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Using specific dystrophin antibodies directed against a conserved C-terminal sequence, we demonstrated that dystrophin of fish white muscle was quickly degraded by 50% within 24h and by 100% within 2 days, in parallel with titin cleavage and alpha-actinin release from Z-disks. These changes were accompanied by sarcolemma detachment from the myofibers in costameres (the structures containing dystrophin) and Zdisks weakening. For muscle stored during 2 to 6 mo before thawing, total dystrophin disappearance was observed at 4ЊC in Ͻ8h. Dystrophin may serve as a marker for stored fish to evaluate post mortem changes or detect a thawing-freezing process.

CapZ is a widely distributed and highly conserved, heterodimeric protein, that nucleates actin polymerization and binds to the barbed ends of actin filaments, preventing the addition or loss of actin monomers. CapZ interaction with actin filaments was shown to be of high affinity and decreased in the presence of PIP2. CapZ was located in nascent Z-lines during skeletal muscle myofibrillogenesis before the striated appearance of thin filaments in sarcomers. In this study, the stabilization and the anchorage of thin filaments were explored through identification of CapZ partners in the Z-line. Fish (sea bass) striated white muscle and its related Z-line proteins were selected since they correspond to the simplest Z-line organization. We report here the interaction between purified CapZ and alpha-actinin, a major component of Z filaments and polar links in Z-discs. Affinity of CapZ for alpha-actinin, estimated by fluorescence and immunochemical assays, is in the microM range. This association was found to be independent of actin and shown to be weakened in the presence of phosphoinositides. Binding contacts on the alpha-actinin molecule lie in the 55 kDa repetitive domain. A model including CapZ/alpha-actinin/titin/actin interactions is proposed considering Luther's 3D Z-line reconstruction.

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