Nucleolar-localized sialidase Neu4 suppresses cell proliferation through destabilizing c-Myc via interacting with ribosomal proteins L5 and L11.

Sialidase Neu4 is a glycosidase that removes sialic acids from nonreducing ends of sugar chains. Recently, Mexican tetra Neu4 (AmNeu4) was identified as a nuclear-localized sialidase. However, the subnuclear localization and biological functions of AmNeu4 have remained unclear. This study demonstrated that AmNeu4 localized in the nucleolus and investigated the significance of its nuclear localization signal (NLS) for the nucleolar localization. Green fluorescent protein (GFP) fused to NLS localized to the nucleolus. Additionally, deletion of the NLS in AmNeu4 abolished its nucleolar localization, and insertion of additional lysine/arginine residues into AmNeu4's NLS induced exclusive nucleolar localization. These results indicated that the nucleolar localization of AmNeu4 was mediated by its NLS. The proliferation of AmNeu4-overexpressing HeLa cells was significantly reduced compared with that of the mock-transfected cells. Moreover, enzymatically inactive AmNeu4 also suppressed cell growth, indicating that the enzymatic activity of AmNeu4 is not required for the suppression of cell growth. We hypothesized that the suppressed cell proliferation is mediated by protein-protein interactions involving AmNeu4. By immunoprecipitation assay, ribosomal proteins L5 and L11 (RPL5 and RPL11, respectively) were identified as AmNeu4 interacting partners. C-MYC protein levels were lower in AmNeu4-overexpressing cells than in mock cells, and these reduced levels were restored by RPL11 knockdown. Coimmunoprecipitation assay further revealed that RPL11 bound to C-MYC in AmNeu4-overexpressing cells. These results suggest that the interaction between AmNeu4 and RPL5/RPL11 delays cell growth through destabilizing C-MYC.