Modification of membrane domain structure and function by membrane proteins
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One of the popular approaches to study cell membranes is to study lipid mixtures phase behaviors. Although the structure of compositionally simple binary and ternary lipid systems have been extensively studied in the last three decades, the effects of proteins on compositional complexity of biological membranes have not been studied in details. Since in some biological membranes, the majority of membrane area is covered by proteins (up to 60–70%), it is reasonable to suppose that membrane proteins would have large effects on bilayer phase behavior. To the best of our knowledge, previously, no 4-component phase diagram, with protein as one of the components, has been reported. This work is the first study of this kind which investigate the effect of polypeptide gramicidin-A on Lo+Ld phase boundaries. In recent years, many studies have focused on the study of lipid rafts, a type of domain structure thought to form spontaneously by lateral phase separation in membranes. Lipid rafts provide domains for self-assembly and transport of specific proteins involved in signaling and trafficking of the cells. To understand the properties of lipid rafts, model lipid bilayers of ternary or quaternary lipid mixtures have been used extensively in the past decade. Model membrane studies have developed the raft hypothesis by providing a picture of plasma membrane domains as coexisting liquid-ordered (Lo) and liquid-disordered (Ld) phases. In this work, the effects of adding 1 mol% of gramicidin-A to Lo+Ld phase boundary of DOPC/DSPC/cholesterol, PLPC/DSPC/Cholesterol and PLPC/DPPC/ Cholesterol ternary lipid mixtures were investigated.