Sticky sugar

Fingers lay fibers n page 553, Canty et al. show how cells use finger-like projections to lay down parallel arrays of collagen fibers that are many times the lengths of the cells. Parallel arrays of long collagen fibers give tendons their resistance and tensile properties. Thousands of collagen molecules self-assemble into fibers, but the fibers need instruction from their environment for alignment—fibers in solution are randomly arranged. The work by Canty et al. shows how long, thin plasma–membrane extensions, called fibripositors, set up this parallel pattern in prenatal mouse tendons. Microscopy studies showed that these fibripositors received mature, fully processed collagen fibers from Golgi-to-plasma membrane carriers (GPCs). The GPCs eventually fused with the tip of the fibripositor, which then retracted, leaving behind its fiber contents. More fibers were added to others at the base of the fibripositor, deep within the cell, near the trans-Golgi network. Collagen fiber assembly was seen two decades ago in microscopy studies, but at the time, the fibripositors appeared to be extracellular invaginations rather than an extension of the cell. The GPCs and fibripositors both align with the long axis O Sticky sugar he 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—extra-cellular 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 T Beads coated with two different glycans (red and green) sort …