Cardiolipin dynamics promote membrane remodeling by mitochondrial OPA1.

Cardiolipin (CL) is a mitochondria-specific phospholipid that forms heterotypic interactions with membrane-shaping proteins and regulates the dynamic remodeling and function of mitochondria. However, the precise mechanisms through which CL influences mitochondrial morphology are not well understood. In this study, employing molecular dynamics (MD) simulations, we determined that CL molecules extensively engage with the paddle domain (PD) of mitochondrial fusion protein Optic Atrophy 1 (OPA1), which controls membrane-shaping mechanisms. Structure-function analysis confirmed the interactions between CL and two conserved motifs of OPA1 at the membrane-binding sites. We further developed a bromine-labeled CL probe to enhance cryoEM contrast and characterized the structure of OPA1 assemblies bound to the CL-brominated lipid bilayers. Our images provide direct evidence of CL enrichment within the OPA1-binding leaflet. Last, we observed a decrease in membrane remodeling activity for OPA1 in lipid compositions with increasing concentrations of monolyso-cardiolipin (MLCL). This suggests that the partial replacement of CL by MLCL, as observed in Barth syndrome-associated mutations of the tafazzin phospholipid transacylase, alters the malleability of the membrane and compromises proper remodeling. Together, these data provide insights into how biological membranes regulate the mechanisms governing mitochondrial homeostasis.