ABSTRACT
To describe and explain the nature and behaviour of MRs, two molecular models are described in the literature. e rst model is called sphingolipid-enriched model, which describes MRs as relatively small structures enriched in cholesterol and sphingolipids, where various proteins aggregate and assemble as a signalling unit that is regulated upon dierent stimuli (Simons and Ikonen 1997). SM composed of a highly hydrophobic ceramide moiety and a hydrophilic phosphorylcholine headgroup is considered a most prevalent sphingolipid in the MRs. Depending upon the tight interaction between the cholesterol-sterol-ring system and the ceramide moiety of the SM, a lateral association between sphingolipids and cholesterol is formed to produce distinct microdomains. In addition, the interactions of cholesterol and SM determine the existing status of MRs on the cell membrane, either in a liquid-ordered or gel-like phase. Without such interactions of cholesterol and SM, many other domains in cell membranes are primarily present in a more disordered uid or liquid phase (Gulbins and Li 2006). Another model for interpretation of the nature and behaviour of MRs is known as the shell model. Based on this model, the generation of MRs is thought to depend on protein-lipid or protein-protein interactions, where MRs consist of lipid shells that are formed by proteins with their preferential types of lipids. It is the protein-protein interaction in such lipid shells to create larger functional units responsible for MR-related activities (Anderson and Jacobson 2002). Moreover, these proteins may be oligomerized to form larger MR platforms, again depending upon protein-lipid interactions or protein-protein interactions (Helms and Zurzolo 2004).
