N-CAM, 43K-RAPSYN, AND S-LAMININ MESSENGER-RNAS ARE CONCENTRATED AT SYNAPTIC SITES IN MUSCLE-FIBERS

Several components of the postsynaptic apparatus are found highly conc entrated at the motor endplate. Studies of the acetylcholine receptor have shown that selective transcription of its genes by synaptic nucle i contributes to its synaptic accumulation. We used the method of in s itu hybridization to study the distribution of mRNAs encoding three ot her proteins localized to the motor endplate, We found preferential sy naptic accumulation of mRNAs for a membrane-associated cell adhesion m olecule (N-CAM) and for an acetylcholine receptor-associated cytoskele tal protein (43K-rapsyn). In contrast, RNAs encoding proteins present throughout the muscle were distributed all along the muscle fiber. RNA encoding a protein concentrated in synaptic basal lamina, s-laminin ( laminin beta 2), was intermediate in distribution, detectable extrasyn aptically but more abundant synaptically, Our data suggest that select ive transcription by synaptic nuclei is a general mechanism that contr ibutes to the concentration of specific proteins in the postsynaptic a pparatus at the neuromuscular junction. (C) 1995 Academic Press, Inc.