In Millimeters proteins were different in the two systems and because their relative electrophoretic mobilities total the FIG. 4. Coelectrophoresis of protein of were different in relation to the mobility of 3H amino acid-labeled NDV (0) and 14C amino acidlabeled NDV nucleocapsid (A). The nucleocapsid was TMV protein. It, therefore, seems likely that the helical incubated with Pronase (100 ,ug/ml) for 30 min at nucleocapsid of NDV contains only a single room temperature prior to sucrose gradient purification (Fig. IB). Electrophoresis was as described fr Fig. 2. kind of protein subunit, i.e., consists of mono- o5000- 392 BKIEL AND DUESBERG J. VIROL. The internal antigens of the enveloped RNA tumor viruses, which have failed to show any detectable symmetry up to date, have also been shown to consist of several distinct proteins (5, 8). On the other hand, the protein subunits of the spherical cores of arboviruses which also failed to show any detectable symmetry, were found to consist of a single type of protein molecule (23). E 51000 ACKNOWLEDG MENTS The atuthors thank W. M. Stanley and H. Rubin for encouragement and support. This investigation was supported by U. S. Public Health Service research grants CA 11426, CA 04774, and CA 05619 fromii the National Cancer Institute. LITERATURE CITED 500 0 10 30 40 50 60 20 Distance Moved In Millimeters FIG. 6. Coelectrophoresis of the total proteins of :H amino acid-labeled influenza virus protein (0) and '4C-amino acid-labeled protein (A) of the three distintct components of the nzucleoprotein of influenza virus (3a). The viral niucleoproteins were isolated and purified as described previously (3a) a,id pooled prior to isolation of the protein. Electrophoresis was for 3 hr and otherwise as described in Fig. 2. meric capsomers like the structurally related TMV. The molecular weight of the capsomer of the nucleocapsid of NDV is about 62,000 or about four times larger than that of TMV. The nucleocapsid of NDV differs from that of TMV in several properties, such as its dissociability by low concentrations of SDS (-0.2 % w/v) and its relative flexibility evident in electron micrographs (1, 12). The helical nucleoprotein of influenza virus, which consists of three distinct subunits (3a), also contains only a single electrophoretic protein component in 0.1 % SDS at pH 8. This result is compatible with Laver's (14) conclusion that the ribonucleoprotein-antigen of influenza A virus consists of a single protein component after electrophoresis on cellulose acetate in 0.4% SDS at pH 8.9. The helical nucleoproteins of two rhabdoviruses (7), vesicular stomatitis virus (10), and rabies virus (22a), which are structurally related to myxoviruses (7) but are not members of the myxovirus group were recently shown to consist of one kind of protein molecule. Thus, all known helical nucleoproteins of RNA viruses consist of monomeric protein subunits. The nucleocapsids of several icosahedral RNA viruses, on the other hand, have been shown to contain capsommers with multiple protein subunits (16, 20). 1. Comiipans, R. W., and P. W. Choppin. 1967. Isolation and properties of the helical nucleocapsid of the parainfluenza virus SV 5. Proc. Nat. Acad. Sci. U.S.A. 57:949-956. 2. de-Thd, G., and T. E. O'Connor. 1966. Structure of a murine leukemia virus after disruption with Tween-ether and comparison with two myxoviruses. Virology 28:713-728. 3. Duesberg, P. H. 1968. Physical properties of Rous sarcoma virus RNA. Proc. Nat. Acad. Sci. U.S.A. 60:1511-1518. 3a. Duesberg, P. H. 1969. Distinct subunits of the ribonucleoprotein of influenza virus. J. Mol. Biol. 42:485-499. 4. Duesberg, P. H., and W. S. Robinson. 1965. Isolation of the nucleic acid of Newcastle disease virus (NDV). Proc. Nat. Acad. Sci. U.S.A. 54:794-800. 5. Duesberg, P. H., H. L. Robinson, W. S. Robinson, R. J. Huebner, and H. C. Turner. 1968. Proteins of Rous sarcoma virus. Virology 36:73-86. 6. Evans, M. J., and D. W. Kingsbury. 1969. Separation of Newcastle disease virus proteins by polyacrylamide gel electrophoresis. Virology 37:597-604. 7. Fenner, F. 1968. Biology of animal viruses. Vol. I. Molecular and cellular biology. Academic Press Inc., New York. 8. Gregoriades, A., and L. J. Old. 1969. Isolation and some characteristics of a group-specific antigen of the murine leukemia viruses. Virology 37:189-202. 9. Hosaka, Y. 1968. Isolation and structure of the nucleocapsid of HVJ. Virology 35:445-457. 10. Kang, C. Y., and L. Prevec. 1969. Proteins of vesicular stomatitis virus. I. Polyacrylamide gel analysis of viral antigens. J. Virol. 3:404-413. 11. Kaper, J. M. 1968. The small RNA viruses of plants, animals and bacteria. A. Physical properties, p. 1-133. In H. Fraenkel-Conrat (ed.), Molecular basis of virology, Reinhold, New York. 12. Kingsbury, D. W., and R. W. Darlington. 1968. Isolation and properties of Newcastle disease virus nucleocapsid. J. Virol. 2:248-255. 13. Klenk, H.-D., and P. W. Choppin. 1969. Chemical composition of the parainfluenza virus SV 5. Virology 37:155-157. 14. Laver, W. G. 1964. Structural studies on the proteirn subunits from three strains of influenza virus. J. Mol. Biol. 9:109-124. 15. Lwoff, A., R. Home, and P. Tournier. 1962. A system of viruses. Cold Spring Harbor Symp. Quant. Biol. 27:51-55. 16. Maizel, J. V., Jr. 1963. Evidence for multiple components in the structural protein of type 1 poliovirus. Biochem. Biophys. Res. Commun. 13:483-489. 17. Martin, R. G., and B. N. Ames. 1961. A method for determining the sedimentation behavior of enzymes: Application to protein mixtures. J. Biol. Chem. 236:1372-1379. 18. Robinson, W. S., and P. H. Duesberg. 1968. The myxoviruses, p. 255-305. In H. Fraenkel-Conrat (ed.), Molecular basis of virology. Reinhold, New York. VOL. 4, 1969 PROTEINS OF NDV 19. Rubin, H. 1960. A virus in chick embryos which induces resistance in vitro to infection with Rous sarcoma virus. Proc. Nat. Acad. Sci. U.S.A. 46:1105-1119. 20. Rueckert, R. R., and P. H. Duesberg. 1966. Non-identical peptide chains in mouse encephalitis virus. J. Mol. Biol. 17:490-502. 21. Schachman, H. K., and M. A. Lauffer. 1949. The hydration size and shape of tobacco mosaic virus. J. Amer. Chem. Soc. 71:536-541. 22. Shapiro, A. L., E. Vi8luela, andJ. V. Maizel, Jr. 1967. Molecular weight estimation of polypeptide chains by electrophoresis 393 in SDS-polyacrylamide gels. Biochem. Biophys. Res. Commun. 28:815-820. 22a. Sokol, F., H. D. Schlumberger, T. J. Wiktor, and H. Koprowski. 1969. Biochemical and biophysical studies on the nucleocapsid and on the RNA of rabies virus. Virology 38:651-665. 23. Strauss, J. H., B. W. Burge, E. R. Pfefferkorn, and J. E. Darnell, Jr. 1968. Identification of the membrane protein and "core" protein of Sindbis virus. Proc. Nat. Acad. Sci. U.S.A. 59:533-537. 24. Waterson, A. P. 1962. Two kinds of myxovirus. Nature 193:1163-1164.