In vitro transport assays show a division of Occ channels into two subfamilies.

<p>Uptake of radiolabeled substrates in total membrane vesicles of <i>E. coli</i> Bl21 omp8 expressing empty plasmid (pB22), Occ channels, and <i>E. coli</i> OmpG or FadL. In addition, uptake mediated by the porin-containing strain C43 (DE3) is shown. Substrates are (A) arginine (0.25 µM, 15 min uptake), (B) benzoate (0.5 µM, 10 min), (C) glucuronate (0.5 µM, 10 min), (D) glucose (0.5 µM, 15 min), and (E) pyroglutamate (0.5 µM, 15 min). 100% specific activities correspond to 199.7±5.5 (A), 40.4±0.7 (B), 49.7±1.1 (C), 36.9±1.4 (D), and 20.7±0.5 (E) pmoles substrate/min/mg protein. Filter backgrounds are subtracted from all measurements.</p

Definition of Occ channel specificity.

<p>Uptake of radiolabeled arginine by OccD1, benzoate by OccK1, and glucuronate by OccK2 was measured in the presence of a 10-fold excess of unlabeled low-molecular weight compounds. In all cases, total levels of uptake are reported, expressed as a percentage of uptake in the absence of unlabeled compound (100%). Substrates containing a carboxyl group are italicized.</p>a<p>Reported values are the average of two or three experiments.</p>b<p>Efficient inhibition values, defined as resulting in <50% transport, are shown in bold.</p

Small substrate-specific pores can mediate efficient substrate uptake.

<p>Comparison of pore sizes of OccD1 (PDB ID: 2 ODJ) and OccK1 with those of the non-specific porins OmpG and OmpF. The surface views are shown from the extracellular side.</p

Schematic model of transport mediated by OccD1.

<p>Substrate selection occurs by matching of opposite charges (blue, positive; red, negative) present on the substrate and in the constriction of the channel.</p