ULK1/2 Restricts the Formation of Inducible SINT-Speckles, Membraneless Organelles Controlling the Threshold of TBK1 Activation

Saul, Vera Vivian; Seibert, Markus; Krüger, Marcus; Jeratsch, Sylvia; Kracht, Michael; Schmitz, Michael Lienhard

TBK1 and several of its interactors including optineurin and mitochondrial antiviral-signaling protein are known to be recruited to membrane-surrounded organelles such as mitochondria (Fang et al., 2017; Moore and Holzbaur, 2016) or upon RNA virus infection to the Golgi apparatus (Pourcelot et al., 2016) . Thus, we asked whether SINTBAD would be found in association with these organelles. Costaining experiments with untreated and arsenite-or heat shock-treated cells showed significant colocalization neither with mitochondria ( Figure 4A ) nor with Golgi, lysosomes, peroxisomes, endosomes, or endoplasmic reticulum (Figure S6) . In addition, we costained speckles with marker proteins of several known MLOs. These experiments showed colocalization of speckles neither with the SG marker proteins eIF4G ( Figure 4B ) or Ras GTPaseactivating protein-binding protein 1 (G3BP1) nor with the P body marker DCP1a ( Figures S7A and S7B) .