This research project will examine the processes that underlie the assembly and disassembly of myofibrils. Myofibrils are composed of sarcomeres, which are repeating units of thin and thick filaments. Thin filaments composed largely of actin filaments are linked to the Z-line, a structure that resembles cell adhesions in protein composition. Among the proteins found in both focal adhesions and Z-line structures are the barbed ends of the actin filaments, focal adhesion kinase, p130 Cas , alpha-actinin, and AFAP-110. AFAP-110, which is the focus of this project, associates with the nonreceptor tyrosine kinase, Src. Recent evidence demonstrates that AFAP-110 recruits Src to the actin cytoskeleton in response either to elevated protein kinase C (PKC) activity or to mechanical stress. Evidence also suggests that AFAP-110 activates Src directly, resulting in elevated phosphotyrosine levels in the cell. One downstream effect of wide- spread activation of Src substrates is the reorganization of the cytoskeleton brought about by the disassembly of cytoskeletal linkages at focal adhesions and Z-lines. Mature myofibrils disappear from myotubes expressing high Src or PKC activity. Conversely, the generation of force across cell adhesions has been shown to cause the formation of actin filament linkages and to stimulate Src activity restricted to the vicinity of the mechanical strain. The properties that make AFAP-110 a candidate for participating in the formation of cytoskeletal linkages include the ability to bind and crosslink actin filaments, the ability to recruit Src to the cytoskeleton, and the ability to activate Src tyrosine kinase. AFAP-110 could be important in the formation of myofibrils during muscle development. To examine the role of AFAP-110 in muscle development, primary cultures of myotubes will be stained for AFAP-110 and the Z-line marker alpha-actinin. The two proteins should colocalize throughout all developmental stages if AFAP-110 plays a role in the formation of cytoskeletal linkages during myofibrillogenesis. Specific inhibitors of Src will be used to test the hypothesis that the myofibril formation requires activation of Src. To examine the disassembly mechanism, myoblasts will be transfected with a mutant form of AFAP-110, AFAP.lzip , which has been shown to activate Src and disrupt actin cytoskeleton of cultured fibroblasts. Preliminary results show that PKC phosphorylation of AFAP-110 results in the activation of Src tyrosine kinase activity and stress fiber disruption in fibroblasts. Transfection with mutants of AFAP-110 that cannot bind PKC or that cannot interact with Src protect fibroblasts from PKC induced stress fiber disruption. Parallel experiments will be carried out to test whether AFAP-110 serves a similar role in muscle cells. The results of these studies will elucidate the mechanisms by which mature myofibrils turn over and new myofibrils form. The work is to be carried out in large part by undergraduate students, who will carry their training onward to careers in science.
