Efflux Pumps Represent Possible Evolutionary Convergence onto the Beta Barrel Fold

SummaryThere are around 100 types of integral outer membrane proteins in each Gram negative bacteria. All of these proteins have the same fold—an up-down β-barrel. It has been suggested that all membrane β-barrels other than lysins are homologous. Here we suggest that β-barrels of efflux pumps have converged on this fold as well. By grouping structurally-solved outer membrane β-barrels (OMBBs) by sequence we find evidence that the membrane environment may have led to convergent evolution of the barrel fold. Specifically, the lack of sequence linkage to other barrels coupled with distinctive structural differences, such as differences in strand tilt and barrel radius, suggest that efflux pumps have evolutionarily converged on the barrel. Finally, we find a possible ancestor for the OMBB efflux pumps as they are related to periplasmic components of the same pumps.

Influence of titanium carbide particles addition on the forging behaviour of powder metallurgy composite steels

This paper evaluates some of the cold-forging features of composite steel preforms of varying titanium carbide contents during cold upsetting under triaxial stress state conditions. A complete experimental investigation is carried out on composite steel preforms of varying titanium carbide contents, namely, 3%TiC and 4%TiC with different lubricating conditions; namely, graphite and zinc stearate and no lubrication. Cylindrical compacts with aspect ratios of 0.42, 0.67, and 1.0 were prepared, sintered, and upset forged at room temperature. The measured barrel radius of curvature is found to have a circular arc because the above relationship is a straight-line one. A relationship is established between the measured barrel radius and the stress ratio parameters of (σθ/σz), (σz/σm), and (σeff/σz) determined under triaxial stress state conditions for both composite steels of varying titanium carbide content. The effect of titanium carbide particle addition in the composite steel, initial preform geometry, and lubricating conditions on the deformation behaviour has been studied, together with the densification route of the composite steel preforms during the deformation.

Some Aspects on Hot Forging Features of P/M Sintered High-Strength Titanium Carbide Composite Steel Preforms Under Different Stress State Conditions

Experiments were carried out to evaluate the hot forging features in the high-strength sintered Powder Metallurgy TiC composite steel performs under different stress states, namely, plane and triaxial stress. Cylindrical compacts with aspect ratios 0.45, 0.75, and 1.25 were prepared, sintered and forged at the temperatures of 1120°C+10°C. The investigation suggests that the experimentally determined relative density has a straight line relationship with the new geometrical shape factor (ρ0∕ρth)(h0∕hf)(3D02∕(2Db2+Dc2)). The measured barrel radius of curvature is found to have a circular arc because the above relation is a straight line one. Relationship is established between the measured barrel radius and the stress ratio parameters namely (σθ∕σz), (σz∕σm) and (σeff∕σz) under plane stress and triaxial stress state conditions. An attempt is also made to establish a relationship between the various stress ratio parameters namely (σθ∕σz), (σz∕σm), and (σeff∕σz) under plane stress and triaxial stress state conditions and the relative density.