Myosin structural and kinetic mechanisms that differentiate fast and slow muscle

描述（由申请人提供）：肌球蛋白和其他运动蛋白将ATP水解的化学能转化为力和运动的动力学和结构机制尚不清楚。在普遍接受的肌球蛋白过桥功能理论中，Pi释放与产生力量的力量冲程有关，但与MgADP释放率相关的步骤被认为是无负荷肌肉缩短速度和振荡功产生的速率限制。然而，最近的证据表明，MgADP释放并不是影响肌肉缩短速度的周期的唯一步骤，我们最近的数据表明，非常快的果蝇肌球蛋白产生振荡功的最佳频率是由Pi释放率而不是MgADP释放率决定的（Swank et al., 2006）。因此，我们将测试我们的动力学假设，即非常快的肌球蛋白的卸载速度和振荡功生产受到与Pi释放相关的步骤的限制，而较慢的肌球蛋白受到与MgADP释放速率相关的步骤的限制。具体目的：(1)通过改变间接飞行肌（IFM）中表达四种速度变化为9倍的果蝇肌球蛋白异构体的MgATP、Pi和MgADP水平来测试我们的振荡功产生的动力学假设，以确定包括速率限制步骤在内的关键过桥速率常数。(2)通过改变转基因表达相同四种肌球蛋白亚型的果蝇皮肤跳跃肌沐浴液中MgATP、Pi和MgADP的水平，检测Pi或ADP的释放是否在纤维水平上限制了卸载速度。我们的动力学假设也将在分子水平上使用肌动蛋白滑动丝试验进行测试。(3)验证我们的结构假设，即肌凝蛋白转换器是决定跨桥速率恒定值的主要区域，而跨桥速率恒定值对于设定肌凝蛋白同型体的缩短速度至关重要。我们将通过与目标1和目标2中描述的相同的分子和纤维实验来验证这一假设，这些实验是用果蝇转换区的其他4个天然版本替换IFM肌球蛋白转换体而制成的肌球蛋白嵌合体。(4)通过替换降低IFM肌球蛋白同型异构体疏水性的氨基酸来验证我们的机制假设，即转化器的疏水性程度对其功能至关重要。意义：我们将确定转换区如何影响MgADP释放和/或Pi释放。这将是非常重要的，因为我们对运动蛋白设定Pi和MgADP释放率的结构机制知之甚少。关于转化器设定流速机制的详细信息将有助于验证最近关于至少8种不同的转化器突变如何导致家族性肥厚性心肌病（FHC）或扩张性心肌病（DCM）的假设。FHC是一种遗传性遗传病，是年轻人猝死的主要原因。

DESCRIPTION (provided by applicant): The kinetic and structural mechanisms by which myosin and other motor proteins convert the chemical energy of ATP hydrolysis into force and motion are far from being understood. In generally accepted theories of myosin cross-bridge function, Pi release is associated with the force-producing power stroke, but steps associated with MgADP release rate are thought to be rate limiting for unloaded muscle shortening velocity and oscillatory work production. However, recent evidence suggests that MgADP release is not the only step of the cycle that influences muscle shortening velocity, and our recent data suggest the optimal frequency of oscillatory work production by very fast Drosophila myosin is set by the Pi release rate rather than the MgADP release rate (Swank et al., 2006). Therefore, we will test our KINETIC HYPOTHESIS that unloaded velocity and oscillatory work production by very fast myosins are limited by steps associated with Pi release while slower myosins are limited by steps associated with MgADP release rate. SPECIFIC AIMS: (1) Test our kinetic hypothesis for oscillatory work production by varying MgATP, Pi and MgADP levels in indirect flight muscle (IFM) transgenically expressing four Drosophila myosin isoforms, which vary 9-fold in velocity, to determine critical cross-bridge rate constants including the rate limiting step. (2) Test if Pi or ADP release limits unloaded velocity at the fiber level by varying MgATP, Pi and MgADP levels in the bathing solution of skinned Drosophila jump muscle transgenically expressing the same four myosin isoforms. Our kinetic hypothesis will also be tested at the molecular level using the actin sliding filament assay. (3) Test our STRUCTURAL HYPOTHESIS that the myosin converter is the primary region responsible for determining cross-bridge rate constant values critical for setting the shortening velocity of myosin isoforms. We will test this hypothesis by performing the same molecular and fiber experiments as described in Aims 1 and 2 on myosin chimeras made by replacing the IFM myosin converter with the other 4 native versions of the Drosophila converter region. (4) Test our MECHANISTIC HYPOTHESIS that the degree of hydrophobicity in the converter is critical to its function by substituting amino acids that decrease the IFM myosin isoform's hydrophobicity. SIGNIFICANCE: We will determine how the converter region influences MgADP release and/or Pi release. This will be highly significant as very little is known about the structural mechanisms by which motor proteins set Pi and MgADP release rates. Details about the converter's mechanism for setting velocity will help test recent hypotheses regarding how at least 8 different mutations in the converter cause either familial hypertrophic cardiomyopathy (FHC) or dilated cardiomyopathy (DCM). FHC is an inherited genetic disease that is a major cause of sudden death among young adults.