Grabe Michael, Neu John, Oster George, Nollert Peter
Department of Physics, University of California, Berkeley, California 94720, USA.
Biophys J. 2003 Feb;84(2 Pt 1):854-68. doi: 10.1016/S0006-3495(03)74904-2.
The difficulty in growing crystals for x-ray diffraction analysis has hindered the determination of membrane protein structures. However, this is changing with the advent of a new method for growing high quality membrane protein crystals from the lipidic cubic phase. Although successful, the mechanism underlying this method has remained unclear. Here, we present a theoretical analysis of the process. We show that it is energetically favorable for proteins embedded in the highly curved cubic phase to cluster together in flattened regions of the membrane. This stabilizes the lamellar phase, permitting its outgrowth from the cubic phase. A kinetic barrier-crossing model is developed to determine the free energy barrier to crystallization from the time-dependent growth of protein clusters. Determining the values of key parameters provides both a rational basis for optimizing the experimental procedure for membrane proteins that have not yet been crystallized and insight into the analogous cubic to lamellar transitions in cells. We also discuss the implications of this mechanism for protein sorting at the exit sites of the Golgi and endoplasmic reticulum and the general stabilization of membrane structures.
用于X射线衍射分析的晶体生长困难阻碍了膜蛋白结构的测定。然而，随着一种从脂质立方相生长高质量膜蛋白晶体的新方法的出现，这种情况正在发生变化。尽管该方法取得了成功，但其背后的机制仍不清楚。在此，我们对该过程进行了理论分析。我们表明，嵌入高度弯曲立方相的蛋白质在膜的扁平区域聚集在一起在能量上是有利的。这稳定了层状相，使其能够从立方相生长出来。开发了一个动力学势垒穿越模型，以根据蛋白质簇随时间的生长来确定结晶的自由能势垒。确定关键参数的值既为优化尚未结晶的膜蛋白的实验程序提供了合理依据，也为深入了解细胞中类似的立方相向层状相转变提供了思路。我们还讨论了这种机制对高尔基体和内质网出口位点的蛋白质分选以及膜结构的总体稳定的影响。
