Condensate formation of the human RNA-binding protein SMAUG1 is controlled by its intrinsically disordered regions and interactions with 14-3-3 proteins

Abstract

SMAUG1 is a human RNA-binding protein that is known to be dysregulated in a wide range of diseases. It is evolutionarily conserved and has been shown to form condensates containing translationally repressed RNAs. This indicates that condensation is central to proper SMAUG1 function; however, the factors governing condensation are largely unknown. In this work, we show that SMAUG1 drives the formation of liquid-like condensates in cells through its non-conventional C-terminal prion-like disordered region. We use biochemical assays to show that this liquid-liquid phase separation is independent of RNA binding and does not depend on other large, disordered regions that potentially harbor several binding sites for partner proteins. Using a combination of computational predictions, structural modeling, in vitro and in cell measurements, we also show that SMAUG1-driven condensation is negatively regulated by direct interactions with members of the 14-3-3 protein family. These interactions are mediated by four distinct phospho-regulated short linear motifs embedded in the disordered regions of SMAUG1, working synergistically. Interactions between SMAUG1 and 14-3-3 proteins drive the dissolution of condensates, alter the dynamics of the condensed state, and are likely to be intertwined with currently unknown regulatory mechanisms. Our results provide information on how SMAUG1 phase separation is regulated and the first known instance of 14-3-3 proteins being able to completely dissolve condensates by directly interacting with a phase separation driver, which might be a general mechanism in cells to regulate biological condensation.

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