The X-ray structure of a sucrose-specific porin (ScrY) from Salmonella typhimurium has been determined by multiple isomorphous replacement at 2.4 A resolution both in its uncomplexed form and with bound sucrose. ScrY is a noncrystallographic trimer of identical subunits, each with 413 structurally well-defined amino acids. A monomer is built up of 18 anti-parallel beta-strands surrounding a hydrophilic pore, with a topology closely similar to that of maltoporin. Two non-overlapping sucrose-binding sites were identified in difference Fourier maps. The higher permeability for sucrose of ScrY as compared to maltoporin is mainly accounted for by differences in their pore-lining residues.

The crystal structure of porin from Rhodobacter capsulatus strain 37b4 has been solved at 3.0 A (1 A = 0. I nm) resolution by multiple isomorphous replacement and solvent-flattening. The three pores of the trimer are well defined in the electron density map. Each pore consists of a Hi-stranded B-barrel which traverses the membrane as a tube. Near its center the tube is narrowed by chain segments protruding from the inner wall of the barrel that form an eye-let with an irregular cross-section of about 6 A by 10 A. The eye-let has an axial length of about 10 A; it defines the exclusion limit for diffusing particles.

The formate transporter FocA was described to switch its mode of operation from a passive export channel at high external pH to a secondary active formate/H(+) importer at low pH. The crystal structure of Salmonella typhimurium FocA at pH 4.0 shows that this switch involves a major rearrangement of the amino termini of individual protomers in the pentameric channel. The amino-terminal helices open or block transport in a concerted, cooperative action that indicates how FocA is gated in a pH-dependent way. Electrophysiological studies show that the protein acts as a specific formate channel at pH 7.0 and that it closes upon a shift of pH to 5.1.

Ammonium transport (Amt) proteins form a ubiquitous family of integral membrane proteins that specifically shuttle ammonium across membranes. In prokaryotes, archaea, and plants, Amts are used as environmental NH 4 + scavengers for uptake and assimilation of nitrogen. In the eukaryotic homologs, the Rhesus proteins, NH 4 + /NH 3 transport is used instead in acid-base and pH homeostasis in kidney or NH 4 + /NH 3 (and eventually CO 2 ) detoxification in erythrocytes. Crystal structures and variant proteins are available, but the inherent challenges associated with the unambiguous identification of substrate and monitoring of transport events severely inhibit further progress in the field. Here we report a reliable in vitro assay that allows us to quantify the electrogenic capacity of Amt proteins. Using solid-supported membrane (SSM)-based electrophysiology, we have investigated the three Amt orthologs from the euryarchaeon Archaeoglobus fulgidus. Af-Amt1 and Af-Amt3 are electrogenic and transport the ammonium and methylammonium cation with high specificity. Transport is pH-dependent, with a steep decline at pH values of ∼5.0. Despite significant sequence homologies, functional differences between the three proteins became apparent. SSM electrophysiology provides a long-sought-after functional assay for the ubiquitous ammonium transporters.ammonium transport proteins | Amt/Rh family | cation transport

The formate/nitrite transporter (FNT) family of integral membrane proteins comprises pentameric channels for monovalent anions that exhibit a broad specificity for small anions such as chloride, the physiological cargo molecules formate, nitrite, and hydrosulfide, and also larger organic acids. Three-dimensional structures are available for the three known subtypes, FocA, NirC, and HSC, which reveal remarkable evolutionary optimizations for the respective physiological context of the channels. FNT channels share a conserved translocation pathway in each protomer, with a central hydrophobic cavity that is separated from both sides of the membrane by a narrow constriction. A single protonable residue, a histidine, plays a key role by transiently protonating the transported anion to allow an uncharged species to pass the hydrophobic barrier. Further selectivity is reached through variations in the electrostatic surface potential of the proteins, priming the formate channel FocA for anion export, whereas NirC and HSC should work bidirectionally. Electrophysiological studies have shown that a broad variety of monovalent anions can be transported, and in the case of FocA, these match exactly the products of mixed-acid fermentation, the predominant metabolic pathway for most enterobacterial species.