Plakophilin-4 (PKP4, also known as p0071) has been identified as a FLCN-interacting protein by yeast-2-hybrid analysis and co-immunoprecipitation studies (Nahorski et al., 2012; Medvetz et al., 2012). PKP4 has been associated with cytokinesis, intercellular junction formation and RhoA signalling, which is discussed in more detail in Section 5. Through the yeast-2-hybrid screen, it is thought that the head domain of PKP4 binds FLCN. The region of FLCN which is involved in the interaction with PKP4 is currently unknown.
Regulatory particle triple-A ATPase 4 (Rpt4) was found to interact with FLCN in both Drosophila and human cells (Gaur et al., 2013). Rpt4 is a component of the 26S proteasome and is required for rRNA synthesis (Ottosen et al. 2002; Fátyol and Grummt, 2008), which is discussed in more detail in Section 5. FLCN interacts with Rpt4, preventing its association with rDNA and thus inhibiting rRNA synthesis (Gaur et al., 2013).
Folliculin interacts with the Rag proteins at the cytosolic surface of lysosomes, and this interaction is dependent on FNIP1 and FNIP2 (Martina et al., 2014, Petit et al., 2013; Tsun et al., 2013). Petit et al. showed FLCN specifically interacts with the GTPase domain of RagA in HeLa cells, suggesting it may function as a guanine nucleotide exchange factor for this protein. However, Tsun et al. showed that the FLCN-FNIP2 complex acts as a GTPase-activating protein towards RagC and RagD in HEK293T cells, catalysing the hydrolysis of RagC bound GTP to GDP. The interaction between FLCN and the Rag proteins was found to activate mTORC1 signalling in response to amino acid stimulation, as shown by increased S6K1 phosphorylation (Martina et al., 2014, Petit et al., 2013; Tsun et al., 2013). Activation of mTORC1 by FLCN led to the phosphorylation of Ser211 of the transcription factor TFEB and Ser311 of TFE3, which regulates the expression of lysosomal genes and autophagy genes (Martina et al., 2014, Petit et al., 2013).
ULK1 and GABARAP
FLCN enhances basal autophagic flux through its interactions with ULK1 and GABARAP (Dunlop et al., 2014). This interaction is mediated by FNIP1 and FNIP2 and inhibited by ULK1, which inhibits FLCN’s interaction with GABARAP by phosphorylating three novel phosphorylation sites at S406, S537 and S542. BHD patient FLCN mutations that truncate the C-terminal end of the protein show reduced binding to GABARAP, suggesting that reduced autophagy is likely to contribute to renal tumorigenesis.
Xia et al. (2015) has identified a role for FLCN in the cytosolic translocation and aggregation of the RNA/DNA binding protein TDP-43. Although TDP-43 is usually shuttled between the nucleus and the cytosol, enhanced translocation out of the nucleus and cytosol aggregation is associated with neuronal loss in ALS and FLTD. Xia et al. report that overexpression of FLCN, which directly interacts with TDP-43, results in enhanced TDP-43 translocation into the cytosol and the formation of stress granules. In contrast a depletion of FLCN increased nuclear deposition and dissociates TDP-43 from other stress granule proteins in the cytosol.