LAUSR

Significance Cytochrome c (cyt c) is an abundant heme protein that, in its native conformation, plays essential roles in mitochondrial bioenergetics and apoptotic signaling in mammalian cells. The dynamic properties of cyt c confers substantial plasticity to produce alternative conformations in vitro, but their detection in cellula remains essentially unexplored. We have restored by de novo sequencing a unique monoclonal antibody (R1D3) recognizing conformationally altered cyt c proteoforms. R1D3 application to different cell models showed that alternative cyt c conformers are translocated from mitochondria to the nuclei without triggering apoptosis. R1D3 represents a unique immunochemical tool for mitochondrial and cell biology, encompassing knowledge of protein dynamics and plasticity with the identification of noncanonical structures and functions of cyt c in vivo.