Spanova Miroslava, Zweytick Dagmar, Lohner Karl, Klug Lisa, Leitner Erich, Hermetter Albin, Daum Günther
Institute of Biochemistry, Graz University of Technology, Graz, Austria.
Biochim Biophys Acta. 2012 Apr;1821(4):647-53. doi: 10.1016/j.bbalip.2012.01.015. Epub 2012 Feb 8.
In a previous study (Spanova et al., 2010, J. Biol. Chem., 285, 6127-6133) we demonstrated that squalene, an intermediate of sterol biosynthesis, accumulates in yeast strains bearing a deletion of the HEM1 gene. In such strains, the vast majority of squalene is stored in lipid particles/droplets together with triacylglycerols and steryl esters. In mutants lacking the ability to form lipid particles, however, substantial amounts of squalene accumulate in organelle membranes. In the present study, we investigated the effect of squalene on biophysical properties of lipid particles and biological membranes and compared these results to artificial membranes. Our experiments showed that squalene together with triacylglycerols forms the fluid core of lipid particles surrounded by only a few steryl ester shells which transform into a fluid phase below growth temperature. In the hem1∆ deletion mutant a slight disordering effect on steryl esters was observed indicated by loss of the high temperature transition. Also in biological membranes from the hem1∆ mutant strain the effect of squalene per se is difficult to pinpoint because multiple effects such as levels of sterols and unsaturated fatty acids contribute to physical membrane properties. Fluorescence spectroscopic studies using endoplasmic reticulum, plasma membrane and artificial membranes revealed that it is not the absolute squalene level in membranes but rather the squalene to sterol ratio which mainly affects membrane fluidity/rigidity. In a fluid membrane environment squalene induces rigidity of the membrane, whereas in rigid membranes there is almost no additive effect of squalene. In summary, our results demonstrate that squalene (i) can be well accommodated in yeast lipid particles and organelle membranes without causing deleterious effects; and (ii) although not being a typical membrane lipid may be regarded as a mild modulator of biophysical membrane properties.
在之前的一项研究中（斯帕诺娃等人，2010年，《生物化学杂志》，第285卷，6127 - 6133页），我们证明了角鲨烯，一种甾醇生物合成的中间体，会在缺失HEM1基因的酵母菌株中积累。在这类菌株中，绝大多数角鲨烯与三酰甘油和甾醇酯一起储存在脂质颗粒/液滴中。然而，在缺乏形成脂质颗粒能力的突变体中，大量角鲨烯会积累在细胞器膜中。在本研究中，我们研究了角鲨烯对脂质颗粒和生物膜生物物理性质的影响，并将这些结果与人工膜进行了比较。我们的实验表明，角鲨烯与三酰甘油一起形成了脂质颗粒的流体核心，周围仅有少数甾醇酯壳层，这些壳层在生长温度以下会转变为流体相。在hem1∆缺失突变体中，观察到甾醇酯有轻微的无序化效应，这表现为高温转变的丧失。同样，在hem1∆突变体菌株的生物膜中，角鲨烯本身的影响也很难确定，因为甾醇和不饱和脂肪酸水平等多种因素会影响膜的物理性质。使用内质网、质膜和人工膜进行的荧光光谱研究表明，并非膜中角鲨烯的绝对水平，而是角鲨烯与甾醇的比例主要影响膜的流动性/刚性。在流体膜环境中，角鲨烯会诱导膜的刚性，而在刚性膜中，角鲨烯几乎没有叠加效应。总之，我们的结果表明，角鲨烯（i）可以很好地容纳在酵母脂质颗粒和细胞器膜中而不会产生有害影响；（ii）尽管不是典型的膜脂，但可被视为生物膜生物物理性质的温和调节剂。
