Different obscurin isoforms localize to distinct sites at sarcomeres

We used four antibodies to regions of obscurin isoforms A and B, encoded by the obscurin gene, to investigate the location of these proteins in skeletal myofibers at resting and stretched lengths. Obscurin A (∼800 kDa) which was recognized by antibodies generated to the N-terminal, Rho-GEF, and the non-modular C-terminal domain that lacks the kinase-like domains, localizes at the level of the M-band. Obscurin B (∼900 kDa) which has the N-terminal, Rho-GEF, and the C-terminal kinase-like domains, localizes at the level of the A/I junction. Additional isoforms, which lack one or more of these epitopes, are present at the Z-disk and Z/I junction.