Porins form aqueous channels that aid the diffusion of small hydrophilic molecules across the outer membrane of Gram-negative bacteria. The crystal structures of matrix porin and phosphoporin both reveal trimers of identical subunits, each subunit consisting of a 16-stranded anti-parallel beta-barrel containing a pore. A long loop inside the barrel contributes to a constriction of the channel where the charge distribution affects ion selectivity. The structures explain at the molecular level functional characteristics and their alterations by known mutations.

Pole development is coordinated with the Caulobacter crescentus cell cycle by two-component signaling proteins. We show that an unusual response regulator, PleD, is required for polar differentiation and is sequestered to the cell pole only when it is activated by phosphorylation. Dynamic localization of PleD to the cell pole provides a mechanism to temporally and spatially control the signaling output of PleD during development. Targeting of PleD to the cell pole is coupled to the activation of a C-terminal guanylate cyclase domain, which catalyzes the synthesis of cyclic di-guanosine monophosphate. We propose that the local action of this novel-type guanylate cyclase might constitute a general regulatory principle in bacterial growth and development. During developmental transitions, localized changes of cellular morphology are mediated by adaptation in levels and arrangement of proteins. Temporal and spatial control often relies on the timed synthesis or activation of transcriptional regulators and on the establishment of gradients through the compartmentalization of signaling complexes. Although the regulatory mechanisms of gene expression are relatively well understood, it is often not clear how morphogenetic changes are controlled and coordinated locally. In prokaryotes, the major paradigm for signal transduction is the two-component regulatory system (Parkinson and Kofoid 1992). On signal input, the first component, a sensor kinase, autophosphorylates on a histidine residue. The second component, a soluble response regulator, often functions as a transcriptional regulator. Its phosphorylation by the cognate histidine kinase on a conserved aspartate residue in the N-terminal receiver domain usually results in increased DNA binding affinity (Parkinson and Kofoid 1992). Here we present evidence that a novel-type response regulator acts at a distinct subcellular site where it contributes to local changes in cell morphology through the production of a novel signaling molecule.The unicellular bacterium Caulobacter crescentus goes through an obligate developmental transition that allows it to switch between a sessile, adhesive, and a motile, planktonic cell during its cell cycle. As a consequence, cell poles are continuously remodeled during cell differentiation to facilitate assembly and removal of motility and surface adherence organelles at the right time and in the correct order. Asymmetry is established in the predivisional cell with a single flagellum, a chemotaxis machinery, and pili being assembled at one pole, whereas the opposite pole consists of a stalk and an adhesive organelle, the holdfast (Fig. 1). As a result, division generates two cell types with distinct properties: a surface-attached stalked cell and a motile swarmer cell. The swarmer progeny first differentiates into a stalked cell before it initiates DNA replication and cell division. During this transition the pili retract, flagella are released, and the adhesive organelles are synthesized at the same pole. Here we investigate the function...

Trimeric maltoporin (LamB protein) facilitates the diffusion of maltodextrins across the outer membrane of Gram-negative bacteria. The crystal structure of maltoporin from Escherichia coli, determined to a resolution of 3.1 angstroms, reveals an 18-stranded, antiparallel beta-barrel that forms the framework of the channel. Three inwardly folded loops contribute to a constriction about halfway through the channel. Six contingent aromatic residues line the channel and form a path from the vestibule to the periplasmic outlet. Soaking of a crystal with maltotriose revealed binding of the sugar to this hydrophobic track across the constriction, which suggests that maltose and linear oligosaccharides may be translocated across the membrane by guided diffusion along this path.

Recent discoveries suggest that a novel second messenger, bis-(335)-cyclic di-GMP (c-diGMP), is extensively used by bacteria to control multicellular behavior. Condensation of two GTP to the dinucleotide is catalyzed by the widely distributed diguanylate cyclase (DGC or GGDEF) domain that occurs in various combinations with sensory and͞or regulatory modules. The crystal structure of the unorthodox response regulator PleD from Caulobacter crescentus, which consists of two CheY-like receiver domains and a DGC domain, has been solved in complex with the product cdiGMP. PleD forms a dimer with the CheY-like domains (the stem) mediating weak monomer-monomer interactions. The fold of the DGC domain is similar to adenylate cyclase, but the nucleotidebinding mode is substantially different. The guanine base is Hbonded to Asn-335 and Asp-344, whereas the ribosyl and ␣-phosphate moieties extend over the ␤2-␤3-hairpin that carries the GGEEF signature motif. In the crystal, c-diGMP molecules are crosslinking active sites of adjacent dimers. It is inferred that, in solution, the two DGC domains of a dimer align in a two-fold symmetric way to catalyze c-diGMP synthesis. Two mutually intercalated c-diGMP molecules are found tightly bound at the stem-DGC interface. This allosteric site explains the observed noncompetitive product inhibition. We propose that product inhibition is due to domain immobilization and sets an upper limit for the concentration of this second messenger in the cell.allosteric regulation ͉ signal transduction ͉ x-ray crystallography

Bis-(3'-5')-cyclic dimeric GMP (c-di-GMP) is a ubiquitous second messenger that regulates cell surface-associated traits in bacteria. Components of this regulatory network include GGDEF and EAL domain-containing proteins that determine the cellular concentrations of c-di-GMP by mediating its synthesis and degradation, respectively. Crystal structure analyses in combination with functional studies have revealed the catalytic mechanisms and regulatory principles involved. Downstream, c-di-GMP is recognized by PilZ domain-containing receptors that can undergo large-scale domain rearrangements on ligand binding. Here, we review recent data on the structure and functional properties of the protein families that are involved in c-di-GMP signalling and discuss the mechanistic implications.