Expression and protective role of two novel NACHT-containing proteins in pathogen infection

Lower vertebrates have been found to possess over 200 NACHT-domain encoding genes; but, to date, very little is known about their functional activity. This article describes the sequences and expression analysis of two zebrafish NACHT-containing proteins, namely NALPL1 and NALPL2. In addition, the functions of zebrafish NALPL1 and NALPL2, which are absent for both amino-terminal effector-binding domain (EBD) and carboxy-terminal ligand-recognition domain (LRD), were investigated for the first time in fish species. The predicted NALPL1 and NALPL2 proteins consist of 651 and 847 amino acids (aa), respectively, with both molecules only containing NACHT domain, which were different from other NACHT-family members. Phylogenetic analysis showed that zebrafish NALPL1 and NALPL2 have a closer relationship with mammalian NALP subfamily than NOD subfamily. The differential expression patterns of NALPL1 and NALPL2 in development stages and organs were observed, suggesting the difference of action phase and effector organ of NALPL1 and NALPL2. When the modulation of NALPL1 and NALPL2 in pathogen infection was analyzed, it was found that the two molecules were upregulated by both bacterial and viral infection. Overexpression of NALPL1 and NALPL2 resulted in significant inhibition for intracellular Edwardsiella tarda growth. Further studies demonstrated that NALPL1 and NALPL2 also contributed to protection against viral infection. These results demonstrate that both NALPL1 and NALPL2 are important intracellular proteins in host surveillance against both bacterial and viral infection. Interestingly, the expression of downstream signaling genes was not affected by the overexpression of NALPL1 or NALPL2, but NOD1 and MDA5 were upregulated by NALPL1 or NALPL2 overexpression, suggesting that they likely act in pathogen infection through the interaction with other PRRs.