Localization of ank1.5 in the sarcoplasmic reticulum precedes that of SERCA and RyR: Relationship with the organization of obscurin in developing sarcomeres

Ank1.5 is a muscle-speciWc isoform of anky- rin1 localized on the sarcoplasmic reticulum (SR) mem- brane that has been shown to interact with obscurin, a sarcomeric protein. We report here studies on the localiza- tion of obscurin and ank1.5 in embryonic and postnatal rodent skeletal muscles. Using two antibodies against epi- topes in the N- and C-terminus of obscurin, two distinct patterns of localization were observed. Before birth, the antibodies against the N- and the C-terminus of obscurin stained the Z-disk and M-band, respectively. At the same time, ank1.5 was detected at the Z-disk, rising the possibil- ity that obscurin molecules at M-band may not be able to interact with ank1.5. Localization of ank1.5 at Z-disks in E14 muscle Wbers revealed that ank1.5 is among the earliest SR proteins to assemble, since its organization preceded that of other SR proteins, like SERCA and RyR. After birth, the antibody against the N-terminus of obscurin stained the M-band while that against the C-terminus stained both M-bands and the Z-disks. Starting from post- natal day 1, ank1.5 was found at the level of both M-bands and Z-disks. Altogether, from these results we infer that exposure of some obscurin epitopes changes during skeletal muscle development, resulting in distinct, antibody-specific, localization pattern. Why this occurs is not clear, yet these data indicate that the organization of obscurin at diVerent locations in the sarcomere changes during muscle development and that this might affect the interaction with ank1.5.