The 5'-terminal structures of poliovirion RNA and poliovirus mRNA differ only in the genome-linked protein VPg

The 5'-terminal, RNase T1-resistant oligonucleotide of poliovirus mRNA has been isolated. Its sequence is pU-U-A-A-A-A-C-A-Gp, which is identical to that of virion RNA except that the genome-linked protein VPg is absent [Nomoto, A., Detjen, B., Pozzatti, R. & Wimmer, E. (1977) Nature 268, 208-213]. Because all newly synthesized viral RNAs are VPg-linked, we propose that VPg is cleaved from progeny RNA at the linkage between protein and nucleic acid prior to polyribosome formation. This may represent a new mode of processing of viral macromolecules. Virion RNA from which VPg has been cleaved proteolytically retains its specific infectivity, an observation suggesting that VPg is not involved in early steps (penetration and translation) of the infectious cycle initiated by RNA.

The location of the polio genome protein in viral RNAs and its implication for RNA synthesis

Evidence is presented that a small protein (VPg) is covalently attached to the 5'-terminal oligonucleotide VPg-pU-U-A-A-A-A-C-A-Gp of the polio genome, to nascent strands of the polio replicative intermediate and to poly(U) of minus strands. A model of polio RNA replication is proposed implicating VPg in initiation of RNA symthesis, possibly as primer.

A protein covalently linked to poliovirus genome RNA

Poliovirion [32P]RNA, after digestion with RNase T2, yields mononucleotides and a labeled compound "X," which is not negatively charged at pH 5. X contains, relative to the label in virion RNA, one to two phosphates and is partially acid insoluble. It can be labeled with tritiated amino acids 3 hr after infection, is insoluble in chloroform/methanol, and can be digested with Pronase. These observations suggest that X is a protein. The protein cannot be removed from the polio genome when the RNA is (i) sedimented through a sucrose gradient in 0.5 M NaCl, (ii) heated to 100 degrees in the presence of sodium dodecyl sulfate followed by sedimentation through a sucrose gradient in 80% dimethylsulfoxide, or (iii) banded in 4 M cesium trichloroacetate. Digestion of the 32P-labeled protein with Pronase yields one major 32P-labeled product, which contains pUp. The protein migrates faster than capsid protein VP4 in a polyacrylamide gel. Our data show that the genome of poliovirus, but not poliovirus mRNA [A. Nomoto, Y. F. Lee, and E. Wimmer (1976) Proc. Natl. Acad. Sci. USA 73, 375-380], is covalently attached to a small virus-coded protein (molecular weight less than 7000), which we call VPg. VPg is probably linked to the 5' end of the polio genome. Possible functions of VPg in viral replication are discussed.

The 5' end of poliovirus mRNA is not capped with m7G(5')ppp(5')Np

Poliovirus was grown in HeLa cells in the presence of phosphorus-32 and actinomycin D. Three to four hours after infection, viral mRNA was recovered from polyribosomes and its identity verified by two-dimensional gel electrophoresis of RNase T1 digests. Digestion of the viral [32P]mRNA with RNase T2 and separation of the products by ion exchange chromatography at pH 5 yielded pUp as possible 5' terminus but no "capping group" of the structure m7G(5')ppp(5')Np. Total cytoplasmic [32P]RNA of HeLa cells, on the other hand, was found to contain capping groups. Neither the capping group nor ppNp or pppNp was found in an RNase T2 digest of poliovirion [32P]RNA, in agreement with previous results [Wimmer, E. (1972) J. Mol. Biol. 68, 537-540]. The data indicate that 5'-terminal m7G(5')ppp(5')Np is absent from poliovirus RNAs and, therefore, is not involved in poliovirus protein synthesis.

Effect of Ricinus communis toxin on cyclic adenosine 3':5'-monophosphate metabolism in Yoshida ascites sarcoma cells

Prostaglandin E1 (2.5 mug/ml) enhanced the level of cyclic adenosine 3':5'-monophosphate (cyclic AMP) three to four times in Yoshida ascites sarcoma (YS) cells cultured in vitro. When Ricinus communis toxin (RC-toxin) was added 30 min after the addition of prostaglandin E1, the enhanced level of cyclic AMP in the YS cells decreased rapidly. Of RC-toxin, 0.2 mug/ml was enough to produce the maximum effect. By addition of 5 mM lactose with RC-toxin, approximately 60% of the RC-toxin effect on the levels of cyclic AMP was abolished. This indicates that the specific binding of RC-toxin on the surface membrane is largely responsible for the observed decrease of the cyclic AMP level. The toxin treatment did not induce either leakage of cyclic AMP from the cell or change in the activity of cyclic AMP phosphodiesterase. However, the treatment of YS cells with RC-toxin caused a decrease of adenylate cyclase activity when the activity was measured at a substrate concentration of 0.15 mM ATP. In contrast, there was little difference with the control when the activity was assayed at a higher ATP concentration, 0.24 mM. It was found that the K-m of adenylate cyclase for ATP was changed by RC-toxin from 0.1 to 0.25 mM, and that the Mg2+ activation of the enzyme observable in untreated cells disappeared. These results suggested that the decrease in the level of cyclic AMP in YS cells induced by RC-toxin can be explained in terms of the change in K-m of the adenylate cyclase activity.