Evolutionary selection of a 19-stranded mitochondrial β-barrel scaffold bears structural and functional significance

Abstract

Transmembrane β-barrels of eukaryotic outer mitochondrial membranes (OMMs) are major channels of communication between the cytosol and mitochondria and are indispensable for cellular homeostasis. A structurally intriguing exception to all known transmembrane β-barrels is the unique odd-stranded, i.e. 19-stranded, structures found solely in the OMM. The molecular origins of this 19-stranded structure and its associated functional significance are unclear. In humans, the most abundant OMM transporter is the voltage-dependent anion channel. Here, using the human voltage-dependent anion channel as our template scaffold, we designed and engineered odd- and even-stranded structures of smaller (V2¹⁶, V2¹⁷, V2¹⁸) and larger (V2²⁰, V2²¹) barrel diameters. Determination of the structure, dynamics, and energetics of these engineered structures in bilayer membranes reveals that the 19-stranded barrel surprisingly holds modest to low stability in a lipid-dependent manner. However, we demonstrate that this structurally metastable protein possesses superior voltage-gated channel regulation, efficient mitochondrial targeting, and in vivo cell survival, with lipid-modulated stability, all of which supersede the occurrence of a metastable 19-stranded scaffold. We propose that the unique structural adaptation of these transmembrane transporters exclusively in mitochondria bears strong evolutionary basis and is functionally significant for homeostasis.