Phosphoinositides (PIPs) act as signaling molecules by recruiting critical effectors to specific subcellular membranes to regulate cell proliferation, apoptosis and cytoskeletal reorganization. These processes require a tight regulation of PIP generation and turnover as well as a high degree of compartmentalization. In addition to the cell compartment specific accumulation of PIPs, it has been suggested that many of the observed physiological functions require selective enrichment of PIPs in microdomains. However, the mechanisms that lead to the formation of PIP-enriched microdomains are still elusive. Our goal in this study was to detail the physiochemical conditions that lead to the formation of phosphoinositide-enriched domains in model membranes. Some factors that have effects on PIP-enriched microdomain formation such as the concentration of cholesterol, temperature and ionic strength were investigated. The induced PIP-enriched domains by some PIP-interacting proteins were also investigated. We have shown that: 1) the nature of the phosphoinositide headgroups affects cholesterol-independent and cholesterol-dependent domain formation. 2) Cholesterol enhances phosphoinositide domain formation through an active interaction that involves cholesterol hydroxyl group/phosphoinositide headgroup interaction. 3) Phosphoinositide domains are in a fluid phase and they are stabilized by mutual hydrogen bond formation between the headgroups. 4) Different phosphatidylinositol derivatives headgroups exhibit different domain properties.
The other aspect of my study was to characterize the biophysical properties of Ceramide 1-phosphate (Cer1P). Ceramide is a well-characterized sphingolipid metabolite and second messenger that participates in numerous biological processes. When ceramide is phosphorylated by ceramide kinase (CERK), Cer1P is obtained. It has recently been proposed that Cer1P is involved in cell survival, cell proliferation, inflammation and phagocytosis. How Cer1P is involved in such a variety of functions hasn’t been fully characterized yet. Therefore knowing the biophysical properties of Cer1P is the first step to understand how the complicated functions involving Cer1P are performed in biological systems. In this study, we have shown that: 1) the order of lipid raft domains is affected by the presence of Cer1P in a manner that is dependent on the ionization state of the lipid. The headgroup of Cer1P might function as an electrostatic switch that drives the lipid in and out of gel phase domains, which is expected to affect the domain morphology and the availability of the lipid for proteins. 2) Cer1P shows some co-localization with PIPs. This co-localization of Cer1P with PIPs may have implications in how Cer1P is involved in the PI3K signaling pathway to promote cell survival and cell proliferation.