The structure of porin from Rhodobacter capsulatus at 0.6 nm resolution

Abstract

The crystal electron density map of porin from Rhodobacter capsulufus 37b4 at 0.6 nm resolution shows that the trimeric molecule consists of 3 merged cylinders as the central part, plus 3 laterally radiating domains. The density shows no prominent a-helices and is consistent with p-pleated sheet structure. The trimer density was dissected into monomers. Three separate pores per trimer with sizes that agree with the exclusion limit of permeating molecules could be identified. The cross-section of the central part as well as the pore distance agree with prior electron microscopy data on other porins.Porin; Membrane protein structure; X-ray structure; (Rhodobacter capsulutus)

The structure of porin from Rhodobacter capsulatus at 1.8 Å resolution

“…All o.helices turned out as right-handed, confirming the chirality derived earlier from the/J-strand twists [18]. The weak density region of previous maps [18,20], which had puzzled us The sequence contains 70 residues with ionogenic side chains. 51 of which are Asp and Glu while there are only 19 His, Lys and Arc.…”

“…An established procedure for the isolation of porin from Rhodebatter capsulatus st ram 37b4 had given rise to crystals diffracting to a medium resolution of about 2,8 A [3,16], the structure of which had Correspondettce address: G,E, Schulz, Institut ffir Organisclle Chemic und Biochemie, Albertstr, 21, D.7800 Freiburg i,Br,, Germany been solved at low [20] and then at medium resolution [Itq. This crystal form is now c~dled 'form-A', By modifylnil the purification procedure [21 ], in particular by dis pensing with EDTA anti replactnl~ SDS by N,N.dimelhyl.dodecylaminoxide (LDAO), we could 8row the new crystal 'form.B' that diffracts to 1,8 ~ resolution [19].…”

Molecular Basis of Bacterial Outer Membrane Permeability Revisited

“…Undoubtedly the most important progress in the study of porins was the elucidation of the three-dimensional structures of trimeric porins by electron diffraction (297)(298)(299)(300)705) and X-ray crystallography. The latter approach had its first success with a Rhodobacter capsulatus trimeric porin (714)(715)(716)(717)(718), an achievement that was quickly followed by the elucidation of the structure of the E. coli OmpF and PhoE porins (152). Important conclusions from these studies include the following (there are recent minireviews on the structure of porins [343,598,599]).…”

“…The ␣-proteobacteria have been divided, by the use of 16S rRNA signature sequences, into three groups, ␣-1, ␣-2, and ␣-3 (738). The porin for which the first three-dimensional structure was determined by crystallography (714,717,718) was from Rhodobacter capsulatus, a member of the ␣-3 group. The monomer produces a 16-strand ␤-barrel, which is assembled into a now classical trimer structure (see "Crystallographic structure of porins" above).…”

Properties of the FhuA channel in the Escherichia coli outer membrane after deletion of FhuA portions within and outside the predicted gating loop

“…Substrates cross the outer membrane of the gram-negative bacterium Escherichia coli by (i) diffusion through porins (2,13,36), (ii) facilitated diffusion, as characterized by maltodextrins through the LamB protein (6,7) or nucleosides through the Tsx protein (5,17,26), and (iii) high-affinity active transport mechanisms, such as the uptake of ferric siderophores and vitamin B 12 . Transport of ferric siderophores and vitamin B 12 is mediated by ligand-specific receptor proteins and requires the electrochemical potential of the cytoplasmic membrane (9) and a protein complex consisting of the TonB, ExbB, and ExbD proteins (Ton system) (14,15,33).…”