Lundbaek J A, Andersen O S
Department of Physiology and Biophysics, Cornell University Medical College, New York, New York 10021.
J Gen Physiol. 1994 Oct;104(4):645-73. doi: 10.1085/jgp.104.4.645.
Lipid metabolites, free fatty acids and lysophospholipids, modify the function of membrane proteins including ion channels. Such alterations can occur through signal transduction pathways, but may also result from "direct" effects of the metabolite on the protein. To investigate possible mechanisms for such direct effects, we examined the alterations of gramicidin channel function by lysophospholipids (LPLs): lysophosphatidylcholine (LPC), lysophosphatidylethanolamine (LPE), lysophosphatidylserine (LPS), and lysophosphatidylinositol (LPI). The experiments were done on planar bilayers formed by diphytanoylphosphatidylcholine in n-decane a system where receptor-mediated effects can be excluded. At aqueous concentrations below the critical micelle concentration (CMC), LPLs can increase the dimerization constant for membrane-bound gramicidin up to 500-fold (at 2 microM). The relative potency increases as a function of the size of the polar head group, but does not seem to vary as a function of head group charge. The increased dimerization constant results primarily from an increase in the rate constant for channel formation, which can increase more than 100-fold (in the presence of LPC and LPI), whereas the channel dissociation rate constant decreases only about fivefold. The LPL effect cannot be ascribed to an increased membrane fluidity, which would give rise to an increased channel dissociation rate constant. The ability of LPC to decrease the channel dissociation rate constant varies as a function of channel length (which is always less than the membrane's equilibrium thickness): as the channel length is decreased, the potency of LPC is increased. LPC has no effect on membrane thickness or the surface tension of monolayers at the air/electrolyte interface. The bilayer-forming glycerolmonooleate does not decrease the channel dissociation rate constant. These results show that LPLs alter gramicidin channel function by altering the membrane deformation energy, and that the changes in deformation energy can be related to the molecular "shape" of the membrane-modifying compounds. Similar alterations in the mechanical properties of biological membranes may form a general mechanism by which one can alter membrane protein function.
脂质代谢物、游离脂肪酸和溶血磷脂可改变包括离子通道在内的膜蛋白功能。这种改变可通过信号转导途径发生，但也可能是代谢物对蛋白质的“直接”作用所致。为了研究这种直接作用的可能机制，我们检测了溶血磷脂（LPLs）对短杆菌肽通道功能的影响，这些溶血磷脂包括溶血磷脂酰胆碱（LPC）、溶血磷脂酰乙醇胺（LPE）、溶血磷脂酰丝氨酸（LPS）和溶血磷脂酰肌醇（LPI）。实验在由二植烷酰磷脂酰胆碱在正癸烷中形成的平面双层膜上进行，该系统可排除受体介导的作用。在水相浓度低于临界胶束浓度（CMC）时，LPLs可使膜结合短杆菌肽的二聚化常数增加高达500倍（在2 microM时）。相对效力随极性头部基团大小的增加而增加，但似乎不随头部基团电荷而变化。二聚化常数的增加主要源于通道形成速率常数的增加，其可增加100倍以上（在LPC和LPI存在时），而通道解离速率常数仅降低约五倍。LPL的作用不能归因于膜流动性的增加，因为膜流动性增加会导致通道解离速率常数增加。LPC降低通道解离速率常数的能力随通道长度而变化（通道长度始终小于膜的平衡厚度）：随着通道长度的减小，LPC的效力增加。LPC对膜厚度或空气/电解质界面单层的表面张力没有影响。形成双层膜的甘油单油酸酯不会降低通道解离速率常数。这些结果表明，LPLs通过改变膜变形能来改变短杆菌肽通道功能，并且变形能的变化可能与膜修饰化合物的分子“形状”有关。生物膜力学性质的类似改变可能形成一种普遍机制，通过该机制可以改变膜蛋白功能。
