Neuromuscular junction (NMJ) assembly is characterized by the clustering and neuronal alignment of acetylcholine receptors (AChRs). In this study we have addressed post-synaptic contributions to assembly that may arise from the NMJ basement membrane with cultured myotubes. We show that the cell surface-binding LG domains of non-neural (muscle) agrin and perlecan promote AChR clustering in the presence of laminin-2. This type of AChR clustering occurs with a several hour lag, requires

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... ific kinase (MuSK), and is accompanied by tyrosine phosphorylation of MuSK and ␤AChR. It also requires conjugation of the agrin or perlecan to laminin together with laminin polymerization. Furthermore, AChR clustering can be mimicked with antibody binding to non-neural agrin, supporting a mechanism of ligand aggregation. Neural agrin, in addition to its unique ability to cluster AChRs through its B/z sequence insert, also exhibits laminin-dependent AChR clustering, the latter enhancing and stabilizing its activity. Finally, we show that type IV collagen, which lacks clustering activity on its own, stabilizes laminin-dependent AChR clusters. These findings provide evidence for cooperative and partially redundant MuSK-dependent functions of basement membrane in AChR assembly that can enhance neural agrin activity yet operate in its absence. Such interactions may contribute to the assembly of aneural AChR clusters that precede neural agrin release as well as affect later NMJ development. . 2 The abbreviations used are: LG domain, laminin G-like domain; NMJ, neuromuscular junction; AChR, acetylcholine receptor; NA, minigene version of neural agrin; nNA, minigene version of non-neural (muscle) agrin; NP, NtA-perlecan LG domain fusion protein; MuSK, muscle-specific kinase; BTX, bungarotoxin; PBS, phosphate-buffered saline; NtA, N-terminal laminin-binding domain.