Vertebrate myosin VIIb is a high duty ratio motor adapted for generating and maintaining tension
Vertebrate myosin VIIb is a high duty ratio motor adapted for generating and maintaining tension
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DOI：
10.1074/jbc.m507667200
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发表时间：
2005-11-25
影响因子：
4.8
通讯作者：
De La Cruz, EM
中科院分区：
文献类型：
--
作者：
Henn, A;De La Cruz, EM
肌肉和非肌肉肌球蛋白的动力学适应在定义这些分子运动酶的独特细胞功能中起着核心作用。非常规的脊椎动物VII类肌球蛋白（myosin VIIb）在极化细胞中高度表达，并定位于高度有序的肌动蛋白丝束，如肠刷状缘和肾脏微绒毛中的肌动蛋白丝束。我们从cDNA文库中克隆了小鼠myosin VIIb，表达并纯化了催化马达结构域，并利用定量平衡和动力学方法表征了其肌动蛋白激活的atp酶循环。肌凝蛋白VIIb稳态atp酶活性缓慢（类似于1 s(-1)），被非常低的肌动蛋白丝浓度激活（k - atp酶类似于0.7 μ M），并受到肌动球蛋白释放ADP的限制。缓慢的ADP解离速率常数产生了长寿命的强结合肌动球蛋白（。）ADP的状态。ADP和肌动蛋白的结合是不耦合的，这使得myosin VIIb在非常低的肌动蛋白浓度下仍能与肌动蛋白和ADP保持强结合。在2 mM ATP和2 μ M肌动蛋白存在的情况下，myosin VIIb的占空比接近0.8。肌动球蛋白VIIb的酶性质适合于产生和维持张力，并有利于肌动蛋白VIIb在将膜表面受体锚定到肌动蛋白细胞骨架上的作用。鉴于脊椎动物VII类肌球蛋白的高度保守性，由正常肌球蛋白VIIa功能破坏引起的耳聋表型可能反映了内耳毛细胞立体纤毛张力的丧失。
Kinetic adaptation of muscle and non-muscle myosins plays a central role in defining the unique cellular functions of these molecular motor enzymes. The unconventional vertebrate class VII myosin, myosin VIIb, is highly expressed in polarized cells and localizes to highly ordered actin filament bundles such as those found in the microvilli of the intestinal brush border and kidney. We have cloned mouse myosin VIIb from a cDNA library, expressed and purified the catalytic motor domain, and characterized its actin-activated ATPase cycle using quantitative equilibrium and kinetic methods. The myosin VIIb steady-state ATPase activity is slow (similar to 1 s (-1)), activated by very low actin filament concentrations ( K-ATPase similar to 0.7 mu M), and limited by ADP release from actomyosin. The slow ADP dissociation rate constant generates a long lifetime of the strong binding actomyosin (.) ADP states. ADP and actin binding is uncoupled, which enables myosin VIIb to remain strongly bound to actin and ADP at very low actin concentrations. In the presence of 2 mM ATP and 2 mu M actin, the duty ratio of myosin VIIb is similar to 0.8. The enzymatic properties of actomyosin VIIb are suited for generating and maintaining tension and favor a role for myosin VIIb in anchoring membrane surface receptors to the actin cytoskeleton. Given the high conservation of vertebrate class VII myosins, deafness phenotypes arising from disruption of normal myosin VIIa function are likely to reflect a loss of tension in the stereocilia of inner ear hair cells.