Nucleotide transport in Rhodobacter capsulatus

Targeting of the pro-apoptotic VDAC-like porin (PorB) of Neisseria gonorrhoeae to mitochondria of infected cells

Infection of cell cultures with Neisseria gonorrhoeae results in apoptosis that is mediated by the PorB porin. During the infection process porin translocates from the outer bacterial membrane into host cell membranes where its channel activity is regulated by nucleotide binding and voltage-dependent gating, features that are shared by the mitochondrial voltage-dependent anion channel (VDAC). Here we show that porin is selectively and efficiently transported to mitochondria of infected cells. Prevention of porin translocation also blocked the induction of apoptosis. Mitochondria of cells treated with porin both in vitro and in vivo were depleted of cytochrome c and underwent permeability transition. Overexpression of Bcl-2 blocked porin-induced apoptosis. The release of cytochrome c occurred independently of active caspases but was completely prevented by Bcl-2. Our data suggest that the Neisseria porin can, like its eukaryotic homologue, function at the mitochondrial checkpoint to mediate apoptosis.

Isolation and properties of an ATP transporter from a strain of Aspergillus niger

A purified ATP transporter from Aspergillus niger did not show release or uptake for any of the nucleotides (ADP or UTP) except ATP. The release and uptake did not result from non-specific binding, but appeared to be concentration-dependent processes. ATP was shown by a double-isotopic technique to be transported across membrane vesicles without degradation. The ATP-transport protein was purified to near homogeneity from the membrane vesicles of a strain of A. niger and its apparent Mr was approximately 60000. The purified protein showed the properties of a membrane-bound protein in that the carrier protein was shown, during the liposome-preparative process, to translocate from the aqueous phase into the lipid bilayer of the liposome, unlike the cytosolic protein glucose-6-phosphate dehydrogenase, which remained confined to the aqueous compartment. Mycobacillin, a lipid-reactive antibiotic, was bound to the transport protein at a site other than the ATP-binding site, leading to its enhanced release or uptake, which was very feeble in absence of the antibiotic.

Spheroplast-derived membrane vesicles from Rhodobacter capsulatus cells catalyzing nucleotide transport

Rodobacter capsulatus cells, which were cultured anaerobically in high light intensity, had fewer foldings in the cytoplasmic membrane than those which were grown in lower light intensities. Spheroplast-derived membrane fractions obtained from cells cultured under high light intensity contained a high yield of large right-side-out membrane vesicles. The right-side-out vesicles catalyzed reversible light-induced proton efflux as did intact cells. Nucleotide transport activity was also catalyzed by these membrane vesicles. This activity was indirectly monitored by measurement of photophosphorylation or hydrolysis of externally added diphospho- and triphosphonucleosides. These enzymatic activities occur inside the cytoplasmic membrane of spheroplasts and membrane vesicles and therefore require the transport of the externally added reagents. The indirect measurements of transport were complemented by the demonstration of direct uptake of radiolabeled nucleotides into the membrane vesicles. These data support the suggestion that a nucleotide transporter located in the cytoplasmic membrane of R. capsulatus bacteria mediates these activities.