Mitochondrial fission protein 1 (Fis1) was identified in yeast as being essential for mitochondrial division or fission and subsequently determined to mediate human mitochondrial and peroxisomal fission. Yet, its exact functions in humans, especially in regard to mitochondrial fission, remains an enigma as genetic deletion of Fis1 elongates mitochondria in some cell types, but not others. Fis1 has also been identified as an important component of apoptotic and mitophagic pathways suggesting the protein may have multiple, essential roles. This review presents current perspectives on the emerging functions of Fis1 and their implications in human health and diseases, with an emphasis on Fis1’s role in both endocrine and neurological disorders.

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Mitochondrial fission protein 1 (FIS1) is conserved in all eukaryotes yet its activity in metazoans is thought divergent from lower eukaryotes like fungi. To address this discrepancy, structure-based sequence alignments revealed a conserved but non-canonical, three-residue insert in a FIS1 turn suggesting a conserved activity. In vertebrate FIS1 this insert is serine (S45), lysine (K46), and tyrosine (Y47). To determine the biological role of this SKY insert, three variants were evaluated for their fold, and tested in HCT116 cells for altered mitochondrial morphology and recruitment of effectors, DRP1 and TBC1D15. Substitution of the SKY insert with three alanine residues (AAA), or deletion of the insert (ΔSKY), did not substantially alter the fold or thermal stability of the protein. Replacing SKY with a canonical turn (ΔSKYD49G) introduced significant conformational heterogeneity by NMR that was removed upon deletion of a known regulatory region, the Fis1 arm. Expression of AAA fragmented mitochondria into perinuclear clumps associated with increased mitochondrial Drp1 similar to the wild-type protein. In contrast, expression of ΔSKY variants elongated mitochondrial networks and reduced mitochondrial Drp1. Co-expression of YFP-TBC1D15 partially rescued mitochondrial morphology and Drp1 recruitment for ΔSKY variants, although ΔSKY variants were markedly unable to support TBC1D15 assembly into punctate structures found upon co-expression with wildtype Fis1 or the AAA variant. Collectively these results show that FIS1 activity can be modulated by conserved residues supporting a generalized model whereby FIS1 is governed by intramolecular interactions between the regulatory FIS1 arm and SKY insert that may be conserved across species.

Since its initial identification in yeast, Fis1, a 17kDa mitochondria‐outer‐membrane protein has been implicated in mitochondrial dynamics, mitophagy, and apoptosis. Despite this importance, the mechanisms by which Fis1 activity is governed in these processes remain unclear. Structural considerations suggests that a highly flexible N‐terminal “arm” may mediate interactions with cognate binding partners. We recently demonstrated that the human Fis1 arm, contrary to previous reports, is capable of assuming conformations akin to its distant relative, yeast Fis1p, suggesting a conserved mode of activity regulation across kingdoms. With this premise, we searched for amino acid sequences using structure and homology‐based sequence alignments, which identified a uniquely conserved region that we hypothesize to be important for regulating Fis1 function. Using MD simulations, IF‐microscopy, and NMR, we show that this region is important for Fis1 fission and mitophagic function.

Fission protein 1 (Fis1) and dynamin‐related protein 1 (Drp1) were initially described as being evolutionarily conserved for mitochondrial fission, yet the role of Fis1 in human fission is unclear and disputed by many. In budding yeast where Fis1 helps to recruit Drp1 from the cytoplasm to mitochondria for fission, an N‐terminal “arm” of Fis1 is required for function. The yeast Fis1 arm interacts intramolecularly with a conserved surface that governs in vitro interactions with yeast Drp1. In human Fis1, NMR and x‐ray structures show a different arm conformation and its importance for human Drp1 recruitment is unknown. Here, we use MD simulations and comparisons to experimental NMR chemical shifts to show the human Fis1 arm can adopt an intramolecular conformation akin to that observed with the yeast molecule. This finding is further supported through intrinsic tryptophan fluorescence and NMR experiments on human Fis1 with and without the arm. Using NMR, the human Fis1 arm is also observed to be sensitive to environmental changes. The importance of these findings are revealed in cellular studies where the removal of the hFis1 arm reduces Drp1 recruitment and mitochondrial fission suggesting an important role for Fis1 in human mitochondrial fission. Support or Funding Information This project was supported by the following National Institutes of Health grants: R01GM067180 (to RBH), and R01HL128240 (to MEW).