Alpha actinin-CapZ, an anchoring complex for thin filaments in Z-line

CapZ is a widely distributed and highly conserved, heterodimeric protein, t hat nucleates actin polymerization and binds to the barbed ends of actin fi laments, preventing the addition or loss of actin monomers. CapZ interactio n with actin filaments was shown to be of high affinity and decreased in th e presence of PIP2. CapZ was located in nascent Z-lines during skeletal mus cle myofibrillogenesis before the striated appearance of thin filaments in sarcomers. In this study, the stabilization and the anchorage of thin filam ents were explored through identification of CapZ partners in the Z-line. F ish (sea bass) striated white muscle and its related Z-line proteins were s elected since they correspond to the simplest Z-line organization. We repor t here the interaction between purified CapZ and alpha-actinin, a major com ponent of Z filaments and polar links in Z-discs. Affinity of CapZ for alph a-actinin, estimated by fluorescence and immunochemical assays, is in the m u m range. This association was found to be independent of actin and shown to be weakened in the presence of phosphoinositides. Binding contacts on th e alpha-actinin molecule lie in the 55 kDa repetitive domain. A model inclu ding CapZ/alpha-actinin/titin/actin interactions is proposed considering Lu ther's 3D Z-line reconstruction.