Molecular adaptation of a leaf-eating bird: stomach lysozyme of the hoatzin

This report describes a lysozyme expressed at high levels in the stomach of the hoatzin, the only known foregut-fermenting bird. Evolutionary comparison places it among the calcium-binding lysozymes rather than among the conventional types. Conventional lysozymes were recruited as digestive enzymes twice in the evolution of mammalian foregut fermenters, and these independently recruited lysozymes share convergent structural changes attributed to selective pressures in the stomach. Biochemical convergence and parallel amino acid replacements are observed in the hoatzin stomach lysozyme even though it has a different genetic origin from the mammalian examples and has undergone more than 300 million years of independent evolution.      

Spectrin domains lose cooperativity in forced unfolding

Spectrin is a multidomain cytoskeletal protein, the component three-helix bundle domains are expected to experience mechanical force in vivo. In thermodynamic and kinetic studies, neighboring domains of chicken brain alpha-spectrin R16 and R17 have been shown to behave cooperatively. Is this cooperativity maintained under force? The effect of force on these spectrin domains was investigated using atomic force microscopy. The response of the individual domains to force was compared to that of the tandem repeat R1617. Importantly, nonhelical linkers (all-beta immunoglobulin domains) were used to avoid formation of nonnative helical linkers. We show that, in contrast to previous studies on spectrin repeats, only 3% of R1617 unfolding events gave an increase in contour length consistent with cooperative two-domain unfolding events. Furthermore, the unfolding forces for R1617 were the same as those for the unfolding of R16 or R17 alone. This is a strong indication that the cooperative unfolding behavior observed in the stopped-flow studies is absent between these spectrin domains when force is acting as a denaturant. Our evidence suggests that the rare double unfolding events result from misfolding between adjacent repeats. We suggest that this switch from cooperative to independent behavior allows multidomain proteins to maintain integrity under applied force.     

Structural origins of fibrin clot rheology

The origins of clot rheological behavior associated with network morphology and factor XIIIa-induced cross-linking were studied in fibrin clots. Network morphology was manipulated by varying the concentrations of fibrinogen, thrombin, and calcium ion, and cross-linking was controlled by a synthetic, active-center inhibitor of FXIIIa. Quantitative measurements of network features (fiber lengths, fiber diameters, and fiber and branching densities) were made by analyzing computerized three-dimensional models constructed from stereo pairs of scanning electron micrographs. Large fiber diameters and lengths were established only when branching was minimal, and increases in fiber length were generally associated with increases in fiber diameter. Junctions at which three fibers joined were the dominant branchpoint type. Viscoelastic properties of the clots were measured with a rheometer and were correlated with structural features of the networks. At constant fibrinogen but varying thrombin and calcium concentrations, maximal rigidities were established in samples (both cross-linked and noncross-linked) which displayed a balance between large fiber sizes and great branching. Clot rigidity was also enhanced by increasing fiber and branchpoint densities at greater fibrinogen concentrations. Network morphology is only minimally altered by the FXIIIa-catalyzed cross-linking reaction, which seems to augment clot rigidity most likely by the stiffening of existing fibers.     

A microcell for the temperature-jump technique

Pregnancy-specific β₁²-glycoprotein was purified to 99% purity by sequential chromatography on carboxymethyl-cellulose (to remove other pregnancy-specific β₁-glycoprotein variants), Cibacron blue 3G-A-Matrex Gel, Procion turquoise MX-G-Matrex Gel, Procion red H-8BN-Matrex Gel, and rabbit anti-normal human male serum-Matrex Gel. The purification was rapid, large-scale, and gave a yield of pregnancy-specific β₁-glycoprotein of 35.1%.     

Specific nuclear envelope transmembrane proteins can promote the location of chromosomes to and from the nuclear periphery

Background

Different cell types have distinctive patterns of chromosome positioning in the nucleus. Although ectopic affinity-tethering of specific loci can be used to relocate chromosomes to the nuclear periphery, endogenous nuclear envelope proteins that control such a mechanism in mammalian cells have yet to be widely identified.

Results

To search for such proteins, 23 nuclear envelope transmembrane proteins were screened for their ability to promote peripheral localization of human chromosomes in HT1080 fibroblasts. Five of these proteins had strong effects on chromosome 5, but individual proteins affected different subsets of chromosomes. The repositioning effects were reversible and the proteins with effects all exhibited highly tissue-restricted patterns of expression. Depletion of two nuclear envelope transmembrane proteins that were preferentially expressed in liver each reduced the normal peripheral positioning of chromosome 5 in liver cells.

Conclusions

The discovery of nuclear envelope transmembrane proteins that can modulate chromosome position and have restricted patterns of expression may enable dissection of the functional relevance of tissue-specific patterns of radial chromosome positioning.     

Overview of microbial biofilms

As the success of this two-issue special section of the Journal of Industrial Microbiology attests, the study of microbial biofilms is truly burgeoning as the uniqueness and the importance of this mode of growth is increasingly recognized. Because of its universality the biofilm concept impacts virtually all of the subdivisions of Microbiology (including Medical, Dental, Agricultural, Industrial and Environmental) and these two issues incorporate contributions from authors in all of these disciplines. Some time ago we reasoned that bacteria cannot possibly be aware (sic) of their precise location, in terms of this spectrum of anthrocentric subspecialties, and that their behavior must be dictated by a standard set of phenotypic responses to environmental conditions in what must seem to them (sic) to be a continuum of very similar aquatic ecosystems. In this overview I will, therefore, stress the common features of microbial biofilms that we should bear in mind as we use this simple universal concept to seek to understand bacterial behavior in literally hundreds of aquatic ecosystems traditionally studied by dozens of subspecies of microbiologists reared in sharply different scientific and academic conventions.