Expression and alternative splicing of N-RAP during mouse skeletal muscle development.

N-RAP alternative splicing and protein localization were studied in developing skeletal muscle tissue from pre- and postnatal mice and in fusing primary myotubes in culture. Messages encoding N-RAP-s and N-RAP-c, the predominant isoforms of N-RAP detected in adult skeletal muscle and heart, respectively, were present in a 5:1 ratio in skeletal muscle isolated from E16.5 embryos. N-RAP-s mRNA levels increased three-fold over the first three weeks of postnatal development, while N-RAP-c mRNA levels remained low. N-RAP alternative splicing during myotube differentiation in culture was similar to the pattern observed in embryonic and neonatal muscle, with N-RAP-s expression increasing and N-RAP-c mRNA levels remaining low. In both developing skeletal muscle and cultured myotubes, N-RAP protein was primarily associated with developing myofibrillar structures containing α-actinin, but was not present in mature myofibrils. The results establish that N-RAP-s is the predominant spliced form of N-RAP present throughout skeletal muscle development.