A global benchmark study using affinity-based biosensors.

To explore the variability in biosensor studies, 150 participants from 20 countries were given the same protein samples and asked to determine kinetic rate constants for the interaction. We chose a protein system that was amenable to analysis using different biosensor platforms as well as by users of different expertise levels. The two proteins (a 50-kDa Fab and a 60-kDa glutathione S-transferase [GST] antigen) form a relatively high-affinity complex, so participants needed to optimize several experimental parameters, including ligand immobilization and regeneration conditions as well as analyte concentrations and injection/dissociation times. Although most participants collected binding responses that could be fit to yield kinetic parameters, the quality of a few data sets could have been improved by optimizing the assay design. Once these outliers were removed, the average reported affinity across the remaining panel of participants was 620 pM with a standard deviation of 980 pM. These results demonstrate that when this biosensor assay was designed and executed appropriately, the reported rate constants were consistent, and independent of which protein was immobilized and which biosensor was used.

Two-step adhesive binding by classical cadherins.

Crystal structures of classical cadherins have revealed two dimeric configurations. In the first, N-terminal beta-strands of EC1 domains 'swap' between partner molecules. The second configuration (the 'X dimer'), also observed for T-cadherin, is mediated by residues near the EC1-EC2 calcium binding sites, and N-terminal beta-strands of partner EC1 domains, though held adjacent, do not swap. Here we show that strand-swapping mutants of type I and II classical cadherins form X dimers. Mutant cadherins impaired for X-dimer formation show no binding in short-time frame surface plasmon resonance assays, but in long-time frame experiments, they have homophilic binding affinities close to that of wild type. Further experiments show that exchange between monomers and dimers is slowed in these mutants. These results reconcile apparently disparate results from prior structural studies and suggest that X dimers are binding intermediates that facilitate the formation of strand-swapped dimers.

T-cadherin structures reveal a novel adhesive binding mechanism.

Vertebrate genomes encode 19 classical cadherins and about 100 nonclassical cadherins. Adhesion by classical cadherins depends on binding interactions in their N-terminal EC1 domains, which swap N-terminal beta-strands between partner molecules from apposing cells. However, strand-swapping sequence signatures are absent from nonclassical cadherins, raising the question of how these proteins function in adhesion. Here, we show that T-cadherin, a glycosylphosphatidylinositol (GPI)-anchored cadherin, forms dimers through an alternative nonswapped interface near the EC1-EC2 calcium-binding sites. Mutations within this interface ablate the adhesive capacity of T-cadherin. These nonadhesive T-cadherin mutants also lose the ability to regulate neurite outgrowth from T-cadherin-expressing neurons. Our findings reveal the likely molecular architecture of the T-cadherin homophilic interface and its requirement for axon outgrowth regulation. The adhesive binding mode used by T-cadherin may also be used by other nonclassical cadherins.

Continuous-flow microfluidic printing of proteins for array-based applications including surface plasmon resonance imaging.

Arraying proteins is often more challenging than creating oligonucleotide arrays. Protein concentration and purity can severely limit the capacity of spots created by traditional pin and ink jet printing techniques. To improve protein printing methods, we have developed a three-dimensional microfluidic system to deposit protein samples within discrete spots (250-microm squares) on a target surface. Our current technology produces a 48-spot array within a 0.5 x 1 cm target area. A chief advantage of this method is that samples may be introduced in continuous flow, which makes it possible to expose each spot to a larger volume of sample than would be possible with standard printing methods. Using Biacore Flexchip (Biacore AB) surface plasmon resonance array-based biosensor as a chip reader, we demonstrate that the microfluidic printer is capable of spotting proteins that are dilute (<0.1 microg/ml) and contain high concentrations of contaminating protein (>10,000-fold molar excess). We also show that the spots created by the microfluidic printer are more uniform and have better-defined borders than what can be achieved with pin printing. The ability to readily print proteins using continuous flow will help expand the application of protein arrays.