A peripheral
membrane protein of approximately 43 kDa (
rapsyn) clusters muscle
nicotinic acetylcholine receptors (AChRs), but molecules relevant to clustering neuronal AChRs have not been identified. Here, we have detected
rapsyn transcripts in the chick nervous system, localized
rapsyn mRNA in ciliary ganglion (CG) neurons, which are known to cluster AChRs, and identified three
rapsyn cDNAs derived from the
ganglion. Our initial Northern blots, performed using a mouse probe, revealed
rapsyn-like transcripts in chick muscle and brain. To develop species-specific probes, we prepared a chick
rapsyn cDNA construct, Ch43K.1, that encodes a
protein having extensive homology to mouse
rapsyn. Using primers designed to anneal near the 5' and 3' boundaries of Ch43K.1, three prominent cDNAs were amplified from chick muscle templates by
reverse transcriptase based-PCR. Products of similar size were also amplified using
cDNA prepared from neuronal tissues expected to contain clustered AChRs (CG and brain), whereas none were detected using templates from tissues not displaying clustered AChRs (sensory ganglia and liver). In situ hybridization confirmed that
rapsyn mRNA is expressed both in chick muscle fibers and in CG neurons. Sequencing the three cDNAs amplified from CG templates revealed the largest to be Ch43K.1, whereas the smaller two may represent splice variants. These findings suggest that multiple
rapsyn-like molecules are involved in clustering the distinct AChRs expressed by muscle fibers and neurons.