Agrin (AGRN) is a multidomain heparan sulfate proteoglycan found in basement membranes, named for its ability to promote aggregation of AChR clusters on the muscle surface directly beneath the nerve terminal (Nitkin et al. 1987). It is a critical organizer of postsynaptic differentiation at the skeletal neuromuscular junction; synaptogenesis is profoundly disrupted in its absence (Gautam et al. 1996, Daniels 2012). Two alternate N-termini exist with differential expression, tissue localization and function. The secreted and predominant longer LN form (Burgess et al. 2000) starts with a secretion signal sequence and a laminin-binding domain (Denzer et al. 1995, Kammerer et al. 1999); the shorter SN form associates with the plasma membrane (Burgess et al. 2000, Neumann et al. 2001). Following the SN or LN regions are 8 follistatin repeats, known to bind growth factors and inhibit proteases in other proteins. The central region has two repeats homologous to domain III of laminin. The C-terminal portion, which is responsible for the molecule's known signaling functions, contains four EGF repeats and three LG (G) domains homologous to those found in laminin alpha chains, neurexins and slits (Timpl et al. 2000).
The LG domains of AGRN bind alpha-dystroglycan (Yamada et al. 1996, Gee et al. 1994, Bowen et al. 1996, Campanelli et al. 1996, Gesemann et al. 1996, Hopf & Hoch 1996).