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 (By similarity). 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. 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). Required to promote FLCN recruitment to lysosomes and interaction with Rag GTPases. 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. Acts as a scaffold to load client protein FLCN onto HSP90AA1/Hsp90 (By similarity). Homodimer and homomultimer. Heterodimer and heteromultimer with FNIP2 (By similarity). Component of the lysosomal folliculin complex (LFC) (By similarity). Localizes to lysosome membrane in amino acid-depleted conditions and relocalizes to the cytosol upon refeeding. Colocalizes with FLCN in the cytoplasm. Belongs to the FNIP family.