Marco-Ferreres Raquel, Arredondo Juan J, Fraile Benito, Cervera Margarita
Departamento de Bioquímica and Instituto Investigaciones Biomédicas, UAM-CSIC, Facultad de Medicina, Universidad Autónoma de Madrid, Arzobispo Morcillo 4, 28029 Madrid, Spain.
Biochem J. 2005 Feb 15;386(Pt 1):145-52. doi: 10.1042/BJ20041240.
Formation of the contractile apparatus in muscle cells requires co-ordinated activation of several genes and the proper assembly of their products. To investigate the role of TnT (troponin T) in the mechanisms that control and co-ordinate thin-filament formation, we generated transgenic Drosophila lines that overexpress TnT in their indirect flight muscles. All flies that overexpress TnT were unable to fly, and the loss of thin filaments themselves was coupled with ultrastructural perturbations of the sarcomere. In contrast, thick filaments remained largely unaffected. Biochemical analysis of these lines revealed that the increase in TnT levels could be detected only during the early stages of adult muscle formation and was followed by a profound decrease in the amount of this protein as well as that of other thin-filament proteins such as tropomyosin, troponin I and actin. The decrease in thin-filament proteins is not only due to degradation but also due to a decrease in their synthesis, since accumulation of their mRNA transcripts was also severely diminished. This decrease in expression levels of the distinct thin-filament components led us to postulate that any change in the amount of TnT transcripts might trigger the down-regulation of other co-regulated thin-filament components. Taken together, these results suggest the existence of a mechanism that tightly co-ordinates the expression of thin-filament genes and controls the correct stoichiometry of these proteins. We propose that the high levels of unassembled protein might act as a sensor in this process.
肌肉细胞中收缩装置的形成需要多个基因的协同激活及其产物的正确组装。为了研究肌钙蛋白T（TnT）在控制和协调细肌丝形成机制中的作用，我们构建了在间接飞行肌中过表达TnT的转基因果蝇品系。所有过表达TnT的果蝇均无法飞行，细肌丝的缺失伴随着肌节超微结构的紊乱。相比之下，粗肌丝基本未受影响。对这些品系的生化分析表明，仅在成年肌肉形成的早期阶段可检测到TnT水平升高，随后该蛋白以及其他细肌丝蛋白（如原肌球蛋白、肌钙蛋白I和肌动蛋白）的量大幅下降。细肌丝蛋白的减少不仅是由于降解，还由于其合成减少，因为它们的mRNA转录本积累也严重减少。不同细肌丝成分表达水平的这种下降使我们推测，TnT转录本数量的任何变化可能会触发其他共同调控的细肌丝成分的下调。综上所述，这些结果表明存在一种机制，该机制紧密协调细肌丝基因的表达并控制这些蛋白质的正确化学计量。我们提出，高水平的未组装蛋白可能在此过程中充当传感器。
