It is known that nuclear lipids play a role in proliferation, differentiation, and apoptotic process. Cellular nuclei contain high levels of phosphatidylcholine and sphingomyelin, which are partially linked with cholesterol and proteins to form lipid-protein complexes. These lipids are also associated with transcription factors and newly synthesized RNA but, up to date, their organization is still unknown. The aim of the present work was to study if these specific lipid-protein interactions could be nuclear membrane microdomains and to evaluate their possible role. The results obtained demonstrate for the first time the existence of nuclear microdomains characterized by a specific lipid composition similar to that of intranuclear lipid-protein complexes previously described. Nuclear microdomain lipid composition changes during cell proliferation when the content of newly synthesized RNA increases. Because previous data show a correlation between nuclear lipids and transcription process, the role of nuclear microdomains in cellular functions is discussed. INTRODUCTIONExtensive research on biological membranes has led to the modification of the Singer-Nicolson fluid-mosaic model and has indicated that certain classes of lipids and proteins are not randomly distributed over the membrane but form distinct microdomains (Lichtenberg et al., 2005) that can exist in equilibrium (Brown, 2006). The most studied class of microdomain are the cholesterol (CHO) and sphingomyelin (SM)-enriched lipid rafts (Edidin, 2003). However, the distinct concepts of lipid rafts, detergent-resistant membranes and liquid-ordered lipid phases are often confused in the current literature (Lichtenberg et al., 2005). Rafts are described as transient detergent-resistant membrane microdomains enriched in sphingolipids and CHO, whereas "detergent-resistant membranes" are formed after detergent treatment and do not correspond to any membrane structure. In contrast, "liquid-ordered lipid phase domains" are the result of interactions between high levels of CHO and phospholipid fatty acyl chains (Lichtenberg et al., 2005). On the other hand, several lines of investigation have supported the idea that detergent-resistant membranes are not detergent-induced artifacts, but do exist as domains in cellular membranes (Brown and London, 1997). As visualization of rafts in living cells is difficult, their existence and function rely on indirect methods such as detergent extraction (Munro, 2003). However, Schuck et al. (2003) have found that various detergents differ considerably in their ability to selectively solubilize membrane proteins and enrich the content of sphingolipids and CHO. Insolubility in detergents like Triton X-100 was observed in lipid bilayers, which exist in physical states where lipid packing is tight (London and Brown, 2000). However, Braccia et al. (2003) have demonstrated that isolation of lipid rafts from microvillar membrane with "conventional" Triton X-100 at low temperature and with Brij 98 at 37°C have essentially similar profiles ...