Beads coated with two different glycans (red and green) sort themselves out.

The sugar coating on cells allows them to find and stick with their brethren, based on results presented on page 529 by Bucior et al. The results show that, like proteins, carbohydrates alone are able to bind strongly to other specific carbohydrates.

The ability of cells to find others like themselves was first shown in sponges, when a mixture of single cells from two different sponges was seen to separate into the original two sets. This self-awareness relies on proteoglycans—extracellular proteins with long carbohydrate chains.

Since part of a proteoglycan is protein, it is easy to imagine that the protein component imparts specificity by forming a binding pocket that recognizes only self-sugars. But the new results show that proteins are dispensable in this process.

The group finds that a mixture of two sets of beads, each set coated with a purified glycan from a different sponge species, sorts itself into the original two sets. Using atomic force microscopy, the authors show that the binding strength between two molecules of the same glycan was three times that of glycans from different species. Forces between the same glycan were similar to that between an antibody–antigen pair.

Optimal binding required calcium at seawater concentrations. Calcium may bind to hydroxyl groups on the hydrocarbons, leaving the hydrogens on the other side of the glycan chain free to create a hydrophobic binding surface. Carbohydrates may thereby form three-dimensional structures in solution, much as proteins do. NMR studies are needed to see what these structures look like, but the authors imagine that the similarity between two of the same molecules allows for the maximum points of contact down the long chain, thus creating a strong zipper-like hold. ▪