Sellers J R, Spudich J A, Sheetz M P
J Cell Biol. 1985 Nov;101(5 Pt 1):1897-902. doi: 10.1083/jcb.101.5.1897.
In smooth muscles there is no organized sarcomere structure wherein the relative movement of myosin filaments and actin filaments has been documented during contraction. Using the recently developed in vitro assay for myosin-coated bead movement (Sheetz, M.P., and J.A. Spudich, 1983, Nature (Lond.)., 303:31-35), we were able to quantitate the rate of movement of both phosphorylated and unphosphorylated smooth muscle myosin on ordered actin filaments derived from the giant alga, Nitella. We found that movement of turkey gizzard smooth muscle myosin on actin filaments depended upon the phosphorylation of the 20-kD myosin light chains. About 95% of the beads coated with phosphorylated myosin moved at velocities between 0.15 and 0.4 micron/s, depending upon the preparation. With unphosphorylated myosin, only 3% of the beads moved and then at a velocity of only approximately 0.01-0.04 micron/s. The effects of phosphorylation were fully reversible after dephosphorylation with a phosphatase prepared from smooth muscle. Analysis of the velocity of movement as a function of phosphorylation level indicated that phosphorylation of both heads of a myosin molecule was required for movement and that unphosphorylated myosin appears to decrease the rate of movement of phosphorylated myosin. Mixing of phosphorylated smooth muscle myosin with skeletal muscle myosin which moves at 2 microns/s resulted in a decreased rate of bead movement, suggesting that the more slowly cycling smooth muscle myosin is primarily determining the velocity of movement in such mixtures.
在平滑肌中，不存在有组织的肌节结构，而在收缩过程中，肌球蛋白丝和肌动蛋白丝的相对运动已有文献记载。利用最近开发的体外检测肌球蛋白包被珠运动的方法（Sheetz, M.P.和J.A. Spudich，1983年，《自然》（伦敦），303:31 - 35），我们能够对磷酸化和未磷酸化的平滑肌肌球蛋白在源自大型藻类丽藻的有序肌动蛋白丝上的运动速率进行定量。我们发现，火鸡砂囊平滑肌肌球蛋白在肌动蛋白丝上的运动取决于20-kD肌球蛋白轻链的磷酸化。大约95%包被有磷酸化肌球蛋白的珠子以0.15至0.4微米/秒的速度移动，具体速度取决于制备情况。对于未磷酸化的肌球蛋白，只有3%的珠子移动，且速度仅约为0.01 - 0.04微米/秒。用平滑肌制备的磷酸酶去磷酸化后，磷酸化的影响完全可逆。对运动速度作为磷酸化水平函数的分析表明，肌球蛋白分子的两个头部都磷酸化是运动所必需的，并且未磷酸化的肌球蛋白似乎会降低磷酸化肌球蛋白的运动速率。将以2微米/秒速度移动的磷酸化平滑肌肌球蛋白与骨骼肌肌球蛋白混合，导致珠子运动速率降低，这表明循环较慢的平滑肌肌球蛋白在这类混合物中主要决定运动速度。
