Pascolini D, Blasie J K
Department of Chemistry, University of Pennsylvania, Philadelphia 19104.
Biophys J. 1988 Oct;54(4):669-78. doi: 10.1016/S0006-3495(88)83002-9.
The calcium uptake reaction kinetics of isolated sarcoplasmic reticulum (SR) vesicles have previously been shown to be at least biphasic over a range of temperatures (26 to 10 degrees C) with a fast phase identified with the formation of E1 approximately P and calcium occlusion and a slow phase with Ca2+ translocation across the membrane and turnover of the Ca2+ ATPase ensemble. At "low" temperatures, namely 0 degrees C or lower, E1 approximately P formation is slowed and E1 approximately P is transiently trapped for at least several seconds, as indicated by the absence of the slow phase for 6 s or more. We now report that a reversible, temperature-induced structural transition occurs at about 2-3 degrees C for the isolated SR membrane. We have investigated the nature of this structural transition utilizing meridional and equatorial x-ray diffraction studies of the oriented SR membrane multilayers in the range of temperatures between 7.5 and -2 degrees C. The phase meridional (lamellar) diffraction has provided the profile structure for the SR membrane at the highest vs. lowest temperature at the same moderate resolution of 16-17 A while the equatorial diffraction has provided information on the average lipid chain packing in the SR membrane plane in the two cases. To identify the contribution of each membrane component in producing the differences between the profile structures at 7.5 and -2 degrees C, step-function models have been fitted to the moderate resolution electron density profiles. Lipid lateral phase separation may be responsible for inducing the structural change in the Ca2+ ATPase, thereby resulting in the slowing of E1 approximately P formation and the transient trapping of E1 approximately P at the "lower" temperatures.
先前已表明，在一系列温度（26至10摄氏度）范围内，分离的肌浆网（SR）囊泡的钙摄取反应动力学至少是双相的，快速相与E1≈P的形成和钙封闭相关，慢速相与Ca2+跨膜转运和Ca2+ ATP酶复合体的周转相关。在“低温”下，即0摄氏度或更低温度时，E1≈P的形成会减慢，并且E1≈P会被短暂捕获至少几秒钟，这表现为慢速相在6秒或更长时间内不存在。我们现在报告，分离的SR膜在约2 - 3摄氏度时会发生可逆的、温度诱导的结构转变。我们利用子午线和赤道X射线衍射研究了取向的SR膜多层在7.5至 - 2摄氏度温度范围内的这种结构转变的性质。在16 - 17埃的相同中等分辨率下，相子午线（层状）衍射提供了SR膜在最高温度与最低温度下的轮廓结构，而赤道衍射提供了这两种情况下SR膜平面中平均脂链堆积的信息。为了确定每种膜成分在产生7.5和 - 2摄氏度轮廓结构差异中的作用，已将阶跃函数模型拟合到中等分辨率的电子密度轮廓上。脂质横向相分离可能是导致Ca2+ ATP酶结构变化的原因，从而导致E1≈P形成减慢以及E1≈P在“较低”温度下被短暂捕获。
