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

A B E C D is required for ULK1 phosphorylation occurring during the cell type-specific induction of autophagy , suggesting a mutual cross talk between these SINT-speckle components. Depletion of ULK1/2 leads to the formation of SINT-speckles in unstimulated cells. As expression of a kinase-inactive ULK1 mutant results in the spontaneous formation of SINT-speckles it is reasonable to assume an important contribution of the kinase function for this process. Also, other protein kinases function to restrict the formation of MLOs. The related kinases HIPK1 and HIPK2 decrease the size and number of PML-NBs during the cell cycle (Berchtold et al., 2018) , and active DYRK3 facilitates the dissolution of several types of MLOs during mitosis (Rai et al., 2018) . Kinase-dependent mechanisms might also contribute to the elimination of SINT-speckles during mitosis, a process that is accompanied by massive SINTBAD phosphorylation (Figures S12A and S12B). MLO composition can be modulated in response to stress (Boulon et al., 2010; Dellaire and Bazett-Jones, 2004) , and accordingly this study reveals the recruitment of SINTBAD to SINT-speckles in response to a variety of cell stresses or after inhibition of the chaperone function of HSPs. Heat shock-induced speckle incorporation of SINTBAD can probably also be explained by the temperature-regulated SINTBAD-HSP70 interaction. The relevance of HSP70 was also shown for the prevention of aberrant SG formation (Mateju et al., 2017) , and it will be interesting to reveal whether HSPs play a general role in the control of MLO integrity, as recently implicated by the analysis of the cellular chaperone network (Rizzolo et al., 2017) . This study revealed the existence of a large functional chaperone supercomplex and its preferential interaction with proteins forming foci or condensates under stress conditions.