Agrin, a molecule produced by motoneurons that induces the aggregation of nicotinic acetylcholine receptors (nAChRs), has recently been structurally characterized. Agrin-related proteins (ARPs) that arise from differential splicing are synthesized by neurons and muscle. The C-terminal region of agrin that instructs muscle to aggregate nAChRs contains three laminin A modules separated by epidermal growth factor-like modules. Alternative splicing in the laminin A modules leads to the formation of at least three ARPs that are devoid of nAChR-aggregating activity. In their N-terminal regions, both agrin and ARPs contain nine follistatin-related modules that, like those in follistatin and in another related protein, osteonectin, may have the capability to bind members of the transforming growth factor β (TGF-β) or platelet-derived growth factor (PDGF) families. This review proposes that these follistatin-like regions of agrin and ARPs might bind and localize growth factors, and thus provide a matrix-bound concentration of them. Beyond agrin's role in inducing AChR aggregation, the function of agrin and ARPs to provide a localized reservoir of growth factors could contribute to the formation and maintenance of the long-lasting synaptic architecture by specifying and limiting the area of influence of these molecules.
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