Ohlendorf D H, Tronrud D E, Matthews B W
Institute of Molecular Biology Howard Hughes Medical Institute and Department of Physics, University of Oregon, Eugene, OR, 97403-1229, USA.
J Mol Biol. 1998 Jul 3;280(1):129-36. doi: 10.1006/jmbi.1998.1849.
The structure of the Cro repressor protein from phage lambda has been refined to a crystallographic R-value of 19.3% at 2.3 A resolution. The re fined model supports the structure as originally described in 1981 and provides a basis for comparison with the Cro-operator complex described in the accompanying paper. Changes in structure seen in different crystal forms and modifications of Cro suggest that the individual subunits are somewhat plastic in nature. In addition, the dimer of Cro suggests a high degree of flexibility, which may be important in forming the Cro-DNA complex. The structure of the Cro subunit as determined by NMR agrees reasonably well with that in the crystals (root-mean-square discrepancy of about 2 A for all atoms). There are, however, only a limited number of intersubunit distance constraints and, presumably for this reason, the different NMR models for the dimer vary substantially among themselves (discrepancies of 1.3 to 5.5 A). Because of this variation it is not possible to say whether the range of discrepancies between the X-ray and NMR Cro dimers (2.9 to 7.5 A) represent a significant difference between the X-ray and solution structures. It has previously been proposed that substitutions of Tyr26 in Cro increase thermal stability by the "reverse hydrophobic effect", i.e. by exposing 40% more hydrophobic surface to solvent in the folded form than in the unfolded state. The refined structure, however, suggests that Tyr26 is equally solvent exposed in the folded and unfolded states. The most stabilizing substitution is Tyr26-->Asp and in this case it appears that interaction with an alpha-helix dipole is at least partly responsible for the enhanced stability.
噬菌体λ的Cro阻遏蛋白结构已被精修至2.3埃分辨率下晶体学R值为19.3%。精修后的模型支持1981年最初描述的结构，并为与随附论文中描述的Cro-操纵基因复合物进行比较提供了基础。在不同晶体形式中观察到的结构变化以及Cro的修饰表明，各个亚基在本质上有些可塑性。此外，Cro二聚体显示出高度的灵活性，这在形成Cro-DNA复合物中可能很重要。通过核磁共振确定的Cro亚基结构与晶体中的结构相当吻合（所有原子的均方根偏差约为2埃）。然而，亚基间距离约束的数量有限，大概是由于这个原因，二聚体的不同核磁共振模型彼此之间差异很大（偏差为1.3至5.5埃）。由于这种变化，无法确定X射线和核磁共振Cro二聚体之间的偏差范围（2.9至7.5埃）是否代表X射线结构和溶液结构之间的显著差异。此前有人提出，Cro中Tyr26的取代通过“反向疏水效应”提高热稳定性，即与未折叠状态相比，折叠形式向溶剂暴露的疏水表面多40%。然而，精修后的结构表明，Tyr26在折叠和未折叠状态下与溶剂的接触程度相同。最稳定的取代是Tyr26→Asp，在这种情况下，与α-螺旋偶极子的相互作用似乎至少部分地导致了稳定性的增强。
