Mutations in murine Mx1: effects on localization and antiviral activity.

Murine Mx1 is a nuclear localized protein of 631 amino acids with antiviral activity against influenza virus. Fourteen mutations in murine Mx1 were constructed in vitro, expressed in chicken embryo fibroblasts via replication-competent retroviruses, and the effects of the mutations on Mx localization and antiviral activity were assayed. The results suggest that a nuclear location is not sufficient for antiviral activity, that there are intricate structural constrains on the Mx protein for antiviral activity and that multiple domains of the Mx protein are required for the characteristic punctate nuclear Mx distribution. These conclusions are based on the findings showing that: (i) none of the mutants retained antiviral activity; (ii) only a mutant with a Leu to Lys substitution at residue 612 within the nuclear targeting signal retained the characteristic punctate nuclear localization of wildtype Mx1; (iii) diffuse nuclear localization was observed for mutants with substitutions of Pro for Leu at residue 619, 626, or both 619 and 626, and deletions of residues 23 to 95, 159 to 185, 369 to 409, 387 to 440, 522 to 560, or 541 to 596; and (iv) cytoplasmic localization was observed for mutants with carboxy-terminal truncations of 15, 30, or 61 amino acids, or a deletion of residues 610 to 624.