Sobieszek Apolinary
Institute for Biomedical Aging Research, Austrian Academy of Sciences, Innsbruck, Austria.
Can J Physiol Pharmacol. 2005 Oct;83(10):899-912. doi: 10.1139/y05-053.
Smooth muscle myosin copurifies with myosin light chain kinase (MLCK) and calmodulin (CaM) as well as with variable amounts of myosin phosphatase. Therefore, myosin filaments formed in vitro also contain relatively high levels of these enzymes. Thus these filaments may be considered to be native-like because they are similar to those expected to exist in vivo. These endogenous enzymes are present at high concentrations relative to myosin, sufficient for rapid phosphorylation and dephosphorylation of the filaments at rates comparable to those observed for contraction and relaxation in intact muscle strips. The phosphorylation by MLCK/CaM complex appears to exhibit some directionality and is not governed by a random diffusional process. For the mixtures of myosin filaments with and without the endogenous MLCK/CaM complex, the complex preferentially phosphorylates its own parent filament at a higher rate than the neighboring filaments. This selective or vectorial-like activation is lost or absent when myosin filaments are dissolved at high ionic strength. Similar vectorial-like activation is exhibited by the reconstituted filament suspensions, but the soluble systems composed of isolated regulatory light chain or soluble myosin head subfragments exhibit normal diffusional kinetic behavior. At physiological concentrations, kinase related protein (telokin) effectively modulates the activation process by reducing the phosphorylation rate of the filaments without affecting the overall phosphorylation level. This results from telokin-induced liberation of the active MLCK/CaM complex from the filaments, so that the latter can also activate the neighboring filaments via a slower diffusional process. When this complex is bound at insufficient levels, this actually results in acceleration of the initial phosphorylation rates. In short, I suggest that in smooth muscle, telokin plays a chaperone role for myosin and its filaments.
平滑肌肌球蛋白与肌球蛋白轻链激酶（MLCK）、钙调蛋白（CaM）以及不同量的肌球蛋白磷酸酶共同纯化。因此，体外形成的肌球蛋白丝也含有相对较高水平的这些酶。所以这些丝状物可被认为是类似天然的，因为它们与预期存在于体内的丝状物相似。相对于肌球蛋白而言，这些内源性酶以高浓度存在，足以使丝状物快速磷酸化和去磷酸化，其速率与完整肌条收缩和舒张时观察到的速率相当。MLCK/CaM复合物的磷酸化似乎表现出一定的方向性，并非由随机扩散过程控制。对于含有和不含有内源性MLCK/CaM复合物的肌球蛋白丝混合物，该复合物优先以比相邻丝更高的速率磷酸化其自身的母丝。当肌球蛋白丝在高离子强度下溶解时，这种选择性或类似矢量的激活作用会丧失或不存在。重组的丝悬浮液也表现出类似矢量的激活作用，但由分离的调节轻链或可溶性肌球蛋白头部亚片段组成的可溶性系统表现出正常的扩散动力学行为。在生理浓度下，激酶相关蛋白（telokin）通过降低丝状物的磷酸化速率而不影响整体磷酸化水平，有效地调节激活过程。这是由于telokin诱导活性MLCK/CaM复合物从丝状物上释放，从而使后者也能通过较慢的扩散过程激活相邻的丝状物。当这种复合物结合水平不足时，实际上会导致初始磷酸化速率加快。简而言之，我认为在平滑肌中，telokin对肌球蛋白及其丝状物起着伴侣作用。
