FNIP2, a second FLCN-interacting protein, first designated KIAA1450 by Nagase et al. (2000), was found by sequencing clones obtained from a size-fractionated human brain cDNA library. Hasumi et al. (2008) subsequently identified it as FNIP2, with Takagi et al. (2008) identifying it as FNIPL (for FNIP1-Like) soon after. Additionally, Komori et al. (2009) cloned mouse FNIP2 (which they called MAPO1), and found that it is required for apoptosis triggered by O6-methylguanine mispairing in DNA. This was confirmed by further studies which found that both FNIP2 and FLCN were necessary for the induction of apoptosis by N-Nitroso-N-methylurea (Lim et al., 2012; Sano et al., 2013).
FNIP1 and FNIP2 are required for FLCN’s localisation to lysosomes during amino acid starvation, where FLCN interacts with the Rag proteins in order to activate mTORC1 signalling once amino acid levels are restored (Petit et al., 2013; Tsun et al., 2013). FLCN’s interaction with and activation of the Rag proteins is also facilitated by FNIP1 and FNIP2 (Petit et al., 2013; Tsun et al., 2013).
FNIP2 may also be a membrane trafficking protein, as it carries a divergent DENN domain similar to that of FLCN (Zhang et al., 2012).
FNIP2 be required for myelination in the central nervous system in mammals, as Weimaraner dogs homozygous for a FNIP2 mutation showed dysmyelination of the spinal cord and developed a tremor phenotype at 12-14 postpartum (Pemberton et al., 2013). FNIP2 expression in oligodendrocytes is controlled by Sox10, a known regulator of oligodendrocyte terminal differentiation (Pemberton et al., 2013).
The accepted nomenclature for this protein is FNIP2, and it is homologous to FNIP1 (49% identity, 74% similarity). As with FNIP1, it is conserved across species and binds AMPK (Hasumi et al., 2008; Takagi et al., 2008). In vitro kinase assays also suggest that FNIP2 is phosphorylated by AMPK (Takagi et al., 2008). FNIP1 and FNIP2 are able to form homo- and heterodimers, as well as multimers (Takagi et al., 2008), suggesting a functional association between these two proteins.
Binding of FLCN to both FNIP1 and FNIP2 is mediated specifically through the C-terminal region of FLCN (Baba et al., 2006; Hasumi et al., 2008; Takagi et al., 2008). In BHD syndrome, the majority of mutations are predicted to introduce a premature stop codon into FLCN, and therefore result in a protein truncation (Schmidt et al., 2005). However, it is unclear whether truncated FLCN is targeted for nonsense-mediated decay, or remains in the cell with altered function. Whatever the outcome, truncating mutations that result in the loss of the C-terminus of FLCN will abolish its ability to interact with FNIP1 and FNIP2, which suggests that this interaction is functionally important.
An interchangeable role for FNIP1 and FNIP2 alongside FLCN in tumour suppression has been suggested by Hasumi et al., (2015). FNIP1 and FNIP2 were found to be partially functionally redundant with either able to interact with FLCN and inhibit tumourigenic growth in mouse kidneys. A complete loss of both FNIP1 and FNIP2 in the kidneys lead to the development of tumours and aberrant mTOR signalling.