Identification and Characterization of Unique Proline-rich Peptides Binding to the Mitochondrial Fission Protein hFis1

Mammalian mitochondrial fission requires at least two proteins, hFis1 and the dynamin-like GTPase DLP1/Drp1. The mitochondrial protein hFis1 is anchored at the outer membrane by a C-terminal transmembrane domain. The cytosolic domain of hFis1 contains six alpha helices [alpha 1-alpha 6] out of which [alpha 2-alpha 5] form tetratricopeptide repeat (TPR)-like motifs. DLP1 and possibly other proteins are thought to interact with the hFis1 TPR region during the fission process. It has also been suggested that the alpha 1-helix regulates protein-protein interactions at the TPR. We performed random peptide phage display screening using the hFis1[alpha 2-alpha 6] as the target and identified ten different peptide sequences. Phage ELISA using mutant hFis1 indicates that the peptide binding requires the alpha 2 and alpha 3 helices and the intact TPR structure. Competition experiments and surface plasmon resonance analyses confirmed that a subset of free peptides enriched with proline residues directly bind to the target. Two of these peptides bind to the alpha 1-containing intact cytosolic domain of hFis1 with decreased affinity. Peptide microinjection into cells abolished the mitochondrial swelling induced by overexpression of alpha 1-deleted hFis1, and significantly decreased cytochrome c release from mitochondria upon apoptotic induction. Our data demonstrate that hFis1 can bind to multiple amino acid sequences selectively, and that the TPR constitutes the main binding region of hFis1, providing a first insight into the hFis1 TPR as a potential therapeutic target.