| Summary: | Birt-Hogg-Dubé (BHD) syndrome is a rare autosomal dominant condition caused by mutations in the FLCN gene and characterised by benign hair follicle tumours, pneumothorax and renal cancer. Folliculin (FLCN), the protein product of FLCN, is a poorly characterised tumour suppressor, currently linked to multiple cellular pathways. Autophagy is an evolutionarily conserved biological process that maintains cellular homeostasis by removing damaged organelles and macromolecules. Although the autophagy kinase, ULK1, is known to initiate autophagy, the underlying mechanisms are still being unravelled and new ULK1 substrates are being identified. In this study, FLCN is identified as a new ULK1 substrate that interacts with the autophagy machinery via GABARAP. FLCN/GABARAP interaction is shown to be modulated by the presence of FNIP1 and FNIP2, as well as ULK1-directed phosphorylation of FLCN. Furthermore, loss of FLCN impairs autophagy while re-expression rescues autophagy. FLCN was found to interact with several Rab small G proteins linked to autophagy, implicating FLCN in vesicular trafficking. Upon generation of FNIP1/2 knockdown cell lines it was revealed that FNIP2 is possibly involved in the transcriptional regulation of FLCN and is involved in FNIP1’s translation and/or stability. Moreover, FNIP1 could be involved in regulation of FNIP2’s transcription via a negative feedback mechanism. FNIP proteins were further implicated in autophagy when FNIP1 and FNIP2 gene expression were found to be significantly increased upon starvation in HK2 cells. Additionally, p62 and GABARAP protein levels demonstrated a significant increase in FNIP2 knockdown cell lines suggestive of impaired autophagy. This work substantially contributes to our understanding of FLCN by linking it directly to autophagy. Furthermore, FLCN and the FNIPs are shown to be involved in multiple protein interactions which could mean that FLCN and its interacting partners might have a more universal housekeeping role, which when lost leads to cancer.
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