RAPSYN AND AGRIN SLOW THE METABOLIC-DEGRADATION OF THE ACETYLCHOLINE-RECEPTOR

Rapsyn is a 43-kDa cytoplasmic protein that clusters nicotinic acetylc holine receptors (AChR) in the postsynaptic membrane. Here we examine the effect of rapsyn-mediated AChR clustering on the metabolic stabili ty of the AChR. When transfected into QT-6 fibroblasts, cell surface A ChRs (alpha, beta, epsilon, and delta subunit combination) pulse label ed with I-125-alpha-bungarotoxin were degraded with a half-life of 16. 4 +/- 1.1 h (mean +/- SEM). Cotransfection of rapsyn with AChR caused extensive AChR clustering and increased AChR half-life to 20.5 +/- 1.0 h. Anti-AChR antibodies such as mab 35 cause an increased AChR degrad ation often associated with myasthenia gravis: 80.8 +/- 2.5% of AChRs labeled at zero time were degraded over a 12-h period. Cotransfection of rapsyn reduced this AChR loss to 66.4 +/- 3.8%. Rapsyn also reduced normal AChR degradation, from 53.2 +/- 2.1 to 44.2 +/- 2.2%. Muscle c ell lines from wild-type myotubes displayed few AChR clusters, but tre atment with neural agrin increased the number of AChR clusters 30-fold . Clustering was accompanied by reductions in AChR degradation (both i n the presence and absence of mab 35) similar in magnitude to those pr oduced by overexpression of rapsyn in QT-6 cells. In rapsyn-deficient myotubes, treatment with neural agrin neither caused AChR clustering n or reduced AChR degradation. Thus neural agrin may slow AChR degradati on by inducing the rapsyn-dependent clustering of AChRs.