Chloride helps collagen build its network

Basement membranes are extracellular scaffolds that organize and strengthen many different tissues in the body. How the various components of basement membranes are assembled into an organized network that can regulate the behavior of overlying cells remains unclear (1), but Cummings et al. now describe how extracellular chloride ions induce the higher order assembly of collagen IV molecules outside the cell (2). Collagen IV is the main component of basement membranes, where it forms a scaffold that binds and organizes numerous other extracellular matrix proteins and growth factors. Collagen IV assembly starts inside cells with the lateral association of three α-chains to form a triple heli-cal collagen " protomer. " After their secretion from the cell, protomers assemble with each other to form a larger network. " But what triggers this? " wonders Billy Hudson, from Vanderbilt University School of Medicine. " Why don't collagen protomers form a network inside the cell? " Collagen IV's C-terminal NC1 domain plays a critical role in the assembly process , as it is involved in both the lateral association of α-chains to form protomers inside the cell, and the end-on association of protomers in the extracellular space. In all, six NC1 domains come together at the interface between two protomer C-termini. Hudson and colleagues , led by Christopher Cummings, Vadim Ped-chenko, and Kyle Brown, noticed that the crystal structure of NC1 hexamers contains chloride at the protomer interface, so they wondered whether this anion, which exists at a much higher concentration outside the cell than inside the cell, might regulate collagen IV assembly. Cummings et al. purifi ed NC1 hexamers from both cultured cells and bovine lens basement membrane. The hexamers dissociated into monomers when dialyzed into chloride-free buffer, and reassembled when chloride levels were restored to physiological concentrations. In isolation, however, NC1 domains don't form a trimeric intermediate , so, to investigate which stage of hexamer assembly relies on chloride, the researchers expressed and purifi ed recom-binant α-chains containing the NC1 domain and a stretch of helical repeats suffi cient to mediate trimerization and protomer formation. " And we could show that chloride isn't required for trimerization, but it is required to put two protomers together, " Hudson explains. Thus, collagen IV α-chains can form protomers inside the cell, but their assembly into larger networks is only triggered when they encounter high concentrations of chloride outside the cell. To understand how this …