A protein hydroxylase couples epithelial membrane biology to nucleolar ribosome biogenesis

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Jumonji-C (JmjC) ribosomal protein hydroxylases are an ancient class of oxygen- and Fe(II)-dependent oxygenases that spawned the wider JmjC family and Histone Lysine Demethylases (KDMs) in eukaryotes. Myc-induced Antigen (MINA) has been implicated in ribosome biogenesis and was assigned as a nucleolar-localized JmjC histidyl hydroxylase of the large ribosomal subunit protein RPL27A, consistent with reports that it supports cell growth and viability in a variety of tumor cell types. Reported roles in diverse aspects of disease biology may be consistent with additional MINA functions, although the molecular mechanisms involved remain unclear. Here, we describe an extra-nucleolar interaction of MINA with the Hinge domain of the membrane-associated guanylate kinase, MPP6. We show that MINA promotes the expression and membrane localization of MPP6 and that the MINA-MPP6 pathway is required for epithelial tight junction integrity and barrier function. The function of MINA in this novel pathway is suppressed by ribosomal RNA transcription and the nucleolar MINA interactome, which sequesters MINA in the nucleoli of growing cells. Our work sheds light on how quiescent cells lose adhesion as they switch to proliferate states associated with increased ribosome biogenesis.

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  1. Review coordinated by Life Science Editors. Reviewed by: Dr. Helen Pickersgill, Life Science Editors Potential Conflicts of Interest: None

    Main point of the paper: The authors discover a new function for a nucleolar protein, a hydroxylase called MINA (a Myc target gene) involved in ribosome biogenesis and linked with lots of diseases (cancer, allergy), at the plasma membrane of epithelial cells. Here, it promotes epithelial tight junction integrity and barrier function, likely together with a guanylate kinase, MPP6.

    Why this is interesting: This may be the first link specifically between ribosome biogenesis and adhesion, which occur in two different subcompartments of the cell, and it is mediated by one protein, MINA. The linking of these processes would enable the careful coordination that is needed between cell division and adhesion, and could thereby help cells transition between two states (i.e., from quiescence to proliferation). So, it’s the multifunctional property of MINA in two different parts of the cell that could help promote an important state transition, which may be why it’s linked to different diseases.

    Background: MINA is primarily known as a nucleolar-localized protein that promotes ribosome biogenesis/function. It catalyzes histidine hydroxylation of the large ribosomal subunit protein RPL27A thereby promoting its activity. Consistent with this, MINA is upregulated in rapidly growing cells, including tumors. However, it is expressed also in normal human tissues, and has also been linked with seemingly ribosome-independent functions, specifically in suppressing cell migration, invasion, and EMT. This suggests it has other, unknown functions, which may or may not be related to each other.

    Transitioning between distinct cell states such as differentiation and proliferation requires extensive coordination of diverse processes. For example, transitioning from quiescence to growth requires increasing metabolic activity and involves coupling processes like the cell cycle and adhesion to enable the cell to successfully divide.

    Results: • MINA depletion in intestinal epithelial cells (Caco-2 cells stably expressing MINA-targeting shRNA) caused flattening and other phenotypes indicative of a role in tight junction integrity and barrier function. This pointed to an unexpected role for a nucleolar protein in epithelial membrane biology. • Pull-downs with a MINA mutant that was unable to localize to the nucleolus in three cell lines (Caco-2, U2OS, and MEFs) together with proteomic screens in Caco-2 cells of the WT and nucleolar-interacting mutant identified MPP6, a member of the MAGUK superfamily of cell adhesion and polarity proteins, as a candidate interacting partner of MINA. • In vitro interaction assays with endogenous and tagged proteins verified the MINA-MPP6 interaction in the nucleoplasm, and showed that it was promoted by RNApol1 inhibition (which increases the pool of extra-nucleolar MINA in subconfluent cells).
    • MINA knockdown redistributes HA-tagged-MPP6 from the plasma membrane to the nucleus, suggesting a functional consequence of this interaction. • shRNA knockdown of MPP6 phenocopied MINA depletion with respect to altered barrier function. i.e., both are implicated in tight junction integrity and epithelial barrier function.

    Remaining questions/points: • It’s not yet demonstrated definitively that MINA works via MPP6 in tight junction integrity and epithelial barrier function. It would help to have more insight into this mechanism – it appears to not be related to MINA’s catalytic activity, and may ‘just’ involve MINA promoting MPP6 membrane localization, but whether this is direct or indirect (and if/how it’s regulated) is unclear. • Whether there is a regulatory link between ribosomal biogenesis and membrane biology via MINA is unclear – i.e., if/how MINA’s two functions are physiologically regulated to coordinate proliferation and adhesion (this may require going beyond cell lines). It could be that these two functions are normally independent (but if you experimentally interfere in vitro, then you see a dependency).