William H. Jennings

Crystal structures of the Toll/Interleukin-1 receptor (TIR) domains from the Brucella protein TcpB and host adaptor TIRAP reveal mechanisms of molecular mimicry.

Background: The Toll/IL-1 receptor (TIR) domains are crucial innate immune signaling modules. Results: The crystal structures of the TIR domains from TcpB and TIRAP reveal similar folds and distinct features. Conclusion: TcpB may mimic the function of TIRAP through their similar TIR domain structures. Significance: These findings suggest mechanisms of bacterial mimicry of host signaling adaptor proteins. The Toll/IL-1 receptor (TIR) domains are crucial innate immune signaling modules. Microbial TIR domain-containing proteins inhibit Toll-like receptor (TLR) signaling through molecular mimicry. The TIR domain-containing protein TcpB from Brucella inhibits TLR signaling through interaction with host adaptor proteins TIRAP/Mal and MyD88. To characterize the microbial mimicry of host proteins, we have determined the X-ray crystal structures of the TIR domains from the Brucella protein TcpB and the host adaptor protein TIRAP. We have further characterized homotypic interactions of TcpB using hydrogen/deuterium exchange mass spectrometry and heterotypic TcpB and TIRAP interaction by co-immunoprecipitation and NF-κB reporter assays. The crystal structure of the TcpB TIR domain reveals the microtubule-binding site encompassing the BB loop as well as a symmetrical dimer mediated by the DD and EE loops. This dimerization interface is validated by peptide mapping through hydrogen/deuterium exchange mass spectrometry. The human TIRAP TIR domain crystal structure reveals a unique N-terminal TIR domain fold containing a disulfide bond formed by Cys89 and Cys134. A comparison between the TcpB and TIRAP crystal structures reveals substantial conformational differences in the region that encompasses the BB loop. These findings underscore the similarities and differences in the molecular features found in the microbial and host TIR domains, which suggests mechanisms of bacterial mimicry of host signaling adaptor proteins, such as TIRAP.

Chlorophyll orientation in crystals of bacteriochlorophyll-protein from green photosynthetic bacteria☆

Abstract Optical properties of single crystals of a bacteriochlorophyll-protein complex from Chloropseudomonas ethylicum were investigated. The absorption spectrum of these crystals is essentially the same as that of the complex in solution. The crystals are weakly dichroic at both the 603-nm ( D = 1 1.21 ) and the 809-nm (D = 1.30) absorbances. This dichroism is confirmed by selective dispersion of birefringence: Negative at 603 nm and positive at 809 nm with respect to the optic axis. The sites of bacteriochlorophyll binding are therefore oriented with respect to the crystal axes. A radial arrangement of the porphyrin sites within each macromolecule is proposed and evaluated.

Paracrystalline aggregates of bacteriochlorophyll protein from green photosynthetic bacteria

Abstract Paracrystalline aggregates of bacteriochlorophyll-protein are observed in preparations of the mother liquor from which crystals have been obtained. These aggregates are confocal in texture but are rigid and fragile as opposed to the classic examples of confocal smectic “liquid crystals.” Observations of birefringence and dichroism indicate that bacteriochlorophyll is molecularly oriented in a filamentous fine texture that constitutes each ordered domain of the aggregates. Electron microscopy of these aggregates shows that this texture is composed of tubules. These tubules are more or less parallel to each other within the small confines of each domain but their lateral spacing is variable and random. Cross sections of the tubules show a hexagonal array of electron-dense elements surrounding an electron-transparent channel. This array seems congruous but not identical to hexagonal units in the lattice observed in cross sections of bacteriochlorophyll-protein crystals. The macromolecular arrangement is considered in terms of the lateral bonding sites in bacteriochlorophyll-protein paracrystals.

A MOLECULAR MODEL FOR A PHOSPHOLIPID-RETINALDEHYDE COMPLEX ABSORBING AT 500 nm

Abstract— –The structure of a complex formed from retinaldehyde and ethanolamine phosphoglyceride has been proposed as a Schiffs base stabilized byπ–πinteraction between the double bond of an oleic acid group (Δ9) and the 9–10 double bond of the retinaldehyde chain. Considerations of stereochemistry and simple MO theory indicate the proposed structure to be energetically favored, and its absorption maximum is predicted by free electron theory.

Some aspects of Schiff bases of retinaldehyde.

Abstract We have previously described an entity, which we have named Complex II, formed from retinaldehyde (RET) (any isomer) and ethanolamine-phosphoglyceride (EPG) which contains unsaturated fatty acid components. Complex II exhibits a 500-nm absorbance maximum in a spectrum similar to that of mammalian rhodopsin. We has thought perhaps it was the true chromophore of the visual pigment but recent evidence suggests it is not. The other generally accepted binding site for retinaldehyde is the amino acid lysine. Since EPG-RET affords a mechanism for forming a substance resembling rhodopsin as we have shown, we thought Complex II might be a suitable model for the lysine-retinal combination that should exist. We have investigated di- and tri-peptides containing 1-lysine and find that only the tri-peptide alpha-t-Boc-1-lysyl-1-alanyl-1-phenylalanine ethyl ester in combination with retinaldehyde in chloroform/methanol 2 1 shows any change in the predicted direction. Complex II has been shown to consist of a Schiff base, internally protonated, and a pi-pi interaction between the RET chain and an unsaturated fatty acid in the EPG. The peptide forms the appropriate unprotonated Schiff base with RET, but protonation does not cause the appearance of the 500-nm band. Apparently the pi-pi interaction is missing. However, in the unprotonated state there is some enhancement of the spectral extinction at 500 nm. We are led to speculate that this may be due to a charge transfer interaction similar to that proposed by Mendelsohn for the Halobacterium halobium pigment.