Laulumaa Saara, Nieminen Tuomo, Raasakka Arne, Krokengen Oda C, Safaryan Anushik, Hallin Erik I, Brysbaert Guillaume, Lensink Marc F, Ruskamo Salla, Vattulainen Ilpo, Kursula Petri
Faculty of Biochemistry and Molecular Medicine, University of Oulu, Oulu, Finland.
European Spallation Source (ESS), Lund, Sweden.
BMC Struct Biol. 2018 Jun 25;18(1):8. doi: 10.1186/s12900-018-0087-2.
Myelin is a multilayered proteolipid sheath wrapped around selected axons in the nervous system. Its constituent proteins play major roles in forming of the highly regular membrane structure. P2 is a myelin-specific protein of the fatty acid binding protein (FABP) superfamily, which is able to stack lipid bilayers together, and it is a target for mutations in the human inherited neuropathy Charcot-Marie-Tooth disease. A conserved residue that has been proposed to participate in membrane and fatty acid binding and conformational changes in FABPs is Phe57. This residue is thought to be a gatekeeper for the opening of the portal region upon ligand entry and egress.
We performed a structural characterization of the F57A mutant of human P2. The mutant protein was crystallized in three crystal forms, all of which showed changes in the portal region and helix α2. In addition, the behaviour of the mutant protein upon lipid bilayer binding suggested more unfolding than previously observed for wild-type P2. On the other hand, membrane binding rendered F57A heat-stable, similarly to wild-type P2. Atomistic molecular dynamics simulations showed opening of the side of the discontinuous β barrel, giving important indications on the mechanism of portal region opening and ligand entry into FABPs. The results suggest a central role for Phe57 in regulating the opening of the portal region in human P2 and other FABPs, and the F57A mutation disturbs dynamic cross-correlation networks in the portal region of P2.
Overall, the F57A variant presents similar properties to the P2 patient mutations recently linked to Charcot-Marie-Tooth disease. Our results identify Phe57 as a residue regulating conformational changes that may accompany membrane surface binding and ligand exchange in P2 and other FABPs.
髓磷脂是包裹在神经系统中特定轴突周围的多层蛋白脂质鞘。其组成蛋白在形成高度规则的膜结构中起主要作用。P2是脂肪酸结合蛋白（FABP）超家族的一种髓磷脂特异性蛋白，能够将脂质双层堆叠在一起，并且是人类遗传性神经病夏科-马里-图斯病（Charcot-Marie-Tooth disease）中突变的靶点。已提出参与FABP中膜和脂肪酸结合及构象变化的一个保守残基是苯丙氨酸57（Phe57）。该残基被认为是配体进出时门户区域打开的守门人。
我们对人P2的F57A突变体进行了结构表征。突变蛋白以三种晶体形式结晶，所有这些形式在门户区域和α2螺旋中均显示出变化。此外，突变蛋白与脂质双层结合后的行为表明其比野生型P2展开程度更大。另一方面，与野生型P2类似，膜结合使F57A具有热稳定性。原子分子动力学模拟显示不连续β桶一侧打开，这为门户区域打开和配体进入FABP的机制提供了重要线索。结果表明Phe57在调节人P2和其他FABP的门户区域打开中起核心作用，并且F57A突变扰乱了P2门户区域的动态互相关网络。
总体而言，F57A变体具有与最近与夏科-马里-图斯病相关的P2患者突变相似的特性。我们的结果确定Phe57是一个调节构象变化的残基，这种构象变化可能伴随P2和其他FABP中膜表面结合和配体交换。
