Fig. 5. Expression patterns of GFP-N-RAP-SR (A,C) and of -actinin in the same fields (B,D). Cardiomyocytes were transfected with GFP-N-RAP-SR at 2 days and viewed 1 day post transfection (A,B) or transfected at 1 day and viewed 3 days post transfection (C,D). In cardiomyocytes transfected after 2 days in culture, GFP-N-RAP-SR is concentrated in myofibril precursors (arrows, A,B), as well as in mature sarcomeres (arrowheads, A,B). The inset (A,B) shows a magnified view of the -actinin striations marked with arrowheads, showing that N-RAP-SR is concentrated within the myofilament-containing region of the sarcomere, but not in the Z-lines marked by -actinin staining. The sarcomeres shown in the inset have an average length of 1.4 µm. Cardiomyocytes transfected with GFP-N-RAP-SR after 1 day in culture rarely form mature myofibrils (C,D).