Binding partner of the GTPase-activating protein FLCN: involved in the cellular response to amino acid availability by regulating the mTORC1 signaling cascade controlling the MiT/TFE factors TFEB and TFE3 (PubMed:23582324). In low-amino acid conditions, component of the lysosomal folliculin complex (LFC) on the membrane of lysosomes, which inhibits the GTPase-activating activity of FLCN, thereby inactivating mTORC1 and promoting nuclear translocation of TFEB and TFE3 (By similarity). Upon amino acid restimulation, disassembly of the LFC complex liberates the GTPase-activating activity of FLCN, leading to activation of mTORC1 and subsequent cytoplasmic retention of TFEB and TFE3 (By similarity). Together with FLCN, regulates autophagy: following phosphorylation by ULK1, interacts with GABARAP and promotes autophagy (By similarity). In addition to its role in mTORC1 signaling, also acts as a co-chaperone of HSP90AA1/Hsp90: inhibits the ATPase activity of HSP90AA1/Hsp90, leading to activate both kinase and non-kinase client proteins of HSP90AA1/Hsp90 (By similarity). Acts as a scaffold to load client protein FLCN onto HSP90AA1/Hsp90 (By similarity). Competes with the activating co-chaperone AHSA1 for binding to HSP90AA1, thereby providing a reciprocal regulatory mechanism for chaperoning of client proteins (By similarity). May play a role in the signal transduction pathway of apoptosis induced by O6-methylguanine-mispaired lesions (PubMed:19137017). Homodimer and homomultimer. Heterodimer and heteromultimer with FNIP1. Interacts (via C-terminus) with FLCN (via C-terminus). Phosphorylated FLCN is preferentially bound. Component of the lysosomal folliculin complex (LFC), composed of FLCN, FNIP1 (or FNIP2), RRAGA/RagA or RRAGB/RagB GDP-bound, RRAGC/RagC or RRAGD/RagD GTP-bound, and Ragulator. Interacts with PRKAA1, PRKAB1 and PRKAG1 subunits of 5'-AMP-activated protein kinase. Interacts with HSP70, HSP90AA1, STIP1, PTGES3, CDC37, BRAF, GCR and CDK4. Colocalizes with FLCN in the cytoplasm. Phosphorylated by AMPK. Mice lacking both Fnip1 and Fnip2 show enlarged polycystic kidneys. Belongs to the FNIP family.