The amino acid sequence of the poliovirus 2C protein contains two highly conserved stretches, GSPGTGKS and MDD, which correspond to the consensus ‘A’ and ‘B’ motifs (GXXXXGKS/T and DD/E, respectively) found in nucleoside triphosphate-binding proteins. To assess the functional importance of these amino acid sequences, we changed conserved and non-conserved amino acids. The replacement of the non-conserved Thr residue with Ser or Ala did not markedly change the virus phenotype. Similarly, replacement of the non-conserved Pro residue by Ala did not abolish virus viability, but changes of this residue to Thr or Asn were not tolerated. No viable mutant could be isolated after transfection of cultured cells with transcripts mutated at the conserved Lys, Ser or Asp residues. However, true revertants were selected from Arg and Ser mutants, from Glu and Gly mutants, and from Ala mutants. Thr mutants not only gave rise to true revertants, but also to two independent isolates of a suppressor mutant, Asn→Tyr. All the lethal mutations resulted in severe inhibition of viral RNA synthesis , although no translational deficiency was detected in a cell-free system. This is the first direct evidence for the functional significance of the nucleoside triphosphatebinding pattern in the poliovirus 2C protein.


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