Smaug1 membraneless organelles regulate mitochondrial function

BOCCACCIO, GRACIELA L.

Sherbrooke

Congreso; Riboclub 20th Anniversary and Canada Gairdner International symposium; 2019

Smaug orthologs are mRNA binding proteins highly conserved in the animal kingdom. Smaug recognizes a wide variety of stem-loops termed Smaug Recognition Elements (SREs), which are present in a large number of mRNAs including nuclear transcripts that encode mitochondrial enzymes. We have previously shown that mammalian Smaug1 forms membraneless organelles (MLOs) termed Smaug-bodies that help localized translation at dendrites (Baez et al JCB 2011; Luchelli et al., JCS 2015). More recently, we performed time-lapse confocal microscopy in live U2OS cells and found that Smaug1 MLOs are highly motile and frequently contact mitochondria. In addition, the simultaneous loss of function of mammalian Smaug1/Samd4a and Smaug2/Samd4b in cell lines disrupted the mitochondrial network and seriously affected the mitochondrial respiration. The phenotype was fully rescued by transfection of a Smaug1 construct or a Smaug1 splicing variant termed ΔEIII, thus suggesting that Smaug1 and Smaug2 isoforms play a rather redundant role. Smaug?s main function is to control mRNAs and we found that deletion of the protein domain involved in RNA binding abrogates the rescue of the phenotype. In addition, we identified several Smaug1 protein regions involved in phase-separation and found that the deletion of a conserved domain located at the N-terminus dramatically affects MLO formation. Strikingly, Smaug1 constructs that show defective body condensation were not able to rescue the mitochondrial phenotype caused by Smaug1 and 2 double KD. We investigated the molecular consequences of impaired Smaug1-body formation and found that the binding to mRNAs that encode enzymes of the electron transport chain (ETC) is moderately affected. Finally, we found that mitochindrial inhibitors immediately induce the dissolution of Smaug1-bodies and the release of bounded mRNAs. We propose that Smaug1 MLOs respond to changes in the energetic metabolism to control mRNAs that encode key mitochondrial proteins, likely coordinating their transport and translation at the mitochondrial periphery.