Metastasis suppressor function of NM23-H1 requires its 3'-5' exonuclease activity

The metastasis suppressor NM23-H1 possesses 3 enzymatic activities in vitro, a nucleoside diphosphate kinase (NDPK), a protein histidine kinase and a more recently characterized 3'-5' exonuclease. Although the histidine kinase has been implicated in suppression of motility in breast carcinoma cell lines, potential relevance of the NDPK and 3'-5' exonuclease to metastasis suppressor function has not been addressed in detail. To this end, site-directed mutagenesis and biochemical analyses of bacterially expressed mutant NM23-H1 proteins have identified mutations that disrupt the 3'-5' exonuclease alone (Glu(5) to Ala, or E(5) A), the NDPK and histidine kinase activities tandemly (Y(52) A, H(118) F) or all 3 activities simultaneously (K(12) Q). Although forced expression of NM23-H1 potently suppressed spontaneous lung metastasis of subcutaneous tumor explants derived from the human melanoma cell line 1205LU, no significant metastasis suppressor activity was obtained with the exonuclease-deficient variants E(5) A and K(12) Q. The H(118) F mutant, which lacked both the NDPK and histidine kinase while retaining the 3'-5' exonuclease, also exhibited compromised suppressor activity. In contrast, each mutant retained the ability to suppress motility and invasive characteristics of 1205LU cells in culture, indicating that the NM23-H1 molecule possesses an additional activity(s) mediating these suppressor functions. These studies provide the first demonstration that the 3'-5' exonuclease activity of NM23-H1 is necessary for metastasis suppressor function and further indicate cooperativity of the 3 enzymatic activities of the molecule on suppression of the metastatic process.