The fluid mosaic membrane model proved to be a very useful hypothesis in explaining many, but certainly not all, phenomena taking place in biological membranes. New experimental data show that the compartmentalization of membrane components can be as important for effective signal transduction as is the fluidity of the membrane. In this work, we pay tribute to the Singer-Nicolson model, which is near its 30th anniversary, honoring its basic features, ''mosaicism'' and ''diffusion,'' which predict the interspersion of proteins and lipids and their ability to undergo dynamic rearrangement via Brownian motion. At the same time, modifications based on quantitative data are proposed, highlighting the often genetically predestined, yet flexible, multilevel structure implementing a vast complexity of cellular functions. This new ''dynamically structured mosaic model'' bears the following characteristics: emphasis is shifted from fluidity to mosaicism, which, in our interpretation, means nonrandom codistribution patterns of specific kinds of membrane proteins forming smallscale clusters at the molecular level and large-scale clusters (groups of clusters, islands) at the submicrometer level. The cohesive forces, which maintain these assemblies as principal elements of the membranes, originate from within a microdomain structure, where lipid-lipid, protein-protein, and protein-lipid interactions, as well as sub-and supramembrane (cytoskeletal, extracellular matrix, other cell) effectors, many of them genetically predestined, play equally important roles. The concept of fluidity in the original model now is interpreted as permissiveness of the architecture to continuous, dynamic restructuring of the molecular-and higherlevel clusters according to the needs of the cell and as evoked by the environment.

The private ␣-chains of IL-2 and IL-15 receptors (IL-2R and IL-15R) share the signaling ␤-and ␥c-subunits, resulting in both common and contrasting roles of IL-2 and IL-15 in T cell function. Knowledge of the cytokine-dependent subunit assembly is indispensable for understanding the paradox of distinct signaling capacities. By using fluorescence resonance energy transfer and confocal microscopy, we have shown that IL-2R␣, IL-15R␣, IL-2͞15R␤ and ␥ csubunits, as well as MHC class I and II glycoproteins formed supramolecular receptor clusters in lipid rafts of the T lymphoma line Kit 225 FT7.10. Fluorescence crosscorrelation microscopy demonstrated the comobility of IL-15R␣ with IL-2R␣ and MHC class I. A model was generated for subunit switching between IL-2R␣ and IL-15R␣ upon the binding of the appropriate cytokine resulting in the formation of high-affinity heterotrimeric receptors. This model suggests a direct role for the ␣-subunits, to which no definite function has been assigned so far, in tuning cellular responses to IL-2 or IL-15. In addition, both ␣-chains were at least partially homodimerized͞oligomerized, which could be the basis of distinct signaling pathways by the two cytokines.