EVIDENCE FOR IN-SITU AND IN-VITRO ASSOCIATION BETWEEN BETA-DYSTROGLYCAN AND THE SUBSYNAPTIC 43K RAPSYN PROTEIN - CONSEQUENCE FOR ACETYLCHOLINE-RECEPTOR CLUSTERING AT THE SYNAPSE

The accumulation of dystrophin and associated proteins at the postsyna ptic membrane of the neuromuscular junction and their co-distribution with nicotinic acetylcholine receptor (AChR) clusters in vitro suggest ed a role for the dystrophin complex in synaptogenesis. Co-transfectio n experiments in which alpha- and beta-dystroglycan form a complex wit h AChR and rapsyn, a peripheral protein required for AChR clustering ( Apel, D. A., Roberds, S, L,, Campbell, K, P,, and Merlie, J, P, (1995) Neuron 15, 115-126), suggested that rapsyn functions as a link betwee n AChR and the dystrophin complex. We have investigated the interactio n between rapsyn and beta-dystroglycan in Torpedo AChR-rich membranes using in situ and in vitro approaches. Cross linking experiments were carried out to study the topography of postsynaptic membrane polypepti des. A cross-linked product of 90 kDa was labeled by antibodies to rap syn and beta-dystroglycan; this demonstrates that these polypeptides a re in close proximity to one another. Affinity chromatography experime nts and ligand blot assays using rapsyn solubilized from Torpedo AChR- rich membranes and constructs containing beta-dystroglycan C-terminal fragments show that a rapsyn-binding site is present in the juxtamembr anous region of the cytoplasmic tail of beta-dystroglycan, These data point out that rapsyn and dystroglycan interact in the postsynaptic me mbrane and thus reinforce the notion that dystroglycan could be involv ed in synaptogenesis.