Micro-segregation induced by bulky-head lipids: formation of characteristic patterns in a giant vesicle

Bulky-head lipids, such as glycolipids, play indispensable roles in cellular membranes. However, little is known about the effects that bulky-head lipids have on the physicochemical properties of the membrane. In this study, we examined the effects of a giant vesicle containing poly(ethylene glycol)-conjugated cholesterol (PEG-Chol), as a model of natural bulky-head lipids, on phase separation in the membrane. We used a lipid combination that included the saturated phospholipid DPPC, the unsaturated phospholipid DOPC and Chol, which is known to cause phase separation into two liquid phases. This phase separation is classified as a first-order phase transition under the criterion of Landau, and thus micro-domains tend to show coarse-graining up to a global pair of domains, i.e., mono-domains, so as to minimize interfacial instability. In contrast to such coarsening in the ternary system, we show here the generation of stable micro-domains in the presence of PEG-Chol above a critical composition. The transition from global- to micro-segregation is interpreted theoretically in terms of the competition between two physical effects, i.e., steric repulsive interaction between the bulky-head groups of PEG-Chol and the cost in line energy along the domain boundaries. Interestingly, among the micro-domain structures, a network pattern of domains appears as an intermediate state in which small domains are connected to each other. We examined the stability of the network pattern under local heating using a focused laser, and confirmed self-recovery of the pattern. Based on these observations, natural bulky-head lipids in cells may also stabilize the domain structure like a lipid raft.