The Long Glu-Rich Segments of Troponin T in Flight Muscles of Birds and Insects

Abstract

The troponin complex plays a central role in the Ca2+-regulation of striated muscle contraction and relaxation. Molecular evolution and comparative structural studies have identified Glu-rich segments specifically expressed in the T subunit of troponin (TnT) in diverse flying species, such as birds and insects. Avian pectoral muscle TnT has a Glu-rich segment in the N-terminal variable region and insect TnT has one in the C-terminal extension. To understand these structural traits of non-homologous evolutionary origins, which may have been selected for analogous functions of flight muscles, will help to understand the mechanisms of striated muscle contraction for the development of therapeutic improvement of skeletal and cardiac muscle functions. We previously demonstrated a potential function of the N-terminal Glu-rich segment of avian pectoral muscle TnT as a myofilament-associated Ca2+-reservoir (Zhang et al., Biochemistry 43:2645-55 2004). To investigate a novel hypothesis that this function reduces the work load of the Ca2+ handling system in myocytes thus saving energy for sustaining the work of muscle especially during long distance flight, we developed a Drosophila line with the Glu rich C-terminal segment of TnT deleted to test its requirement for muscle function and flight capacity. Engineered drosophila TnT proteins are expressed in E. coli for the characterization of changes in biochemical activities. The results showed that the C-terminal truncated Drosophila TnT retains the capacity of binding to troponin I, troponin C and tropomyosin. Tpnt gene-modified flies expressing the Glu-rich C-terminal segment deleted TnT have been generated for functional studies. Together with the development and characterization of genetically modified mice expressing an insect muscle-like TnT in the heart, our study aims at establishing a novel mechanism to improve cardiac efficiency for the treatment of heart failure.