N-CAM, 43K-rapsyn, and S-laminin mRNAs are concentrated at synaptic sites in muscle fibers.

Several components of the postsynaptic apparatus are found highly concentrated at the motor endplate. Studies of the acetylcholine receptor have shown that selective transcription of its genes by synaptic nuclei contributes to its synaptic accumulation. We used the method of in situ hybridization to study the distribution of mRNAs encoding three other proteins localized to the motor endplate. We found preferential synaptic accumulation of mRNAs for a membrane-associated cell adhesion molecule (N-CAM) and for an acetylcholine receptor-associated cytoskeletal 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 extrasynaptically but more abundant synaptically. Our data suggest that selective transcription by synaptic nuclei is a general mechanism that contributes to the concentration of specific proteins in the postsynaptic apparatus at the neuromuscular junction.