New N - RAP - binding partners α - actinin , filamin and Krp 1 detected by yeast two - hybrid screening : implications for myofibril assembly

N-RAP is an actin binding LIM protein expressed in skeletal and cardiac muscle tissues (Luo et al., 1997). In adult mice, N-RAP is found at the longitudinal ends of striated muscles at sites of mechanical coupling between the myofibrils and the cell membrane (Luo et al., 1997). These regions are the myotendinous junctions in skeletal muscle and the intercalated disks in heart muscle, and ultrastructural data show that N-RAP co-localizes with the terminal actin bundles that link the myofibrils to the membrane in these regions (Herrera et al., 2000; Zhang et al., 2001). On the basis of the localization and binding affinities of N-RAP, we proposed that N-RAP serves as a link between the terminal actin of the myofibril and the protein complexes at the cell membrane (Luo et al., 1999; Luo et al., 1997). N-RAP has also been implicated in the earliest steps of myofibril assembly (Carroll et al., 2001; Carroll and Horowits, 2000). Although subsequent steps are controversial, many investigators agree that the earliest myofibril precursors originate near the membrane, where α-actinin and actin Time-lapse studies of living cardiomyocytes expressing α-actinin fused to GFP suggest that the closely