Cancer metastasis‐associated protein 1 localizes to the nucleolus and regulates pre‐rRNA synthesis in cancer cells

Metastasis‐associated protein 1 (MTA1) is a critical component of the nucleosome remodeling and histone deacetylase (NuRD) complex. MTA1 has several biological functions, and it is closely associated with the malignant properties of human cancers; however, the mechanisms and subcellular localization of MTA1 in cells remain unclear. Some initial studies indicated that MTA1 was absent from the nucleolus; however, several NuRD components were recently found to be present in the nucleolus, where they regulate preribosomal RNA (pre‐rRNA) transcription. In this study, we demonstrated that MTA1 is definitely localized to the nucleolus and regulates pre‐rRNA transcription, which is consistent with the recent reports on NuRD. To determine if MTA1 was present in the nucleolus, we utilized the following complementary molecular approaches: immunofluorescence, GFP‐tag tracking, immunoelectron microscopy, and immunoprecipitation (IP). To examine the role of MTA1 in rRNA synthesis, we performed quantitative polymerase chain reaction analysis. We revealed that both endogenous and exogenous MTA1 showed apparent granule‐like nucleolar subcellular localization. MTA1 interacts with two major resident nucleolar proteins, nucleolin and nucleophosmin. Immunofluorescent colocalization analyses showed that MTA1 localizes to the fibrillarin‐deficient regions of the nucleolus, and Co‐IP experiments indicated that there was no interaction between MTA1 and fibrillarin; further, fibrillarin was not identified in the MTA1 interactome. Loss‐ and gain‐of‐function studies indicated that MTA1 promotes pre‐rRNA transcription in cancer cells. Collectively, our data identify MTA1 as a novel nucleolar protein, and activation of pre‐rRNA transcription in cancer cells may be an alternative mechanism by which MTA1 promotes malignancies. In the current study, by tracking both the endogenous and exogenous metastasis‐associated protein 1 (MTA1), we uncover a granule‐like distribution pattern of MTA1 in the nucleolus. MTA1 interacts with the major resident nucleolar proteins NCL and NPM1, but not with FBL which was identified mutually exclusive with MTA1 for the nucleolar occupation. Functionally, MTA1 facilitates the transcription of preribosomal RNA in cancer cells.