Siegman M J, Mooers S U, Li C, Narayan S, Trinkle-Mulcahy L, Watabe S, Hartshorne D J, Butler T M
Department of Physiology, Jefferson Medical College, Philadelphia, PA 19107, USA.
J Muscle Res Cell Motil. 1997 Dec;18(6):655-70. doi: 10.1023/a:1018683823020.
A unique property of smooth muscle is its ability to maintain force with a very low expenditure of energy. This characteristic is highly expressed in molluscan smooth muscles, such as the anterior byssus retractor muscle (ABRM) of Mytilus edulis, during a contractile state called 'catch'. Catch occurs following the initial activation of the muscle, and is characterized by prolonged force maintenance in the face of a low [Ca2+]i, high instantaneous stiffness, a very slow cross-bridge cycling rate, and low ATP usage. In the intact muscle, rapid relaxation (release of catch) is initiated by serotonin, and mediated by an increase in cAMP and activation of protein kinase A. We sought to determine which proteins undergo a change in phosphorylation on a time-course that corresponds to the release of catch in permeabilized ABRM. Only one protein consistently satisfied this criterion. This protein, having a molecular weight of approximately 600 kDa and a molar concentration about 30 times lower than the myosin heavy chain, showed an increase in phosphorylation during the release of catch. Under the mechanical conditions studied (rest, activation, catch, and release of catch), changes in phosphorylation of all other proteins, including myosin light chains, myosin heavy chain and paramyosin, are minimal compared with the cAMP-induced phosphorylation of the approximately 600 kDa protein. Under these conditions, somewhat less than one mole of phosphate is incorporated per mole of approximately 600 kDa protein. Inhibition of A kinase blocked both the cAMP-induced increase in phosphorylation of the protein and the release of catch. In addition, irreversible thiophosphorylation of the protein prevented the development of catch. In intact muscle, the degree of phosphorylation of the protein increases significantly when catch is released with serotonin. In muscles pre-treated with serotonin, a net dephosphorylation of the protein occurs when the muscle is subsequently put into catch. We conclude that the phosphorylation state of the approximately 600 kDa protein regulates catch.
平滑肌的一个独特特性是其能够以极低的能量消耗维持张力。这种特性在软体动物的平滑肌中高度体现，比如紫贻贝的前足丝牵缩肌（ABRM），在一种称为“捕捉”的收缩状态下。“捕捉”发生在肌肉最初激活之后，其特征是在低[Ca2+]i、高瞬时刚度、极慢的横桥循环速率和低ATP消耗的情况下长时间维持张力。在完整肌肉中，5-羟色胺引发快速松弛（捕捉的解除），并由cAMP增加和蛋白激酶A的激活介导。我们试图确定在通透的ABRM中，哪些蛋白质在与捕捉解除相对应的时间进程中发生磷酸化变化。只有一种蛋白质始终符合这一标准。这种蛋白质分子量约为600 kDa，摩尔浓度比肌球蛋白重链低约30倍，在捕捉解除过程中磷酸化增加。在所研究的力学条件（静息、激活、捕捉和捕捉解除）下，与cAMP诱导的约600 kDa蛋白质的磷酸化相比，包括肌球蛋白轻链、肌球蛋白重链和副肌球蛋白在内的所有其他蛋白质的磷酸化变化极小。在这些条件下，每摩尔约600 kDa蛋白质掺入的磷酸盐略少于一摩尔。A激酶的抑制阻止了cAMP诱导的该蛋白质磷酸化增加以及捕捉的解除。此外，该蛋白质的不可逆硫代磷酸化阻止了捕捉的形成。在完整肌肉中，当用5-羟色胺解除捕捉时，该蛋白质的磷酸化程度显著增加。在用5-羟色胺预处理的肌肉中，当肌肉随后进入捕捉状态时，该蛋白质会发生净去磷酸化。我们得出结论，约600 kDa蛋白质的磷酸化状态调节捕捉。
