The mitochondria-targeted peptide SS-31 binds lipid bilayers and modulates surface electrostatics as a key component of its mechanism of action

This 2020 Journal of Biological Chemistry paper is the most rigorous biophysical re-examination of the SS-31 mechanism of action and refines the foundational cardiolipin-binding framework that Birk 2013 established. The investigators — a collaboration between the Szeto group at Weill Cornell, the Alder group at University of Connecticut, and computational laboratories — combined biophysical experimental methods (steady-state fluorescence, calorimetry, electrochemistry) with all-atom molecular-dynamics simulations to characterise SS-31 binding to model membranes containing varying proportions of cardiolipin, phosphatidylserine, and other anionic and zwitterionic phospholipids. Key findings: SS-31 partitions into the membrane interfacial region with affinity and lipid-binding density that are directly proportional to bilayer surface charge — meaning the peptide binds anionic membranes (which include cardiolipin-rich mitochondrial inner membranes) generally rather than cardiolipin selectively. The peptide does not destabilise bilayer structure even at high concentrations. Most importantly, SS-31 modulates the surface-electrostatic properties of the bilayer by down-regulating surface potential, which the authors propose affects calcium distribution at the membrane interface and protein–lipid interactions that organise the electron-transport-chain supercomplexes. The paper proposes a refined mechanism: SS-31 protects mitochondria by stabilising the electrostatic environment of the inner membrane, not by interacting with cardiolipin per se as a specific receptor.