ELECTRON-MICROSCOPIC STRUCTURE OF AGRIN AND MAPPING OF ITS BINDING-SITE IN LAMININ-1

Agrin is a large, multidomain heparan sulfate proteoglycan that is ass ociated with basement membranes of several tissues, Particular splice variants of agrin are essential for the formation of synaptic structur es at the neuromuscular junction. The binding of agrin to laminin appe ars to be required for its localization to synaptic basal lamina and o ther basement membranes. Here, electron microscopy was used to determi ne the structure of agrin and to localize its binding site in laminin- l, Agrin appears as an similar to 95 mn long particle that consists of a globular, N-terminal laminin-binding domain, a central rod predomin antly formed by the follistatin-like domains and three globular, C-ter minal laminin G-like domains, In a few cases, heparan sulfate glycosam inoglycan chains were seen emerging from the central portion of the co re protein, Moreover, we show that agrin binds to the central region o f the three-stranded, coiled-coil oligomerization domain in the long a rm of laminin-1, which mediates subunit assembly of the native laminin molecule, In summary, our data show for the first time a protein-prot ein interaction of the extracellular matrix that involves a coiled-coi l domain, and they assign a novel role to this domain of laminin-1, Ba sed on this, we propose that agrin associates with basal lamina in a p olarized way.