The Mitochondrial Porin, VDAC, Has Retained the Ability to Be Assembled in the Bacterial Outer Membrane

Beta-barrel proteins are present in the outer membranes (OMs) of Gram-negative bacteria, mitochondria, and chloroplasts. Their assembly requires a machinery of which the central component, called Omp85 (BamA) in bacteria and Tob55 (Sam50) in mitochondria, is evolutionarily conserved. An open question is whether the signals in beta-barrel OM proteins required for assembly via this multicomponent machinery are also conserved. To address this question, we have expressed in Escherichia coli the mitochondrial porin voltage-dependent anion channel (VDAC) from Neurospora crassa fused to a bacterial signal sequence for transport across the bacterial inner membrane. The protein was assembled in the bacterial OM where it formed pores. Assembly of VDAC was dependent on its beta-signal, which is required for assembly in the mitochondrial OM, and on the bacterial Omp85 assembly machinery. These results demonstrate that the basic mechanism of beta-barrel assembly in the OMs of bacteria and mitochondria is conserved.