Bacterial Porin Disrupts Mitochondrial Membrane Potential and Sensitizes Host Cells to Apoptosis

Abstract

The bacterial PorB porin, an ATP-binding β-barrel protein of pathogenic Neisseria gonorrhoeae, triggers host cell apoptosis by an unknown mechanism. PorB is targeted to and imported by host cell mitochondria, causing the breakdown of the mitochondrial membrane potential (ΔΨm). Here, we show that PorB induces the condensation of the mitochondrial matrix and the loss of cristae structures, sensitizing cells to the induction of apoptosis via signaling pathways activated by BH3-only proteins. PorB is imported into mitochondria through the general translocase TOM but, unexpectedly, is not recognized by the SAM sorting machinery, usually required for the assembly of β-barrel proteins in the mitochondrial outer membrane. PorB integrates into the mitochondrial inner membrane, leading to the breakdown of ΔΨm. The PorB channel is regulated by nucleotides and an isogenic PorB mutant defective in ATP-binding failed to induce ΔΨm loss and apoptosis, demonstrating that dissipation of ΔΨm is a requirement for cell death caused by neisserial infection.

Structural basis for solute transport, nucleotide regulation, and immunological recognition of Neisseria meningitidis PorB

“…3C), and is located such that it would prevent pore closure by protein loops. This binding location is consistent with mutagenesis studies showing residues equivalent to Lys42 and Lys100 in the N. gonorrhoeae PorB affect nucleotide binding (9,10) and with the observations that interaction of PorB with ATP reduces the single-channel conductance (10) and increases channel open probability (9). The AMP-PNP binding site overlaps with the sugar-binding site and putative cation and anion translocation pathways to different extents (Fig.…”

“…Taken together, this likely affects the identity of solutes that can traverse the pore directly because it may alter size cutoff for solutes and may shift the selectivity to more positively charged solutes. Supporting a direct effect of pNTP on solute translocation, previous single-channel bilayer recordings of PorB performed in the presence and absence of ATP show that ATP indeed decreases single-channel conductance (9,10).…”

“…During infection, PorB targets to the mitochondria (7,8) where it likely inserts into the inner membrane (9). Once inserted, these porins noncovalently bind ATP (10).…”

“…Once inserted, these porins noncovalently bind ATP (10). Functionally, the presence of ATP reduces the single-channel conductance and increases the open probability (9). In addition, PorB directly interacts with the host mitochondrial voltage-dependent anion channel (VDAC) (7).…”

“…In addition, PorB directly interacts with the host mitochondrial voltage-dependent anion channel (VDAC) (7). Importantly, the insertion of PorB into mitochondrial membranes is known to influence apoptosis (7)(8)(9)(11)(12)(13). Because both the mitochondrial membrane potential and VDAC are important for apoptosis (14), PorB may influence the initiation of apoptosis at multiple checkpoints.…”