Insights into the autotransport process of a trimeric autotransporter,Yersinia Adhesin A (YadA)

Trimeric autotransporter adhesins (TAAs) are a subset of a larger protein family called the type V secretion systems. They are localized on the cell surface of Gram‐negative bacteria, function as mediators of attachment to inorganic surfaces and host cells, and thus include important virulence factors. Yersinia adhesin A (YadA) from Yersinia enterocolitica is a prototypical TAA that is used extensively to study the structure and function of the type Vc secretion system. A solid‐state NMR study of the membrane anchor domain of YadA previously revealed a flexible stretch of small residues, termed the ASSA region, that links the membrane anchor to the stalk domain. In this study, we present evidence that single amino acid proline substitutions produce two different conformers of the membrane anchor domain of YadA; one with the N‐termini facing the extracellular surface, and a second with the N‐termini located in the periplasm. We propose that TAAs adopt a hairpin intermediate during secretion, as has been shown before for other subtypes of the type V secretion system. As the YadA transition state intermediate can be isolated from the outer membrane, future structural studies should be possible to further unravel details of the autotransport process.     

Inward-facing glycine residues create sharp turns in β-barrel membrane proteins

The transmembrane region of outer-membrane proteins (OMPs) of Gram-negative bacteria are almost exclusively β-barrels composed of between 8 and 26 β-strands. To explore the relationship between β-barrel size and shape, we modeled and simulated engineered variants of the Escherichia coli protein OmpX with 8, 10, 12, 14, and 16 β-strands. We found that while smaller barrels maintained a roughly circular shape, the 16-stranded variant developed a flattened cross section. This flat cross section impeded its ability to conduct ions, in agreement with previous experimental observations. Flattening was determined to arise from the presence of inward-facing glycines at sharp turns in the β-barrel. An analysis of all simulations revealed that glycines, on average, make significantly smaller angles with residues on neighboring strands than all other amino acids, including alanine, and create sharp turns in β-barrel cross sections. This observation was generalized to 119 unique structurally resolved OMPs. We also found that the fraction of glycines in β-barrels decreases as the strand number increases, suggesting an evolutionary role for the addition or removal of glycine in OMP sequences.     

Paranodal Schwann cell mitochondria in spinal roots of the cat. An ultrastructural morphometric analysis

We have calculated the number of paranodal Schwann cell mitochondria in adult feline ventral and dorsal lumbar spinal roots using ultrastructural serial section analysis. Distinct accumulations of paranodal mitochondria were noted in nerve fibres more than 4-5 mm in diameter. The calculated number of paranodal mitochondria increased linearly with fibre diameter from a few hundred up to 20 000-30 000 per node. A linear increase in the number of paranodal mitochondria per node also appeared as a function of nodal variables such as ‘nodal axon membrane area’, ‘nodal Schwann cell membrane area’, and ‘node gap extracellular volume’. In large fibres (D=15-18 mm), a calculated number of about 20 000 paranodal Schwann cell mitochondria were accumulated at each node of Ranvier and related to nodal axon membrane area of about 20 mm2. Our calculations indicate that, on the average, 1000 paranodal Schwann cell mitochondria with a total volume of 6.7 mm3, a total outer membrane area of 250 mm2 and a total inner membrane area of 580 mm2 projected to each mm2 of the nodal axon membrane via the nodal Schwann cell brush border.